Exclusive male care despite extreme female promiscuity and low paternity in a marine snail

Partial paternity does not always select for female-biased care

The theoretical literature predicts that parentage differences between the sexes, due to females mating with multiple males, select males to provide less parental care and females to care more for the offspring. We formulate simple evolutionary games to question the generality of this prediction. We find that the relationship between paternal care and fitness gained from extra-pair matings is important. A trade-off between these two quantities is required for partial paternity and complete maternity to bias the evolutionary stable strategy (ESS) toward more female care. We argue that this trade-off has been implicitly or explicitly assumed in most previous theories. However, if there is no trade-off between paternal care and extra-pair matings, parentage differences do not influence the ESS sex roles. Moreover, it is also possible for these two quantities to have a positive relationship, in which case we predict selection for male care is possible. We support these predictions using agent-based simulations. We also consider the possibility that caring males have greater opportunities to guard their paternity, and find that this mechanism can also select for male-biased care. Hence, we derive the conditions under which male care may be selected despite partial paternity and complete maternity.

The evolution of paternal care: a role for microbes?

Paternal care, particularly in cases of uncertain paternity, carries significant costs. Extensive research, both theoretical and experimental, has explored the conditions in which paternal care behaviour would be favoured. Common explanations include an adjustment of care with uncertainty in paternity and limited accuracy in parentage assessment. Here, we propose a new explanation that microbes may play a role in the evolution of paternal care among their hosts. Using computational models, we demonstrate that microbes associated with increased paternal care could be favoured by natural selection. We find that microbe-induced paternal care could evolve under wider conditions than suggested by genetic models. Moreover, we show that microbe-induced paternal care is more likely to evolve when considering paternal care interactions that increase microbial transmission, such as feeding and grooming. Our results imply that factors affecting the composition of host microbiome may also alter paternal behaviour. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Multiple and Extra-Pair Mating in a Pair-Living Hermaphrodite, the Intertidal Limpet Siphonaria gigas

Pair-living is a common social system found across animal taxa, and the relationship between pair-living and reproduction varies greatly among species. Siphonaria gigas, hermaphroditic pulmonate gastropods, often live in pairs in the rocky intertidal zone of the tropical Eastern Pacific. Combining genetic parentage analysis using four polymorphic microsatellite loci with behavioral observations from a 10-week field study, we provide the first description of the mating system of a Siphonaria species incorporating genetic data. S. gigas mated both within-pair and extra-pair and three out of four paired S. gigas individuals produced egg masses with extra-pair paternity. Multiple paternity was detected, but at a relatively low frequency (19% of egg masses) compared to other marine gastropods. Behavioral data indicate one potential advantage of pair-living: paired S. gigas produced almost twice as many egg masses as their solitary counterparts over four reproductive cycles. These observations, together with constraints on the movement of S. gigas, suggest that pairing may ensure mate access and increase reproductive success.

Brood Reduction, Not Poecilogony, in a Vermetid Gastropod with Two Developmental Outcomes within Egg Capsules

A small vermetid gastropod broods capsules containing nurse eggs and embryos that develop into small veligers. A few of these veligers continue development and growth while nurse eggs and developmentally arrested sibling veligers disappear. Survivors hatch as crawling pediveligers and juveniles. None of the veligers, if removed from capsules, swim in a directed way or withdraw into their shells, indicating that even the developing veligers are unsuited for extracapsular life until they can crawl. The shells of arrested veligers decalcify while their siblings grow. Few of the developmentally arrested veligers that were isolated from siblings and fed algal cells resumed detectable growth. Nurse eggs rather than cannibalism provide most of the food, but full growth of developing veligers depends on limited sharing; arrest of some siblings is a necessary adjunct of the nurse-egg feeding. Here, two developmental outcomes for larvae produced by developmental arrest of some (often termed poecilogony) serves instead as a means of brood reduction. Brood reduction is often attributed to family conflicts resulting from genetic differences. Another hypothesis is that a mother who cannot accurately sort numbers of nurse eggs and developing eggs into capsules could rely on brood reduction to adjust food for her offspring. At the extreme, an entirely random packaging would produce a binomial distribution of embryos in capsules, a very uneven distribution of food per embryo, and some capsules with no embryos. Males have yet to be found in this species, but even if reproduction is asexual, selection could still favor brood reduction.

Sperm competition games when males invest in paternal care

Sperm competition games investigate how males partition limited resources between pre- and post-copulatory competition. Although extensive research has explored how various aspects of mating systems affect this allocation, male allocation between mating, fertilization and parental effort has not previously been considered. Yet, paternal care can be energetically expensive and males are generally predicted to adjust their parental effort in response to expected paternity. Here, we incorporate parental effort into sperm competition games, particularly exploring how the relationship between paternal care and offspring survival affects sperm competition and the relationship between paternity and paternal care. Our results support existing expectations that (i) fertilization effort should increase with female promiscuity and (ii) paternal care should increase with expected paternity. However, our analyses also reveal that the cost of male care can drive the strength of these patterns. When paternal behaviour is energetically costly, increased allocation to parental effort constrains allocation to fertilization effort. As paternal care becomes less costly, the association between paternity and paternal care weakens and may even be absent. By explicitly considering variation in sperm competition and the cost of male care, our model provides an integrative framework for predicting the interaction between paternal care and patterns of paternity.

Give one species the task to come up with a theory that spans them all: what good can come out of that?

Does the progress in understanding evolutionary theory depend on the species that is doing the investigation? This question is difficult to answer scientifically, as we are dealing with an n = 1 scenario: every individual who has ever written about evolution is a human being. I will discuss, first, whether we get the correct answer to questions if we begin with ourselves and expand outwards, and second, whether we might fail to ask all the interesting questions unless we combat our tendencies to favour taxa that are close to us. As a whole, the human tendency to understand general biological phenomena via 'putting oneself in another organism's shoes' has upsides and downsides. As an upside, our intuitive ability to rethink strategies if the situation changes can lead to ready generation of adaptive hypotheses. Downsides occur if we trust this intuition too much, and particular danger zones exist for traits where humans are an unusual species. I argue that the levels of selection debate might have proceeded differently if human cooperation patterns were not so unique, as this brings about unique challenges in biology teaching; and that theoretical insights regarding inbreeding avoidance versus tolerance could have spread faster if we were not extrapolating our emotional reactions to incest disproportionately depending on whether we study animals or plants. I also discuss patterns such as taxonomic chauvinism, i.e. less attention being paid to species that differ more from human-like life histories. Textbooks on evolution reinforce such biases insofar as they present, as a default case, systems that resemble ours in terms of life cycles and other features (e.g. gonochorism). Additionally, societal norms may have led to incorrect null hypotheses such as females not mating multiply.

The seminal receptacle and implications for reproductive processes in the invasive gastropod Crepidula fornicata

The calyptraeid gastropod Crepidula fornicata is the object of considerable research attention, due to its invasive status in the North-Eastern Atlantic, its introduction to habitats throughout the Northern hemisphere, and its scientific interest as a model organism for the study of developmental and reproductive processes in the Metazoa. Since the knowledge concerning the structural foundations for its reproductive processes is surprisingly weak, we investigated the seminal receptacle, a key structure in the reproductive biology of other metazoans, using histology, scanning electron and transmission electron microscopy. The seminal receptacle consists of 9-11 lobes, each subdivided into small, narrow lobules. The inner epithelium of the lobules appears to be highly dynamic, characterised by the perforation and attachment of received spermatozoa, the progressive degeneration of this epithelium, and the concomitant detachment of the spermatozoa. The allocation of spermatozoa to many different lobules, in different phases, may explain the extended reproductive season of C. fornicata, and thereby contribute to its colonizing and invasive success. The same compartmentalisation, as well as the complete covering of the inner epithelium of the lobules by spermatozoa and the large amount of spermatozoan debris in the lumina, suggest that the C. fornicata seminal receptacle may be a site of sperm competition in this polyandrous species.

Sexual conflict between parents: offspring desertion and asymmetrical parental care

Parental care is an immensely variable social behavior, and sexual conflict offers a powerful paradigm to understand this diversity. Conflict over care (usually considered as a type of postzygotic sexual conflict) is common, because the evolutionary interests of male and female parents are rarely identical. I investigate how sexual conflict over care may facilitate the emergence and maintenance of diverse parenting strategies and argue that researchers should combine two fundamental concepts in social behavior to understand care patterns: cooperation and conflict. Behavioral evidence of conflict over care is well established, studies have estimated specific fitness implications of conflict for males or females, and experiments have investigated specific components of conflict. However, studies are long overdue to reveal the full implications of conflict for both males and females. Manipulating (or harming) the opposite sex seems less common in postzygotic conflicts than in prezygotic conflicts because by manipulating, coercing, or harming the opposite sex, the reproductive interest of the actor is also reduced. Parental care is a complex trait, although few studies have yet considered the implications of multidimensionality for parental conflict. Future research in parental conflict will benefit from understanding the behavioral interactions between male and female parents (e.g., negotiation, learning, and coercion), the genetic and neurogenomic bases of parental behavior, and the influence of social environment on parental strategies. Empirical studies are needed to put sexual conflict in a population context and reveal feedback between mate choice, pair bonds and parenting strategies, and their demographic consequences for the population such as mortalities and sex ratios. Taken together, sexual conflict offers a fascinating avenue for understanding the causes and consequences of parenting behavior, sex roles, and breeding system evolution.

Correlated evolution in parental care in females but not males in response to selection on paternity assurance behaviour

According to classical parental care theory males are expected to provide less parental care when offspring in a brood are less likely to be their own, but empirical evidence in support of this relationship is equivocal. Recent work predicts that social interactions between the sexes can modify co-evolution between traits involved in mating and parental care as a result of costs associated with these social interactions (i.e. sexual conflict). In burying beetles (Nicrophorus vespilloides), we use artificial selection on a paternity assurance trait, and crosses within and between selection lines, to show that selection acting on females, not males, can drive the co-evolution of paternity assurance traits and parental care. Males do not care more in response to selection on mating rate. Instead, patterns of parental care change as an indirect response to costs of mating for females.

Paternity and gregariousness in the sex-changing sessile marine gastropod Crepidula convexa: comparison with other protandrous Crepidula species

In sex-changing animals with internal fertilization, gregarious behavior may increase mating opportunities and the frequency of multiple paternity, thus increasing maternal reproductive success. Crepidula convexa is a direct-developing protandrous gastropod characterized by only modest gregarious behavior compared with previously studied members of the genus: females are frequently found isolated. Using 6 microsatellite markers, we analyzed paternity profiles in 10 broods (25 embryos per mother). The number of assigned fathers varied among families from 1 to 4 fathers per brood. Interestingly, polyandry was not detected in solitary females but only in females grouped with conspecific individuals. Overall, we found an average of 1.8 fathers per brood, but this increased to 2.6 fathers per brood when considering only the nonisolated females. Among 18 unambiguously identified fathers, only 5 were collected in our samples, suggesting substantial male mobility. Comparison with previous paternity analyses in Crepidula fornicata and Crepidula coquimbensis revealed that polyandry appears as a common trait of these sex-changing gastropods despite their different grouping behaviors and life histories. As expected, the level of polyandry was nevertheless lower in the modestly gregarious C. convexa.

Microsatellite evidence for high frequency of multiple paternity in the marine gastropod Rapana venosa

BACKGROUND

Inferring of parentage in natural populations is important in understanding the mating systems of a species, which have great effects on its genetic structure and evolution. Muricidae, a large group (approximately 1,600 species) of marine gastropods, are poorly investigated in patterns of multiple paternity and sperm competition based on molecular techniques. The veined Rapa whelk, Rapana venosa, a commercially important muricid species with internal fertilization, is an ideal species to study the occurrence and frequency of multiple paternity and to facilitate understanding of their reproductive strategies.

METHODOLOGY/PRINCIPAL FINDINGS

We developed five highly polymorphic microsatellites in R. venosa and applied them to identify multiple paternity in 19 broods (1381 embryos) collected from Dandong, China. Multiple paternity was detected in 17 (89.5%) of 19 broods. The number of sires per brood ranged from 1 to 7 (4.3 on average). Of the 17 multiply sired broods, 16 (94.1%) were significantly skewed from equal paternal contributions, and had a dominant sire which was also dominant in each assayed capsule.

CONCLUSIONS

Our results indicate that a high level of multiple paternity occurs in the wild population of R. venosa. Similar patterns of multiple paternity in the 2-6 assayed capsules from each brood imply that fertilization events within the body of a female occur mostly (but not entirely) as random draws from a "well-but-not-perfectly blended sperm pool" of her several mates. Strongly skewed distributions of fertilization success among sires also suggest that sperm competition and/or cryptic female choice might be important for post-copulatory paternity biasing in this species.

Kinship and the evolution of social behaviours in the sea

Until recently, little attention has been paid to the existence of kin structure in the sea, despite the fact that many marine organisms are sessile or sedentary. This lack of attention to kin structure, and its impacts on social evolution, historically stems from the pervasive assumption that the dispersal of gametes and larvae is almost always sufficient to prevent any persistent associations of closely related offspring or adults. However, growing evidence, both theoretical and empirical, casts doubt on the generality of this assumption, not only in species with limited dispersal, but also in species with long dispersive phases. Moreover, many marine organisms either internally brood their progeny or package them in nurseries, both of which provide ample opportunities for kinship to influence the nature and outcomes of social interactions among family members. As the evidence for kin structure within marine populations mounts, it follows that kin selection may play a far greater role in the evolution of both behaviours and life histories of marine organisms than is presently appreciated.