Eco-evo-devo of the lemur syndrome: did adaptive behavioral plasticity get canalized in a large primate radiation?

Benefits but not the dual functions of submissive signals differ between two Malagasy primates

Many animals use formalized signals to communicate dominance relationships. In some primates, such as macaques, the function of such signals varies with dominance style. Despotic species produce unidirectional submission signals that have a dual function: in conflict contexts, they signal a willingness to withdraw, whereas in peaceful contexts, they indicate the agreement to subordination. More despotic species produce these calls to a lesser extent than less despotic species. Here, we investigated whether the use of unidirectional submission signals is also related to dominance style in two lemur species and whether signalling subordination stabilizes social relationships at the group level. Ring-tailed lemurs (Lemur catta) exhibit a more despotic dominance hierarchy than Verreaux's sifakas (Propithecus verreauxi). We observed social interactions in 75 dyads of Verreaux's sifakas and 118 dyads of ring-tailed lemurs. Both species used unidirectional submissive calls that have a dual function, potentially suggesting convergent evolution of the function of these signals in independent primate lineages. However, signalling subordination did not stabilize social relationships at the group level in both species. Additionally, subordination occurred more frequently in dyads of the more despotic ring-tailed lemurs than in Verreaux's sifakas, indicating opposite patterns to macaques in the coevolution of social traits with dominance style. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics'.

Evidence for female dominance in the Milne-Edwards' sportive lemur (Lepilemur edwardsi)

Female dominance over males is more frequent in Malagasy lemurs than in other primate clades, but patterns of female-male dominance vary among species, and few data are available for one particularly species-rich genus, Lepilemur. We investigated the types, temporal distribution, and outcome of female-male agonistic conflicts in the Milne-Edwards' sportive lemur (Lepilemur edwardsi). Ten L. edwardsi belonging to five social units were equipped with radio collars and observed during 79 focal follows between May and November 1998. We quantified agonistic conflicts, monthly conflict rates, and documented the winner and context for all conflicts. Female-male agonistic conflicts (N = 162) occurred at a mean rate of 0.21 conflicts/hour within groups. Agonistic conflicts peaked during the birth season and occurred mostly in unknown or infant proximity contexts. Females won 96% of all decided agonistic conflicts (N = 154). Other outcomes occurred only when females were with infants during the birth season. In that context, one female sometimes withdrew from her pair partner, and another female withdrew or fled from an extra-group male whose attacks eventually led to infanticide. Our results suggest female dominance in pair-living L. edwardsi. We hypothesize that elevated female aggression may convey fitness benefits to female Lepilemur because it generally allows females to intervene quickly and efficiently on behalf of their infants against nonfather males who may approach the newborn infants when they are parked in vegetation.

Male-biased dominance in greater bamboo lemurs (Prolemur simus)

Intersexual dominance relationships in virtually all lemur species have been reported to be female-biased. Although a claim of male dominance in greater bamboo lemurs (Prolemur simus) which was not supported by data is unusual against this background, it is in line with recent studies on other lemur species that suggest the existence of a continuum of intersexual dominance relationships. We therefore studied the details of agonistic interactions among adults of one captive group of P. simus at Cologne Zoo. This very preliminary study confirmed male-biased dominance because the adult male of the study group won all agonistic interactions with all three adult females, and the male was never dominated by any of the females. This result raises several interesting questions about the mechanisms and evolution of intersexual dominance relationships in group-living lemurs and should encourage similar future studies of additional groups of this species - ideally in the wild.

Long-term female bias in sex ratios across life stages of Harpy Eagle, a large raptor exhibiting reverse sexual size dimorphism

The primary (PSR), secondary (SSR) and adult (ASR) sex ratios of sexually reproducing organisms influence their life histories. Species exhibiting reversed sexual size dimorphism (RSD) may imply a higher cost of female production or lower female survival, thus generating biases in PSR, SSR and/or ASR towards males. The Harpy Eagle is the world's largest eagle exhibiting RSD. This species is found in the Neotropical region and is currently threatened with extinction. We used molecular markers to determine the sex of 309 Harpy Eagles spanning different life stages-eaglets, subadults and adults-from 1904 to 2021 within the Amazon Rainforest and Atlantic Forest. Sex ratios for all life stages revealed a female-biased deviation across all periods and regions. Our results suggest that the population bias towards females is an evolutionary ecological pattern of this species, and SSR and ASR likely emerged from the PSR. This natural bias towards females may be compensated by an earlier sexual maturation age of males, implying a longer reproductive lifespan and a higher proportion of sexually active males. A better understanding of the Harpy Eagle's life history can contribute to understanding sex-role evolution and enable more appropriate conservation strategies for the species.

Dynamics of intersexual dominance in a highly dimorphic primate

Intersexual dominance, which is measured by the probability that members of one sex elicit submission of members of the other sex during agonistic interactions, is often skewed in favor of males. However, even in sexually dimorphic species, several factors may influence intersexual dominance. Here, we use an 8-year dataset to examine the dynamics of intersexual dominance in wild-living mandrills (Mandrillus sphinx). Mandrills exhibit an extreme male-biased sexual size dimorphism but females show pronounced kin-differentiated social relationships and occasionally form coalitions against males. We established intersexual hierarchies across consecutive 6-month time blocks, representing either mating or birth seasons. Although females appeared to outrank 11% of males, they elicited male submission in only 2% of agonistic interactions against males. This discrepancy is likely due to the temporary residency of most males in the exceptionally large mandrill groups, the sexually coercive male mating strategies and the scarce number of agonistic interactions within most dyads, that may limit hierarchical inferences. In a second step, we found that the intersexual hierarchy mixes the intrasexual ones respecting their respective order. Females outranked mostly young and old males during the mating (vs. birth) season and social integration was positively correlated to dominance status in both sexes. In a third step, we found that females win more conflicts against young or old males which are closer to them in the intersexual hierarchy. These results extend our understanding of female-male dominance relationships by indicating that female mandrills occasionally outrank males who are considerably larger than them, and that a combination of demographic and social factors can influence the intersexual hierarchy.

Coevolution of social and communicative complexity in lemurs

The endemic lemurs of Madagascar (Lemuriformes: Primates) exhibit great social and communicative diversity. Given their independent evolutionary history, lemurs provide an excellent opportunity to identify fundamental principles in the coevolution of social and communicative traits. We conducted comparative phylogenetic analyses to examine patterns of interspecific variation among measures of social complexity and repertoire sizes in the vocal, olfactory and visual modality, while controlling for environmental factors such as habitat and number of sympatric species. We also examined potential trade-offs in signal evolution as well as coevolution between body mass or brain size and communicative complexity. Repertoire sizes in the vocal, olfactory and visual modality correlated positively with group size, but not with environmental factors. Evolutionary changes in social complexity presumably antedated corresponding changes in communicative complexity. There was no trade-off in the evolution of signals in different modalities and neither body mass nor brain size correlated with any repertoire size. Hence, communicative complexity coevolved with social complexity across different modalities, possibly to service social relationships flexibly and effectively in pair- and group-living species. Our analyses shed light on the requirements and adaptive possibilities in the coevolution of core elements of social organization and social structure in a basal primate lineage. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Sex and dominance: How to assess and interpret intersexual dominance relationships in mammalian societies

The causes and consequences of being in a particular dominance position have been illuminated in various animal species, and new methods to assess dominance relationships and to describe the structure of dominance hierarchies have been developed in recent years. Most research has focused on same-sex relationships, however, so that intersexual dominance relationships and hierarchies including both sexes have remained much less studied. In particular, different methods continue to be employed to rank males and females along a dominance hierarchy, and sex biases in dominance are still widely regarded as simple byproducts of sexual size dimorphism. However, males and females regularly compete over similar resources when living in the same group, and sexual conflict takes a variety of forms across societies. These processes affect the fitness of both sexes, and are mitigated by intersexual hierarchies. In this study, we draw on data from free-ranging populations of nine species of mammals that vary in the degree to which members of one sex dominate members of the other sex to explore the consequences of using different criteria and procedures for describing intra- and intersexual dominance relationships in these societies. Our analyses confirmed a continuum in patterns of intersexual dominance, from strictly male-dominated species to strictly female-dominated species. All indices of the degree of female dominance were well correlated with each other. The rank order among same-sex individuals was highly correlated between the intra- and intersexual hierarchies, and such correlation was not affected by the degree of female dominance. The relative prevalence of aggression and submission was sensitive to variation in the degree of female dominance across species, with more submissive signals and fewer aggressive acts being used in societies where female dominance prevails. Thus, this study provides important insights and key methodological tools to study intersexual dominance relationships in mammals.

Reproductive Senescence in Two Lemur Lineages

The relationship between age and reproductive performance is highly variable across species. Humans and some cetaceans exhibit an extreme form of reproductive senescence in that female reproduction ceases years or even decades before average life expectancy is reached. However, neither the existence of reproductive senescence in some taxa nor its absence in others is fully understood. Comparative data from other long-lived mammals may contribute to a more comprehensive understanding of the evolution of menopause, but data from wild primates, in particular, are scarce. We therefore investigated age-related female reproductive performance in two wild sympatric populations of Malagasy primates: Verreaux’s sifakas (Propithecus verreauxi) and redfronted lemurs (Eulemur rufifrons), which have a maximal longevity of more than 20 years. Based on 25 years of long-term demographic data, we extracted information on reproductive output of 38 female Verreaux’s sifakas and 42 female redfronted lemurs. We modeled variation in female reproductive performance and interbirth intervals as a function of age, the number of adult females within a group to account for female competition, and rainfall as a proxy for annual variation in food availability. We also compared our results for these two species with data on captive populations of the same two genera that are buffered from fluctuations in environmental variables. Our analyses disclosed statistical evidence for reproductive senescence in three out of four populations (captive Coquerel’s sifakas, wild redfronted lemurs, and captive red lemurs) but not for wild Verreaux’s sifakas. Compared to wild populations, reproductive senescence was therefore not less pronounced in captive animals, even though the latter are buffered from environmental adversities. In wild redfronted lemurs, mothers were more likely to give birth in years with more rainfall, but neither the number of co-resident females, nor annual rainfall did predict variation in the probability of giving birth in wild Verreaux’s sifakas. Thus, our study contributes valuable comparative information on reproductive senescence in a basal group of primates, and offers insights into the modulating effects of environmental, social and phylogenetic factors on patterns and dynamics of age-specific female reproduction.

Sex bias in intergroup conflict and collective movements among social mammals: male warriors and female guides

Intergroup conflict is a major evolutionary force shaping animal and human societies. Males and females should, on average, experience different costs and benefits for participating in collective action. Specifically, among mammals, male fitness is generally limited by access to mates whereas females are limited by access to food and safety. Here we analyse sex biases among 72 species of group-living mammals in two contexts: intergroup conflict and collective movements. Our comparative phylogenetic analyses show that the modal mammalian pattern is male-biased participation in intergroup conflict and female-biased leadership in collective movements. However, the probability of male-biased participation in intergroup conflicts decreased and female-biased participation increased with female-biased leadership in movements. Thus, female-biased participation in intergroup conflict only emerged in species with female-biased leadership in collective movements, such as in spotted hyenas and some lemurs. Sex differences are probably attributable to costs and benefits of participating in collective movements (e.g. towards food, water, safety) and intergroup conflict (e.g. access to mates or resources, risk of injury). Our comparative review offers new insights into the factors shaping sex bias in leadership across social mammals and is consistent with the 'male warrior hypothesis' which posits evolved sex differences in human intergroup psychology. This article is part of the theme issue 'Intergroup conflict across taxa'.

The eco-evolutionary landscape of power relationships between males and females

In animal societies, control over resources and reproduction is often biased towards one sex. Yet, the ecological and evolutionary underpinnings of male-female power asymmetries remain poorly understood. We outline a comprehensive framework to quantify and predict the dynamics of male-female power relationships within and across mammalian species. We show that male-female power relationships are more nuanced and flexible than previously acknowledged. We then propose that enhanced reproductive control over when and with whom to mate predicts social empowerment across ecological and evolutionary contexts. The framework explains distinct pathways to sex-biased power: coercion and male-biased dimorphism constitute a co-evolutionary highway to male power, whereas female power emerges through multiple physiological, morphological, behavioural, and socioecological pathways.

The Island of Female Power? Intersexual Dominance Relationships in the Lemurs of Madagascar

The extant primates of Madagascar (Lemuriformes) represent the endpoints of an adaptive radiation following a single colonization event more than 50 million years ago. They have since evolved a diversity of life history traits, ecological adaptations and social systems that rivals that of all other living primates combined. Their social systems are characterized by a unique combination of traits, including the ability of adult females to dominate adult males. In fact, there is no other group of mammals in which female dominance is so widespread. Yet, recent research has indicated that there is more interspecific variation in lemur intersexual relationships than previously acknowledged. Here, we therefore review and summarize the relevant literature, quantifying the extent of sex-bias in intersexual dominance relations documented in observational and experimental studies in captivity and the wild. Female dominance is often, but not always, implemented by spontaneous male submission in the absence of female aggression and linked to female sexual maturation. We connect the available evidence to the hypotheses that have been proposed to explain the evolution of female dominance among lemurs. The occurrence of female dominance in all lemur families and the interspecific variation in its extent indicate that it has evolved soon after lemurs colonized Madagascar – presumably in response to particular ecological challenges – and that it has since been reduced in magnitude independently in some taxa. Our study contributes important comparative information on sex roles from an independent primate radiation and provides general insights into the conditions, opportunities and obstacles in the evolution of female-biased power.

Nine Levels of Explanation : A Proposed Expansion of Tinbergen's Four-Level Framework for Understanding the Causes of Behavior

Tinbergen's classic "On Aims and Methods of Ethology" (Zeitschrift für Tierpsychologie, 20, 1963) proposed four levels of explanation of behavior, which he thought would soon apply to humans. This paper discusses the need for multilevel explanation; Huxley and Mayr's prior models, and others that followed; Tinbergen's differences with Lorenz on "the innate"; and Mayr's ultimate/proximate distinction. It synthesizes these approaches with nine levels of explanation in three categories: phylogeny, natural selection, and genomics (ultimate causes); maturation, sensitive period effects, and routine environmental effects (intermediate causes); and hormonal/metabolic processes, neural circuitry, and eliciting stimuli (proximate causes), as a respectful extension of Tinbergen's levels. The proposed classification supports and builds on Tinbergen's multilevel model and Mayr's ultimate/proximate continuum, adding intermediate causes in accord with Tinbergen's emphasis on ontogeny. It requires no modification of Standard Evolutionary Theory or The Modern Synthesis, but shows that much that critics claim was missing was in fact part of Neo-Darwinian theory (so named by J. Mark Baldwin in The American Naturalist in 1896) all along, notably reciprocal causation in ontogeny, niche construction, cultural evolution, and multilevel selection. Updates of classical examples in ethology are offered at each of the nine levels, including the neuroethological and genomic findings Tinbergen foresaw. Finally, human examples are supplied at each level, fulfilling his hope of human applications as part of the biology of behavior. This broad ethological framework empowers us to explain human behavior-eventually completely-and vindicates the idea of human nature, and of humans as a part of nature.

Selection on adaptive and maladaptive gene expression plasticity during thermal adaptation to urban heat islands

Phenotypic plasticity enables a single genotype to produce multiple phenotypes in response to environmental variation. Plasticity may play a critical role in the colonization of novel environments, but its role in adaptive evolution is controversial. Here we suggest that rapid parallel regulatory adaptation of Anolis lizards to urban heat islands is due primarily to selection for reduced and/or reversed heat-induced plasticity that is maladaptive in urban thermal conditions. We identify evidence for polygenic selection across genes of the skeletal muscle transcriptome associated with heat tolerance. Forest lizards raised in common garden conditions exhibit heat-induced changes in expression of these genes that largely correlate with decreased heat tolerance, consistent with maladaptive regulatory response to high-temperature environments. In contrast, urban lizards display reduced gene expression plasticity after heat challenge in common garden and a significant increase in gene expression change that is congruent with greater heat tolerance, a putatively adaptive state in warmer urban environments. Genes displaying maladaptive heat-induced plasticity repeatedly show greater genetic divergence between urban and forest habitats than those displaying adaptive plasticity. These results highlight the role of selection against maladaptive regulatory plasticity during rapid adaptive modification of complex systems in the wild.

The energy allocation trade-offs underlying life history traits in hypometabolic strepsirhines and other primates

Life history, brain size and energy expenditure scale with body mass in mammals but there is little conclusive evidence for a correlated evolution between life history and energy expenditure (either basal/resting or daily) independent of body mass. We addressed this question by examining the relationship between primate free-living daily energy expenditure (DEE) measured by doubly labeled water method (n = 18 species), life history variables (maximum lifespan, gestation and lactation duration, interbirth interval, litter mass, age at first reproduction), resting metabolic rate (RMR) and brain size. We also analyzed whether the hypometabolic primates of Madagascar (lemurs) make distinct energy allocation tradeoffs compared to other primates (monkeys and apes) with different life history traits and ecological constraints. None of the life-history traits correlated with DEE after controlling for body mass and phylogeny. In contrast, a regression model showed that DEE increased with increasing RMR and decreasing reproductive output (i.e., litter mass/interbirth interval) independent of body mass. Despite their low RMR and smaller brains, lemurs had an average DEE remarkably similar to that of haplorhines. The data suggest that lemurs have evolved energy strategies that maximize energy investment to survive in the unusually harsh and unpredictable environments of Madagascar at the expense of reproduction.

Evolutionary stasis of the pseudoautosomal boundary in strepsirrhine primates

Sex chromosomes are typically comprised of a non-recombining region and a recombining pseudoautosomal region. Accurately quantifying the relative size of these regions is critical for sex-chromosome biology both from a functional and evolutionary perspective. The evolution of the pseudoautosomal boundary (PAB) is well documented in haplorrhines (apes and monkeys) but not in strepsirrhines (lemurs and lorises). Here, we studied the PAB of seven species representing the main strepsirrhine lineages by sequencing a male and a female genome in each species and using sex differences in coverage to identify the PAB. We found that during primate evolution, the PAB has remained unchanged in strepsirrhines whereas several recombination suppression events moved the PAB and shortened the pseudoautosomal region in haplorrhines. Strepsirrhines are well known to have much lower sexual dimorphism than haplorrhines. We suggest that mutations with antagonistic effects between males and females have driven recombination suppression and PAB evolution in haplorrhines.

The Diversity of Primates: From Biomedicine to Conservation Genomics

Until now, the field of primate genomics has focused on two major themes: understanding human evolution and advancing biomedical research. We propose that it is now time for a third theme to receive attention: conservation genomics. As a result of anthropogenic effects, the majority of primate species have become threatened with extinction. A more robust primate conservation genomics will allow for genetically informed population management. Thanks to a steady decline in the cost of sequencing, it has now become feasible to sequence whole primate genomes at the population level. Furthermore, technological advances in noninvasive genomic methods have made it possible to acquire genome-scale data from noninvasive biomaterials. Here, we review recent advances in the analysis of primate diversity, with a focus on genomic data sets across the radiation.

Mid-Cenozoic climate change, extinction, and faunal turnover in Madagascar, and their bearing on the evolution of lemurs

Background

Was there a mid-Cenozoic vertebrate extinction and recovery event in Madagascar and, if so, what are its implications for the evolution of lemurs? The near lack of an early and mid-Cenozoic fossil record on Madagascar has inhibited direct testing of any such hypotheses. We compare the terrestrial vertebrate fauna of Madagascar in the Holocene to that of early Cenozoic continental Africa to shed light on the probability of a major mid-Cenozoic lemur extinction event, followed by an “adaptive radiation” or recovery. We also use multiple analytic approaches to test competing models of lemur diversification and the null hypothesis that no unusual mid-Cenozoic extinction of lemurs occurred.

Results

Comparisons of the terrestrial vertebrate faunas of the early Cenozoic on continental Africa and Holocene on Madagascar support the inference that Madagascar suffered a major mid-Cenozoic extinction event. Evolutionary modeling offers some corroboration, although the level of support varies by phylogeny and model used. Using the lemur phylogeny and divergence dates generated by Kistler and colleagues, RPANDA and TESS offer moderate support for the occurrence of unusual extinction at or near the Eocene-Oligocene (E-O) boundary (34 Ma). TreePar, operating under the condition of obligate mass extinction, found peak diversification at 31 Ma, and low probability of survival of prior lineages. Extinction at the E-O boundary received greater support than other candidate extinctions or the null hypothesis of no major extinction. Using the lemur phylogeny and divergence dates generated by Herrera & Dàvalos, evidence for large-scale extinction diminishes and its most likely timing shifts to before 40 Ma, which fails to conform to global expectations.

Conclusions

While support for large-scale mid-Cenozoic lemur extinction on Madagascar based on phylogenetic modeling is inconclusive, the African fossil record does provide indirect support. Furthermore, a major extinction and recovery of lemuriforms during the Eocene-Oligocene transition (EOT) would coincide with other major vertebrate extinctions in North America, Europe, and Africa. It would suggest that Madagascar’s lemurs were impacted by the climate shift from “greenhouse” to “ice-house” conditions that occurred at that time. This could, in turn, help to explain some of the peculiar characteristics of the lemuriform clade.

Morphometric signals of population decline in diademed sifakas occupying degraded rainforest habitat in Madagascar

Anthropogenic habitat change can have varied impacts on primates, including both negative and positive outcomes. Even when behavioural shifts are seen, they may reflect decreased health, or simply behavioural flexibility; understanding this distinction is important for conservation efforts. This study examines habitat-related variation in adult and immature morphometrics among diademed sifakas (Propithecus diadema). We collected morphometric data from sifakas at Tsinjoarivo, Madagascar (19 years, 188 captures, 113 individuals). Captures spanned 12 groups, five within continuous forest ("CONT"), and seven in degraded fragments ("FRAG") where sifakas have lower nutritional intakes. Few consistent differences were found between CONT and FRAG groups. However, using home range quality as a covariate rather than a CONT/FRAG dichotomy revealed a threshold: the two FRAG groups in the lowest-quality habitat showed low adult mass and condition (wasting), and low immature mass and length (stunting). Though less-disturbed fragments apparently provide viable habitat, we suggest the sifakas in the most challenging habitats cannot evolve fast enough to keep up with such rapid habitat change. We suggest other long-lived organisms will show similar morphometric "warning signs" (wasting in adults, stunting in immatures); selected morphometric variables can thus be useful at gauging vulnerability of populations in the face of anthropogenic change.

Evolutionary dynamics of sexual size dimorphism in non-volant mammals following their independent colonization of Madagascar

As predicted by sexual selection theory, males are larger than females in most polygynous mammals, but recent studies found that ecology and life history traits also affect sexual size dimorphism (SSD) through evolutionary changes in either male size, female size, or both. The primates of Madagascar (Lemuriformes) represent the largest group of mammals without male-biased SSD. The eco-evo-devo hypothesis posited that adaptations to unusual climatic unpredictability on Madagascar have ultimately reduced SSD in lemurs after dispersing to Madagascar, but data have not been available for comparative tests of the corresponding predictions that SSD is also absent in other terrestrial Malagasy mammals and that patterns of SSD changed following the colonization of Madagascar. We used phylogenetic methods and new body mass data to test these predictions among the four endemic radiations of Malagasy primates, carnivorans, tenrecs, and rodents. In support of our prediction, we found that male-biased SSD is generally absent among all Malagasy mammals. Phylogenetic comparative analyses further indicated that after their independent colonization of Madagascar, SSD decreased in primates and tenrecs, but not in the other lineages or when analyzed across all species. We discuss several mechanisms that may have generated these patterns and conclude that neither the eco-evo-devo hypothesis, founder effects, the island rule nor sexual selection theory alone can provide a compelling explanation for the observed patterns of SSD in Malagasy mammals.

Female Power in Primates and the Phenomenon of Female Dominance

Sex-biased power structures are common in human and nonhuman primate societies. “Female dominance” is a term applied to a wide range of female-biased power structures. However, the full extent of this variation remains obscure because an adequate vocabulary of power has not been adopted consistently. Female power occurs throughout primates and other animals, even in male-dominant societies, but the legacy of patriarchy persists in primatologists’ use of language and implicit assumptions about intersexual power. While explanations for the occurrence of female power can be accommodated within existing ethological theory, many hypotheses seeking to explain the evolution of female power are narrowly focused on particular taxa. Theories about primate social evolution would benefit from a synthesis of the disparate literature on power, increased emphasis on intersexual social relationships, and comparative studies that include the full behavioral diversity of primates and other mammals.

Sex roles and adult sex ratios: insights from mammalian biology and consequences for primate behaviour

Theoretical models and empirical studies in various taxa have identified important links between variation in sex roles and the number of adult males and females (adult sex ratio (ASR)) in a population. In this review, I examine these relationships in non-human primates. Because most existing theoretical models of the evolution of sex roles focus on the evolutionary origins of sex-biased behaviour, they offer only a general scaffold for predicting variation in sex roles among and within species. I argue that studies examining sex role variation at these more specific levels need to take social organization into account to identify meaningful levels for the measurement of ASR and to account for the fact that ASR and sex roles mutually influence each other. Moreover, taxon-specific life-history traits can constrain sex role flexibility and impact the operational sex ratio (OSR) by specifying the minimum length of female time outs from reproduction. Using examples from the primate literature, I highlight practical problems in estimating ASR and OSR. I then argue that interspecific variation in the occurrence of indirect forms of paternal care might indeed be linked to variation in ASR. Some studies also indicate that female aggression and bonding, as well as components of inter-sexual relationships, are sensitive to variation in ASR. Thus, links between primate sex roles and sex ratios merit further study, and such studies could prompt the development of more specific theoretical models that make realistic assumptions about taxon-specific life history and social organization.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'.

Lifetime development of behavioural phenotype in the house mouse (Mus musculus)

With each trajectory taken during the ontogeny of an individual, the number of optional behavioural phenotypes that can be expressed across its life span is reduced. The initial range of phenotypic plasticity is largely determined by the genetic material/composition of the gametes whereas interacting with the given environment shapes individuals to adapt to/cope with specific demands. In mammalian species, the phenotype is shaped as the foetus grows, depending on the environment in the uterus, which in turn depends on the outer environment the mother experiences during pregnancy. After birth, a complex interaction between innate constitution and environmental conditions shapes individual lifetime trajectories, bringing about a wide range of diversity among individual subjects.

In laboratory mice inbreeding has been systematically induced in order to reduce the genetic variability between experimental subjects. In addition, within most laboratories conducting behavioural phenotyping with mice, breeding and housing conditions are highly standardised. Despite such standardisation efforts a considerable amount of variability persists in the behaviour of mice. There is good evidence that phenotypic variation is not merely random but might involve individual specific behavioural patterns consistent over time. In order to understand the mechanisms and the possible adaptive value of the maintenance of individuality we review the emergence of behavioural phenotypes over the course of the life of (laboratory) mice. We present a literature review summarizing developmental stages of behavioural development of mice along with three illustrative case studies. We conclude that the accumulation of environmental differences and experiences lead to a “mouse individuality” that becomes increasingly stable over the lifetime.