PHYLOGENETIC PERSPECTIVES IN THE EVOLUTION OF PARENTAL CARE IN RAY-FINNED FISHES

The evolution of sex roles: The importance of ecology and social environment

Males and females often have different roles in reproduction, although the origin of these differences has remained controversial. Explaining the enigmatic reversed sex roles where males sacrifice their mating potential and provide full parental care is a particularly long-standing challenge in evolutionary biology. While most studies focused on ecological factors as the drivers of sex roles, recent research highlights the significance of social factors such as the adult sex ratio. To disentangle these propositions, here, we investigate the additive and interactive effects of several ecological and social factors on sex role variation using shorebirds (sandpipers, plovers, and allies) as model organisms that provide the full spectrum of sex role variation including some of the best-known examples of sex-role reversal. Our results consistently show that social factors play a prominent role in driving sex roles. Importantly, we show that reversed sex roles are associated with both male-skewed adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses provide general support for sex ratios driving sex role variations rather than being a consequence of sex roles. Together, these important results open future research directions by showing that different mating opportunities of males and females play a major role in generating the evolutionary diversity of sex roles, mating system, and parental care.

Divergence in Reproductive Behaviors Is Associated with the Evolutionary Loss of Parental Care

AbstractThe mechanisms underlying the divergence of reproductive strategies between closely related species are still poorly understood. Additionally, it is unclear which selective factors drive the evolution of reproductive behavioral variation and how these traits coevolve, particularly during early divergence. To address these questions, we quantified behavioral differences in a recently diverged pair of Nova Scotian three-spined stickleback (Gasterosteus aculeatus) populations, which vary in parental care, with one population displaying paternal care and the other lacking this. We compared both populations, and a full reciprocal F1 hybrid cross, across four major reproductive stages: territoriality, nesting, courtship, and parenting. We identified significant divergence in a suite of heritable behaviors. Importantly, F1 hybrids exhibited a mix of behavioral patterns, some of which suggest sex linkage. This system offers fresh insights into the coevolutionary dynamics of reproductive behaviors during early divergence and offers support for the hypothesis that coevolutionary feedback between sexual selection and parental care can drive rapid evolution of reproductive strategies.

Evolutionary trade-offs between testes size and parenting in Neotropical glassfrogs

In males, large testes size signifies high sperm production and is commonly linked to heightened sperm competition levels. It may also evolve as a response to an elevated risk of sperm depletion due to multiple mating or large clutch sizes. Conversely, weapons, mate or clutch guarding may allow individuals to monopolize mating events and preclude sperm competition, thereby reducing the selection of large testes. Herein, we examined how paternal care, sexual size dimorphism (SSD), weaponry and female fecundity are linked to testes size in glassfrogs. We found that paternal care was associated with a reduction in relative testes size, suggesting an evolutionary trade-off between testes size and parenting. Although females were slightly larger than males and species with paternal care tended to have larger clutches, there was no significant relationship between SSD, clutch size and relative testes size. These findings suggest that the evolution of testes size in glassfrogs is influenced by sperm competition risk, rather than sperm depletion risk. We infer that clutch guarding precludes the risk of fertilization by other males and consequently diminishes selective pressure for larger testes. Our study highlights the prominent role of paternal care in the evolution of testes size in species with external fertilization.

Fertilization Mode Covaries with Body Size

AbstractThe evolution of internal fertilization has occurred repeatedly and independently across the tree of life. As it has evolved, internal fertilization has reshaped sexual selection and the covariances among sexual traits, such as testes size, and gamete traits. But it is unclear whether fertilization mode also shows evolutionary associations with traits other than primary sex traits. Theory predicts that fertilization mode and body size should covary, but formal tests with phylogenetic control are lacking. We used a phylogenetically controlled approach to test the covariance between fertilization mode and adult body size (while accounting for latitude, offspring size, and offspring developmental mode) among 1,232 species of marine invertebrates from three phyla. Within all phyla, external fertilizers are consistently larger than internal fertilizers: the consequences of fertilization mode extend to traits that are only indirectly related to reproduction. We suspect that other traits may also coevolve with fertilization mode in ways that remain unexplored.

The evolution of monogamy in cichlids and marine reef fishes

Although several hypotheses have been proposed to explain the emergence of social monogamy, its origin is still intensely debated. Monogamy has many potential drivers, but evolutionary causality among them remains unclear. Using phylogenetic comparative methods within a Bayesian framework we explored the evolution of monogamy in cichlids and in marine reef fishes because, while both groups are characterised by unusually high incidence of social monogamy, they face very different ecological challenges. For each group, we examined four classic hypotheses that explain the evolution of monogamy: female dispersal, male mate guarding, female–female intolerance, and the biparental care hypotheses. We also explored whether the ecological traits of diet and shelter use are evolutionarily coupled with these hypotheses or with monogamy. First, we found that the evolution of monogamy was predicted by male territoriality in cichlids and simultaneous male and female territoriality in marine reef fishes. We suggest that these results provide support for the male mate guarding hypothesis in cichlids and female–female intolerance hypothesis in marine reef fishes. Second, we demonstrate clear evidence against the biparental care hypothesis, as biparental care was a consequence, not a cause, of monogamy in our analyses. Third, as female dispersal drove the loss of monogamy in both cichlids and marine reef fishes, this suggests the female dispersal hypothesis is not driving the evolution of monogamy in either group. These findings in two highly-monogamous fish taxa largely support prior findings from primate and bird comparative studies and provide novel large-scale evidence for a link between mate guarding and the evolution of monogamy.

Influence of microhabitat, fecundity, and parental care on the evolution of sexual size dimorphism in Caribbean Eleutherodactylus frogs

Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intrasexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean Eleutherodactylus frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean Eleutherodactylus females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female-biased SSD in Caribbean Eleutherodactylus results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of SSD.

Life history and the evolutionary loss of parental care

Parental care has been gained and lost evolutionarily multiple times. While many studies have focused on the origin of care, few have explored the evolutionary loss of care. Understanding the loss of parental care is important as the conditions that favour its loss will not necessarily be the opposite of those that favour the evolution of care. Evolutionary hysteresis (the case in which evolution depends on the history of a system) could create a situation in which it is relatively challenging to lose care once it has evolved. Here, using a mathematical approach, we explore the evolutionary loss of parental care in relation to basic life-history conditions. Our results suggest that parental care is most likely to be lost when egg and adult death rates are low, eggs mature quickly, and the level of care provided is high. We also predict evolutionary hysteresis with respect to egg maturation rate: as egg maturation rate decreases, it becomes increasingly more costly to lose care than to gain it. This suggests that once care is present, it will be particularly challenging for it to be lost if eggs develop slowly.

Sexual Dimorphism in the Chinese Endemic Species Hynobius maoershanensis (Urodela: Hynobiidae)

Simple Summary

In the present study, we examined the sexual dimorphism of Hynobius maoershanensis. The results showed that it exhibits sexual shape dimorphism, with five morphological traits being male-biased and one being female-biased. The observed sexual shape dimorphism between males and females could be explained using the sexual selection and fecundity theory hypotheses.

Abstract

Sexual dimorphism is common in most vertebrate species and has diverse manifestations. The study of sexual dimorphism has critical significance for evolutionary biological and ecological adaptation. In this study, we analysed the morphometric data of Hynobius maoershanensis, a rare and endangered species, to examine sexual dimorphism in size and shape. A total of 61 H. maoershanensis individuals (9 adult females and 52 adult males) were used in this study. We measured 14 morphological variables and weight of each individual. Analysis of covariance using snout–vent length (SVL) as the covariate showed significant differences in head width (HW), tail length (TL), tail height (TH), forelimb length (FLL), hindlimb length (HLL) and space between axilla and groin (AGS) between the male and female. The female AGS was greater than that of the male, whereas males had greater HW, TL, TH, FLL and HLL than females. The findings show that sexual dimorphism is present in terms of shape but not in terms of size. The wider head of the male could improve mating success, and its thicker limbs and longer tail might facilitate courtship. The females’ wider AGS may increase reproductive output. Our results support sexual dimorphism in H. maoershanensis, which could be explained by the sexual selection and fecundity theory hypothesis.

The economics of brain size evolution in vertebrates

Across the animal kingdom, we see remarkable variation in brain size. This variation has even increased over evolutionary time. Traditionally, studies aiming to explain brain size evolution have looked at the fitness benefits of increased brain size in relation to its increased cognitive performance in the social and/or ecological domain. However, brains are among the most energetically expensive tissues in the body and also require an uninterrupted energy supply. If not compensated, these energetic demands inevitably lead to a reduction in energy allocation to other vital functions. In this review, we summarize how an increasing number of studies show that to fully comprehend brain size evolution and the large variation in brain size across lineages, it is important to look at the economics of brains, including the different pathways through which the high energetic costs of brains can be offset. We further show how numerous studies converge on the conclusion that cognitive abilities can only drive brain size evolution in vertebrate lineages where they result in an improved energy balance through favourable ecological preconditions. Cognitive benefits that do not directly improve the organism's energy balance can only be selectively favoured when they produce such large improvements in reproduction or survival that they outweigh the negative energetic effects of the large brain.

The costs and benefits of larger brains in fishes

The astonishing diversity of brain sizes observed across the animal kingdom is typically explained in the context of trade-offs: the benefits of a larger brain, such as enhanced cognitive ability, are balanced against potential costs, such as increased energetic demands. Several hypotheses have been formulated in this framework, placing different emphasis on ecological, behavioural, or physiological aspects of trade-offs in brain size evolution. Within this body of work, there exists considerable taxonomic bias towards studies of birds and mammals, leaving some uncertainty about the generality of the respective arguments. Here, we test three of the most prominent such hypotheses, the 'expensive tissue', 'social brain' and 'cognitive buffer' hypotheses, in a large dataset of fishes, derived from a publicly available resource (FishBase). In accordance with predictions from the 'expensive tissue' and the 'social brain' hypothesis, larger brains co-occur with reduced fecundity and increased sociality in at least some Classes of fish. Contrary to expectations, however, lifespan is reduced in large-brained fishes, and there is a tendency for species that perform parental care to have smaller brains. As such, it appears that some potential costs (reduced fecundity) and benefits (increased sociality) of large brains are near universal to vertebrates, whereas others have more lineage-specific effects. We discuss our findings in the context of fundamental differences between the classically studied birds and mammals and the fishes we analyse here, namely divergent patterns of growth, parenting and neurogenesis. As such, our work highlights the need for a taxonomically diverse approach to any fundamental question in evolutionary biology.

The genetic basis of a novel reproductive strategy in Sulawesi ricefishes: How modularity and a low number of loci shape pelvic brooding

The evolution of complex phenotypes like reproductive strategies is challenging to understand, as they often depend on multiple adaptations that only jointly result in a specific functionality. Sulawesi ricefishes (Adrianichthyidae) evolved a reproductive strategy termed as pelvic brooding. In contrast to the more common transfer brooding, female pelvic brooders carry an egg bundle connected to their body for weeks until the fry hatches. To examine the genetic architecture of pelvic brooding, we crossed the pelvic brooding Oryzias eversi and the transfer brooding Oryzias nigrimas (species divergence time: ∼3.6 my). We hypothesize, that a low number of loci and modularity have facilitated the rapid evolution of pelvic brooding. Traits associated to pelvic brooding, like rib length, pelvic fin length, and morphology of the genital papilla, were correlated in the parental species but correlations were reduced or lost in their F1 and F2 hybrids. Using the Castle-Wright estimator, we found that generally few loci underlie the studied traits. Further, both parental species showed modularity in their body plans. In conclusion, morphological traits related to pelvic brooding were based on a few loci and the mid-body region likely could evolve independently from the remaining body parts. Both factors presumably facilitated the evolution of pelvic brooding.

Integrative Studies of the Effects of Mothers on Offspring: An Example from Wild North American Red Squirrels

Animal species vary in whether they provide parental care or the type of care provided, and this variation in parental care among species has been a common focus of comparative studies. However, the proximate causes and ultimate consequences of within-species variation in parental care have been less studied. Most studies about the impacts of within-species variation in parental care on parental fitness have been in primates, whereas studies in laboratory rodents have been invaluable for understanding what causes inter-individual variation in parental care and its influence on offspring characteristics. We integrated both of these perspectives in our long-term study of North American red squirrels (Tamiasciurus hudsonicus) in the Yukon, Canada, where we have focused on understanding the impacts of mothers on offspring. This includes documenting the impacts that mothers or the maternal environment itself has on their offspring, identifying how changes in maternal physiology impact offspring characteristics, the presence of individual variation in maternal attentiveness toward offspring before weaning and its fitness consequences, and postweaning maternal care and its fitness consequences. We provide an overview of these contributions to understanding the impacts mothers have on their offspring in red squirrels using an integrative framework and contrast them with studies in the laboratory.

Evolutionary change in the construction of the nursery environment when parents are prevented from caring for their young directly

Parental care can be partitioned into traits that involve direct engagement with offspring and traits that are expressed as an extended phenotype and influence the developmental environment, such as constructing a nursery. Here, we use experimental evolution to test whether parents can evolve modifications in nursery construction when they are experimentally prevented from supplying care directly to offspring. We exposed replicate experimental populations of burying beetles (Nicrophorus vespilloides) to different regimes of posthatching care by allowing larvae to develop in the presence (Full Care) or absence of parents (No Care). After only 13 generations of experimental evolution, we found an adaptive evolutionary increase in the pace at which parents in the No Care populations converted a dead body into a carrion nest for larvae. Cross-fostering experiments further revealed that No Care larvae performed better on a carrion nest prepared by No Care parents than did Full Care larvae. We conclude that parents construct the nursery environment in relation to their effectiveness at supplying care directly, after offspring are born. When direct care is prevented entirely, they evolve to make compensatory adjustments to the nursery in which their young will develop. The rapid evolutionary change observed in our experiments suggests there is considerable standing genetic variation for parental care traits in natural burying beetle populations-for reasons that remain unclear.

External reproductive structures of male Surfperches (Embiotocidae): Urogenital papilla

Fishes exhibit a dynamic array of reproductive diversity. Among the various forms of reproduction, internal fertilization has evolved at least 13 times and has been observed in 2-3% of teleost species, half of which are marine. Internal fertilization has led to the evolution of male intromittent organs to aid in sperm transfer during copulation. The intromittent organs of poeciliids have been widely investigated, but little research has focused on the intromittent organs of marine fishes. The surfperches are internally fertilizing, viviparous fishes with a ventrally located papilla. In this article, the external morphology of 19 Embiotocidae species was examined. Additionally, paraffin histology was used to investigate the walleye surfperch (Hyperprosopon argenteum), black perch (Embiotoca jacksoni), and dwarf perch (Micrometrus minimus) papilla to aid in the identification of tissue composition to determine its function. External morphology was documented via macro photography and males were dissected to observe the relationship between the testes, spermatic duct, and papilla. We provide histological evidence that the urogenital papilla is directly connected to the spermatic duct and testis and likely involved in spermatozeugmata transfer. Our research also determined that the papillae were composed of both urinary and reproductive tissues, thus we propose the papilla be referred to as a urogenital papilla. Our study contributes to an overall understanding of intromittent organs of marine fishes, particularly in the surfperches, which has not been previously documented.

Sex Reversal and Performance in Fitness-Related Traits During Early Life in Agile Frogs

Sex reversal is a mismatch between genetic sex (sex chromosomes) and phenotypic sex (reproductive organs and secondary sexual traits). It can be induced in various ectothermic vertebrates by environmental perturbations, such as extreme temperatures or chemical pollution, experienced during embryonic or larval development. Theoretical studies and recent empirical evidence suggest that sex reversal may be widespread in nature and may impact individual fitness and population dynamics. So far, however, little is known about the performance of sex-reversed individuals in fitness-related traits compared to conspecifics whose phenotypic sex is concordant with their genetic sex. Using a novel molecular marker set for diagnosing genetic sex in agile frogs (Rana dalmatina), we investigated fitness-related traits in larvae and juveniles that underwent spontaneous female-to-male sex reversal in the laboratory. We found only a few differences in early life growth, development, and larval behavior between sex-reversed and sex-concordant individuals, and altogether these differences did not clearly support either higher or lower fitness prospects for sex-reversed individuals. Putting these results together with earlier findings suggesting that sex reversal triggered by heat stress may be associated with low fitness in agile frogs, we propose the hypothesis that the fitness consequences of sex reversal may depend on its etiology.

Life history, mating dynamics and the origin of parental care

Parental care, mating dynamics and life history co-evolve. Understanding the diversity of reproductive patterns found in nature is a major focus of evolutionary ecology research. Previous research suggests that the origin of parental care of eggs will be favoured when egg and adult death rates and juvenile survival are relatively high. However, the previous research that explored the link between care and life history did not account for among-species variation in mating dynamics. As mating dynamics are generally expected to influence care, we explore, theoretically, the life-history conditions (stage-specific rates of maturation and survival) that favour parental care across three mating scenarios: reproductive rate (1) is unaffected by males (assuming that some males are present), (2) increases as male abundance increases or (3) decreases as male abundance increases. Across scenarios, all forms of care were most strongly favoured when egg and adult death rates, juvenile survival and female egg maturation rates were relatively high. When reproductive rate was unaffected by male abundance or increased as male abundance increased, as we might expect in systems in which females are mate-limited, all forms of care were most strongly favoured when male egg maturation rate (i.e. the rate at which male eggs develop, mature and hatch) was moderate or high. When greater male abundance inhibited reproduction, which might occur in systems with intense male-male competition, all forms of care were most strongly favoured when male egg maturation rate was low-to-moderate. These results suggest that life history affects the evolution of parental care, and sex-specific life history can interact with mating dynamics to influence the origin of care.

Sexual Dichromatism Is Decoupled from Diversification over Deep Time in Fishes

AbstractSexually selected traits have long been thought to drive diversification, but support for this hypothesis has been persistently controversial. In fishes, sexually dimorphic coloration is associated with assortative mating and speciation among closely related species, as shown in classic studies. However, it is unclear whether these results can generalize to explain diversity patterns across ray-finned fishes, which contain the majority of vertebrate species and 96% of fishes. Here, we use phylogenetic approaches to test for an association between sexual dichromatism and diversification rates (speciation minus extinction) in ray-finned fishes. We assembled dichromatism data for 10,898 species, a data set of unprecedented size. We found no difference in diversification rates between monochromatic and dichromatic species when including all ray-finned fishes. However, at lower phylogenetic scales (within orders and families), some intermediate-sized clades did show an effect of dichromatism on diversification. Surprisingly, dichromatism could significantly increase or decrease diversification rates. Moreover, we found no effect in many of the clades initially used to link dichromatism to speciation in fishes (e.g., cichlids) or an effect only at shallow scales (within subclades). Overall, we show how the effects of dichromatism on diversification are highly variable in direction and restricted to certain clades and phylogenetic scales.

Complex sexually dimorphic traits shape the parallel evolution of a novel reproductive strategy in Sulawesi ricefishes (Adrianichthyidae)

Background

Pelvic brooding is a form of uni-parental care, and likely evolved in parallel in two lineages of Sulawesi ricefishes. Contrary to all other ricefishes, females of pelvic brooding species do not deposit eggs at a substrate (transfer brooding), but carry them until the fry hatches. We assume that modifications reducing the costs of egg carrying are beneficial for pelvic brooding females, but likely disadvantageous in conspecific males, which might be resolved by the evolution of sexual dimorphism via sexual antagonistic selection. Thus we hypothesize that the evolution of pelvic brooding gave rise to female-specific skeletal adaptations that are shared by both pelvic brooding lineages, but are absent in conspecific males and transfer brooding species. To tackle this, we combine 3D-imaging and morphometrics to analyze skeletal adaptations to pelvic brooding.

Results

The morphology of skeletal traits correlated with sex and brooding strategy across seven ricefish species. Pelvic brooding females have short ribs caudal of the pelvic girdle forming a ventral concavity and clearly elongated and thickened pelvic fins compared to both sexes of transfer brooding species. The ventral concavity limits the body cavity volume in female pelvic brooders. Thus body volumes are smaller compared to males in pelvic brooding species, a pattern sharply contrasted by transfer brooding species.

Conclusions

We showed in a comparative framework that highly similar, sexually dimorphic traits evolved in parallel in both lineages of pelvic brooding ricefish species. Key traits, present in all pelvic brooding females, were absent or much less pronounced in conspecific males and both sexes of transfer brooding species, indicating that they are non-beneficial or even maladaptive for ricefishes not providing extended care. We assume that the combination of ventral concavity and robust, elongated fins reduces drag of brooding females and provides protection and stability to the egg cluster. Thus ricefishes are one of the rare examples where environmental factors rather than sexual selection shaped the evolution of sexually dimorphic skeletal adaptations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01791-z.

Phylogenetic supertree reveals detailed evolution of SARS-CoV-2

Corona Virus Disease 2019 (COVID-19) caused by the emerged coronavirus SARS-CoV-2 is spreading globally. The origin of SARS-Cov-2 and its evolutionary relationship is still ambiguous. Several reports attempted to figure out this critical issue by genome-based phylogenetic analysis, yet limited progress was obtained, principally owing to the disability of these methods to reasonably integrate phylogenetic information from all genes of SARS-CoV-2. Supertree method based on multiple trees can produce the overall reasonable phylogenetic tree. However, the supertree method has been barely used for phylogenetic analysis of viruses. Here we applied the matrix representation with parsimony (MRP) pseudo-sequence supertree analysis to study the origin and evolution of SARS-CoV-2. Compared with other phylogenetic analysis methods, the supertree method showed more resolution power for phylogenetic analysis of coronaviruses. In particular, the MRP pseudo-sequence supertree analysis firmly disputes bat coronavirus RaTG13 be the last common ancestor of SARS-CoV-2, which was implied by other phylogenetic tree analysis based on viral genome sequences. Furthermore, the discovery of evolution and mutation in SARS-CoV-2 was achieved by MRP pseudo-sequence supertree analysis. Taken together, the MRP pseudo-sequence supertree provided more information on the SARS-CoV-2 evolution inference relative to the normal phylogenetic tree based on full-length genomic sequences.

Genomic landscape of reproductive isolation in Lucania killifish: The role of sex loci and salinity

Adaptation to different environments can directly and indirectly generate reproductive isolation between species. Bluefin killifish (Lucania goodei) and rainwater killifish (L. parva) are sister species that have diverged across a salinity gradient and are reproductively isolated by habitat, behavioural, extrinsic and intrinsic post-zygotic isolation. We asked if salinity adaptation contributes indirectly to other forms of reproductive isolation via linked selection and hypothesized that low recombination regions, such as sex chromosomes or chromosomal rearrangements, might facilitate this process. We conducted QTL mapping in backcrosses between L. parva and L. goodei to explore the genetic architecture of salinity tolerance, behavioural isolation and intrinsic isolation. We mapped traits relative to a chromosome that has undergone a centric fusion in L. parva (relative to L. goodei). We found that the sex locus appears to be male determining (XX-XY), was located on the fused chromosome and was implicated in intrinsic isolation. QTL associated with salinity tolerance were spread across the genome and did not overly co-localize with regions associated with behavioural or intrinsic isolation. This preliminary analysis of the genetic architecture of reproductive isolation between Lucania species does not support the hypothesis that divergent natural selection for salinity tolerance led to behavioural and intrinsic isolation as a by-product. Combined with previous studies in this system, our work suggests that adaptation as a function of salinity contributes to habitat isolation and that reinforcement may have contributed to the evolution of behavioural isolation instead, possibly facilitated by linkage between behavioural isolation and intrinsic isolation loci on the fused chromosome.

The developmental support hypothesis: adaptive plasticity in neural development in response to cues of social support

Across mammals, cues of developmental support, such as touching, licking or attentiveness, stimulate neural development, behavioural exploration and even overall body growth. Why should such fitness-related traits be so sensitive to developmental conditions? Here, we review what we term the 'developmental support hypothesis', a potential adaptive explanation of this plasticity. Neural development can be a costly process, in terms of time, energy and exposure. However, environmental variability may sometimes compromise parental care during this costly developmental period. We propose this environmental variation has led to the evolution of adaptive plasticity of neural and behavioural development in response to cues of developmental support, where neural development is stimulated in conditions that support associated costs. When parental care is compromised, offspring grow less and adopt a more resilient and stress-responsive strategy, improving their chances of survival in difficult conditions, similar to existing ideas on the adaptive value of early-life programming of stress. The developmental support hypothesis suggests new research directions, such as testing the adaptive value of reduced neural growth and metabolism in stressful conditions, and expanding the range of potential cues animals may attend to as indicators of developmental support. Considering evolutionary and ecologically appropriate cues of social support also has implications for promoting healthy neural development in humans. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

The evolution and physiology of male pregnancy in syngnathid fishes

The seahorses, pipefishes and seadragons (Syngnathidae) are among the few vertebrates in which pregnant males incubate developing embryos. Syngnathids are popular in studies of sexual selection, sex-role reversal, and reproductive trade-offs, and are now emerging as valuable comparative models for the study of the biology and evolution of reproductive complexity. These fish offer the opportunity to examine the physiology, behavioural implications, and evolutionary origins of embryo incubation, independent of the female reproductive tract and female hormonal milieu. Such studies allow us to examine flexibility in regulatory systems, by determining whether the pathways underpinning female pregnancy are also co-opted in incubating males, or whether novel pathways have evolved in response to the common challenges imposed by incubating developing embryos and releasing live young. The Syngnathidae are also ideal for studies of the evolution of reproductive complexity, because they exhibit multiple parallel origins of complex reproductive phenotypes. Here we assay the taxonomic distribution of syngnathid parity mode, examine the selective pressures that may have led to the emergence of male pregnancy, describe the biology of syngnathid reproduction, and highlight pressing areas for future research. Experimental tests of a range of hypotheses, including many generated with genomic tools, are required to inform overarching theories about the fitness implications of pregnancy and the evolution of male pregnancy. Such information will be widely applicable to our understanding of fundamental reproductive and evolutionary processes in animals.

Anisogamy selects for male-biased care in self-consistent games with synchronous matings

We reexamine the influential parental investment hypothesis proposed by Trivers for the causal relationship between anisogamy and widespread female-biased parental care. We build self-consistent versions of Maynard Smith's simple evolutionary game between males and females over parental care, and incorporate consequences of anisogamy for gamete production and its trade-off with parental care, and for patterns of mate limitation. As male mating opportunities are limited by females, frequency-dependent selection acts on male strategies. Assuming synchrony of matings in the population, our analytical models find either symmetric sex roles or male-biased care as an evolutionarily stable strategy (ESS), in contrast to Trivers' hypothesis. We simulate evolution in asynchronously mating populations and find that diverse parental roles, including female care, can be ESS depending on the parameters. When caring males can also remate, or when females can increase the clutch size by deserting, there is stronger selection for male-biased care. Hence, we argue that the mating-caring trade-off for males is neither a necessary consequence of anisogamy nor sufficient to select for female-biased care. Instead, the factors excluded from our models-costly competitive traits, sexual selection, and partial parentage-may be necessary for the parental investment hypothesis to work.

Novel life history strategy in a deep sea fish challenges assumptions about reproduction in extreme environments

The deep ocean is frequently assumed to be a homogeneous system lacking the same diverse life history strategies found in shallower waters. However, as our methods for exploring the deep ocean improve, common assumptions about dispersal, reproduction and behavior are constantly being challenged. Fishes exhibit the most diverse reproductive strategies among vertebrates. Understanding life history strategies in deep-sea environments is lacking for many species of fishes. Here, we report a novel reproductive strategy where a fish (Parazen pacificus) provides parental care via mouth brooding. This behavior is observed from a specimen collected with eggs present in the buccal cavity, along with other specimens exhibiting pre-brooding morphologies. This is the first description of this unique life history trait in a deep-sea fish and fills in a gap in the larval literature for this family of fishes and prompts further investigation into other novel reproductive modes of deep-sea fauna.

Sexual dimorphism in Belostoma angustum Lauck (Insecta: Heteroptera: Belostomatidae) may be related to paternal care

The structures involved in parental care are often dimorphic. Female Belostoma angustum water bugs lay eggs on the hemelytra of their mates, where the eggs are brooded until hatching. Males use their hind legs to carry, aerate and protect the eggs. After controlling for covariance between variables, we fitted a series of structural equation models (SEMs) and evaluated the existence of sexual dimorphism in the size of the body and hind legs, in the shape and centroid size of the hemelytrum, and among the static allometry slopes of the size-related differences. Landmarks were used to capture phenotypic variation, by eliminating all non-shape variations with a Procrustes superimposition. Neither the shape of the hemelytrum nor its centroid size was related significantly to the aforementioned linear body measurements. Instead, the differences in the size of the hind legs were mediated by body dimensions only in males. We also found that males were wider and had longer heads than females, according to the SEM intercept values. Our findings suggest that sexual dimorphism in B. angustum may be related to a balance between sexual role reversal and viability costs.

Where are all the moms? External fertilization predicts the rise of male parental care in bony fishes

Parental care shows remarkable variation across the animal kingdom, but while maternal and biparental care are common in terrestrial organisms, male-only care dominates in aquatic species that provide care. Using the most complete phylogenetic tree of bony fishes to date, we test whether the opportunity for external fertilization in aquatic environments can explain the more frequent evolution of male care in this group. We show that paternal care has evolved at least 30 times independently in fish and is found exclusively in externally fertilizing species. Male care is positively associated with pair spawning, suggesting that confidence in paternity is an important determinant of the evolution of care. Crucially, while female care is constrained by other forms of reproductive investment, male care occurs more frequently when females invest heavily in gamete production. Our results suggest that moving control of fertilization outside of the female reproductive tract raises male confidence in parentage and increases the potential for paternal care, highlighting that in an aquatic environment in which fertilization is external, paternal care is an effective reproductive strategy.

Reversals in complex traits uncovered as reticulation events: Lessons from the evolution of parity-mode, chromosome morphology, and maternal resource transfer

Complex traits include, among many others, the evolution of eyes, wings, body forms, reproductive modes, human intelligence, social behavior, diseases, and chromosome morphology. Dollo's law states that the evolution of complex traits is irreversible. However, potential exceptions have been proposed. Here, we investigated whether reticulation, a simple and elegant means by which complex characters may be reacquired, could account for suggested reversals in the evolution of complex characters using two datasets with sufficient genetic coverage and a total of five potential reversals. Our analyses uncovered a potential reversal in the evolution of parity mode and a potential reversal in the evolution of placentotrophy of fish (Cyprinodontiformes) as reticulation events. Moreover, in a reptile that exhibits a potential reversal from viviparity to oviparity (Zootoca vivipara), reticulation provided the most parsimonious explanation for sex chromosome evolution. Therefore, three of the five studied potential reversals were unraveled as reticulation events. This constitutes the first evidence that accounting for reticulation can fundamentally influence the interpretation of the evolution of complex traits, that testing for reticulation is crucial for obtaining robust phylogenies, and that complex ancestral characters may be reacquired through hybridization with a lineage that still exhibits the trait. Hybridization, rather than reappearance of ancestral traits by means of small evolutionary steps, may thus account for suggested exceptions to Dollo's law. Consequently, ruling out reticulation is required to claim the evolutionary reversal of complex characters and potential exceptions to Dollo's rule.

The evolution of parental care diversity in amphibians

Parental care is extremely diverse across species, ranging from simple behaviours to complex adaptations, varying in duration and in which sex cares. Surprisingly, we know little about how such diversity has evolved. Here, using phylogenetic comparative methods and data for over 1300 amphibian species, we show that egg attendance, arguably one of the simplest care behaviours, is gained and lost faster than any other care form, while complex adaptations, like brooding and viviparity, are lost at very low rates, if at all. Prolonged care from the egg to later developmental stages evolves from temporally limited care, but it is as easily lost as it is gained. Finally, biparental care is evolutionarily unstable regardless of whether the parents perform complementary or similar care duties. By considering the full spectrum of parental care adaptations, our study reveals a more complex and nuanced picture of how care evolves, is maintained, or is lost.

Parental care can take many forms but how this diversity arises is not well understood. Here, the authors compile data for over 1300 amphibian species and show that different forms of care evolve at different rates, prolonged care can be easily reduced, and biparental care is evolutionarily unstable.

Coevolution influences the evolution of filial cannibalism, offspring abandonment and parental care

Understanding evolutionary patterns of parental investment and care has been a long-standing focus in studies of evolutionary and behavioural ecology. Indeed, patterns of investment and care are highly diverse, and fully understanding such diversity has been challenging. Recently, several studies have highlighted the need to consider coevolutionary dynamics in studies of parental care, as parental care is likely to co-occur and co-originate with a range of other traits. Two traits that commonly co-occur with parental care are offspring abandonment (the termination of parental investment prior to full independence in offspring) and filial cannibalism (the consumption of one's offspring). Here, we use a mathematical framework to explore how co-occurrence and coevolution among care, abandonment and cannibalism can influence the life-history conditions under which care is expected to evolve. Our results suggest that in some cases, the evolution of parental care can be inhibited by offspring abandonment and filial cannibalism. In other cases, abandonment and filial cannibalism that benefits parents can promote the evolution of parental care. It is particularly interesting that behaviours that seem so contrary to care-that is, eating or abandoning one's young-can in some cases broaden the conditions under which care can evolve. In general, our findings highlight that considering co-occurrence and coevolutionary dynamics between two or more traits is essential to understanding the evolution of trait diversity.

Mating Suppresses Alarm Response in Zebrafish

Mating and flight from threats are innate behaviors that enhance species survival [1, 2]. Stimuli to these behaviors often are contemporaneous and conflicting [3, 4]. Both how such conflicts are resolved and where in the brain such decisions are made are poorly understood. For teleosts, olfactory stimuli are key elements of mating and threat responses [5-7]. For example, zebrafish manifest a stereotypical escape response when exposed to an alarm substance released from injured conspecific skin ("skin extract") [8, 9]. We find that when mating, fish ignore this threatening stimulus. Water conditioned by the mating fish ("mating water") suffices to suppress much of the alarm-response behavior. By 2-photon imaging of calcium transients [10], we mapped the regions of the brain responding to skin extract and to mating water. In the telencephalon, we found regions where the responses overlap, one region (medial Dp) to be predominantly activated by skin extract, and another, Vs, to be predominantly activated by mating water. When mating water and skin extract were applied simultaneously, the alarm-specific response was suppressed, while the mating-water-specific response was retained, corresponding to the dominance of mating over flight behavior. The choice made, for reproduction over escape, is opposite to that of mammals, presumably reflecting how the balance affects species survival.

Gonad pathology and intersex severity in pelagic (Tilapia zilli) and benthic (Neochanna diversus and Clarias gariepinus) species from a pesticide-impacted agrarian catchment, south-south Nigeria

Pesticides are distributed to different degrees in surface water and sediment, thus, risks of toxicity and adverse impacts to physiology of resident species could be determined by their microhabitat (sub-habitat) associations, either water column or sediment. River Owan receives pesticide input from diffuse sources from adjacent farmlands. Surface water, sediment and fish samples [(Pelagic: Tilapia zilli (n = 92) and benthic: Neochanna diversus (n = 59), Clarias gariepinus (n = 68) were collected within the catchment area across seasons for 18-months (August 2016-January 2018) and measured for pesticide levels. Testicular and Ovarian tissue samples across the three species were also examined for pathological alterations. Individual pesticide concentrations in surface water and sediment exceeded international allowable limits, while concentration of pesticide residues in tissues of benthic species was higher compared to pelagic fish. Histopathological assessment revealed a higher incidence of ovarian disruption including atretic follicles, intersex and disorganization of ovarian structure in benthic (bottom-dwelling) fish (C. gariepinus and N. diversus) compared to pelagic (water-column) fish (T. zilli). Males benthic fish species also recorded more severe anomalies, compared to pelagic fish. The damages and anomalies observed in ovarian and testicular tissue indicate chronic responses to pollutant exposure, and implicates the elevation of pesticide concentrations in surface water and sediment above permissible limits. The higher incidence of anomalies recorded for benthic species compared to pelagic species, indicates greater risks of reproductive disruption and could be associated with the microhabitat preferences (water-column or sediment).

Parental care and the evolution of terrestriality in frogs

Frogs and toads (Anura) exhibit some of the most diverse parental strategies in vertebrates. Identifying the evolutionary origins of parenting is fundamental to understanding the relationships between sexual selection, social evolution and parental care systems of contemporary Anura. Moreover, parenting has been hypothesized to allow the invasion of terrestrial habitats by the ancestors of terrestrial vertebrates. Using comprehensive phylogenetic analyses of frogs and toads based on data from over 1000 species that represent 46 out of 55 Anura families, we test whether parental care is associated with terrestrial reproduction and several life-history traits. Here, we show that both the duration of care and offspring protection by males and females have coevolved with terrestrial reproduction. Sexual size dimorphism is also related to care, because the large male size relative to female size is associated with increased paternal care. Furthermore, increased egg size and reduced clutch volume are associated with increased care in bivariate but not in multivariate analyses, suggesting that the relationships between care, egg size and clutch volume are mediated by terrestrial reproduction. Taken together, our results suggest that parenting by males and females has coevolved, and complex parenting traits have evolved several times independently in Anura in response to breeding in terrestrial environments.

Sex roles and sexual selection: lessons from a dynamic model system

Our understanding of sexual selection has greatly improved during the last decades. The focus is no longer solely on males, but also on how female competition and male mate choice shape ornamentation and other sexually selected traits in females. At the same time, the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection, and their evolutionary consequences. Here, I review insights from a model system with exceptionally dynamic sexual selection, the two-spotted goby fish Gobiusculus flavescens. The species displays a complete reversal of sex roles over a 3-month breeding season. The reversal is driven by a dramatic change in the operational sex ratio, which is heavily male-biased at the start of the season and heavily female-biased late in the season. Early in the season, breeding-ready males outnumber mature females, causing males to be highly competitive, and leading to sexual selection on males. Late in the season, mating-ready females are in excess, engage more in courtship and aggression than males, and rarely reject mating opportunities. With typically many females simultaneously courting available males late in the season, males become selective and prefer more colorful females. This variable sexual selection regime likely explains why both male and female G. flavescens have ornamental colors. The G. flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluctuations in mating competition. Future work should explore whether sexual selection is equally dynamic on a spatial scale, and related spatiotemporal dynamics.

Not all sex ratios are equal: the Fisher condition, parental care and sexual selection

The term 'sex roles' encapsulates male-female differences in mate searching, competitive traits that increase mating/fertilization opportunities, choosiness about mates and parental care. Theoretical models suggest that biased sex ratios drive the evolution of sex roles. To model sex role evolution, it is essential to note that in most sexually reproducing species (haplodiploid insects are an exception), each offspring has one father and one mother. Consequently, the total number of offspring produced by each sex is identical, so the mean number of offspring produced by individuals of each sex depends on the sex ratio (Fisher condition). Similarly, the total number of heterosexual matings is identical for each sex. On average, neither sex can mate nor breed more often when the sex ratio is even. But equally common in which sex ratio? The Fisher condition only applies to some reproductive measures (e.g. lifetime offspring production or matings) for certain sex ratios (e.g. operational or adult sex ratio; OSR, ASR). Here, we review recent models that clarify whether a biased OSR, ASR or sex ratio at maturation (MSR) have a causal or correlational relationship with the evolution of sex differences in parental care and competitive traits-two key components of sex roles. We suggest that it is more fruitful to understand the combined effect of the MSR and mortality rates while caring and competing than that of the ASR itself. In short, we argue that the ASR does not have a causal role in the evolution of parental care. We point out, however, that the ASR can be a cue for adaptive phenotypic plasticity in how each sex invests in parental care.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Parental care compromises feeding in the pumpkinseed (Lepomis gibbosus)

Providing parental care is potentially costly. Costs can arise through elevated energy expenditure or from an increased risk of mortality. A cost of parental care can also occur because a parent is compromised in their ability to forage. We used pumpkinseed Lepomis gibbosus, a fish with an alternative male mating strategy, to test whether parental males differed in their feeding in comparison with females and cuckolder males. To address this question, we examined the stomach contents of female, cuckolder male, and parental male pumpkinseed during the breeding season over an entire diel cycle. We showed that parental males had a lower total weight of food in their stomachs in comparison with females, while cuckolder males did not. Parental males also had a lower weight and number of chironomids in their stomachs. The temporal pattern of feeding of parental males diverged from that of females, and they had a lower probability of pupal chironomids in their stomachs, which implies spatial segregation in foraging. Parental males had a greater probability of conspecific eggs in their stomachs than females, while the probability of egg cannibalism did not differ between cuckolder males and females. Overall, these finding meet predictions in accordance with an assumption that parental care and territoriality can compromise feeding.

Patterns of parental care in Neotropical glassfrogs: fieldwork alters hypotheses of sex-role evolution

Many animals provide parental care to offspring. Parental sex-roles vary extensively across taxa, and such patterns are considered well documented. However, information on amphibians is lacking relative to other vertebrate groups. We combine natural history observations with functional and historical analyses to examine the evolution of egg care in glassfrogs (Centrolenidae). Parental care was considered rare and predominately provided by males. Our field observations of 40 species revealed that care occurs throughout the family, and the caregiving sex changes across lineages. We discovered that a brief period of maternal care is widespread and occurs in species previously thought to lack care. Using a combination of female-removal experiments, prey-choice tests with egg-eating katydids, and parental disturbance-tolerance assays, we confirm the adaptive benefits of short-term maternal care in wild Cochranella granulosa and Teratohyla pulverata. To examine historical transitions between caregiving sexes, we assembled a molecular phylogeny and estimated ancestral care states using our data and the literature. We assessed patterns indicative of sex-specific constraints by testing whether transitions between the sexes are associated with changes in care levels. Our analyses support that male-only care evolved 2-3 times from female-only care, and this change is associated with substantial increases in care levels - a pattern supporting the hypothesis that male-only care evolved via constraints on maternal expenditure. Many groups of amphibians remain poorly studied, with emerging evidence indicating that care patterns are more diverse than currently appreciated. Natural history remains fundamental to uncovering this diversity and generating testable hypotheses of sex-role evolution.

Understanding the origin of number sense: a review of fish studies

The ability to use quantitative information is thought to be adaptive in a wide range of ecological contexts. For nearly a century, the numerical abilities of mammals and birds have been extensively studied using a variety of approaches. However, in the last two decades, there has been increasing interest in investigating the numerical abilities of teleosts (i.e. a large group of ray-finned fish), mainly due to the practical advantages of using fish species as models in laboratory research. Here, we review the current state of the art in this field. In the first part, we highlight some potential ecological functions of numerical abilities in fish and summarize the existing literature that demonstrates numerical abilities in different fish species. In many cases, surprising similarities have been reported among the numerical performance of mammals, birds and fish, raising the question as to whether vertebrates' numerical systems have been inherited from a common ancestor. In the second part, we will focus on what we still need to investigate, specifically the research fields in which the use of fish would be particularly beneficial, such as the genetic bases of numerical abilities, the development of these abilities and the evolutionary foundation of vertebrate number sense.This article is part of a discussion meeting issue 'The origins of numerical abilities'.

Biparental care is predominant and beneficial to parents in the burying beetle Nicrophorus orbicollis (Coleoptera: Silphidae)

Parenting strategies can be flexible within a species, and may have varying fitness effects. Understanding this flexibility and its fitness consequences is important for understanding why parenting strategies evolve. Here, we investigate the fitness consequences of flexible parenting in the burying beetle Nicrophorus orbicollis, a species known for its advanced provisioning behaviour of regurgitated vertebrate carrion to offspring by both sexes. We show that even when a parent is freely allowed to abandon the carcass at any point in time, biparental post-hatching care is the most common pattern of care adopted in N. orbicollis. Furthermore, two parents together raised more offspring than single parents of either sex, showing that the presence of the male can directly influences parental fitness even in the absence of competitors. This contrasts with studies in other species of burying beetle, where biparental families do not differ in offspring number. This may explain why biparental care is more common in N. orbicollis than in other burying beetles. We suggest how fitness benefits of two parents may play a role in the evolution and maintenance of flexible biparental care in N. orbicollis.