In multi-male groups where females mate promiscuously, male-infant associations have rarely been studied. However, recent studies have shown that males selectively support their offspring during agonistic conflicts with other juveniles and that father’s presence accelerates offspring maturation. Furthermore, it was shown that males invest in unrelated infants to enhance future mating success with the infant’s mother. Hence, infant care might provide fitness gain for males. Here we investigate male-infant associations in rhesus macaques (Macaca mulatta), a primate with low paternity certainty as females mate with multiple partners and males ensure paternity less efficiently through mate-guarding. We combined behavioral data with genetic paternity analyses of one cohort of the semifree-ranging population of Cayo Santiago (Puerto Rico) and recorded affiliative and aggressive interactions between focal subjects and adult males from birth to sexual maturation (0–4 years) of focal subjects. Our results revealed, that 9.6% of all interactions of focal subjects involved an adult male and 94% of all male-infant interactions were affiliative, indicating the rareness of male-infant aggression. Second and most interestingly, sires were more likely to affiliate with their offspring than non-sires with unrelated infants. This preference was independent of mother’s proximity and emphasized during early infancy. Male-infant affiliation rose with infant age and was pronounced between adult males and male rather than female focal subjects. Overall our results suggest that male-infant affiliation are also an important component in structuring primate societies and affiliation directed towards own offspring presumably represent low cost paternal care.
male-infant association; rhesus macaques; paternal care; paternity uncertainty; kin recognition
The hypothalamic-pituitary-adrenal (HPA) axis modulates individuals’ physiological responses to social stress, which is an inevitable aspect of the daily lives of group-living animals. Previous nonhuman primate studies have reported that sex, age, rank and reproductive condition influence cortisol levels under stressful conditions. In this study we investigated cortisol responses to stress among 70 multiparous, free-ranging female rhesus macaques (Macaca mulatta) on the island of Cayo Santiago, PR. Plasma cortisol samples were collected in two consecutive years under similar conditions. Twenty-two females were sampled both years, and most of those females were lactating in only one of the years. Individual differences in cortisol levels were stable across years, even though reproductive condition changed for most individuals. No relationship was found between age or social rank and cortisol levels. Of the females that changed reproductive conditions, cortisol levels were higher when they were lactating than when they were cycling, and the amount of change in cortisol from cycling to lactating was greatest for low-ranking individuals. Heightened reactivity to stress during lactation may be the result of concerns about infant safety, and such concerns may be higher among low-ranking mothers than among higher ranking mothers. Psychosocial stress and hyperactivation of the HPA axis during lactation can suppress immune function and increase vulnerability to infectious diseases, thus explaining why adult females in the free-ranging rhesus macaque population on Cayo Santiago have a higher probability of mortality during the birth season than during the mating season.
reproductive condition; cortisol; rank; rhesus macaques
Evolutionary explanations for life history diversity are based on the idea of costs of reproduction, particularly on the concept of a trade-off between age-specific reproduction and parental survival, and between expenditure on current and future offspring. Such trade-offs are often difficult to detect in population studies of wild mammals. Terminal investment theory predicts that reproductive effort by older parents should increase, because individual offspring become more valuable to parents as the conflict between current versus potential future offspring declines with age. In order to demonstrate this phenomenon in females, there must be an increase in maternal expenditure on offspring with age, imposing a fitness cost on the mother. Clear evidence of both the expenditure and fitness cost components has rarely been found. In this study, we quantify costs of reproduction throughout the lifespan of female antechinuses. Antechinuses are nocturnal, insectivorous, forest-dwelling small (20–40 g) marsupials, which nest in tree hollows. They have a single synchronized mating season of around three weeks, which occurs on predictable dates each year in a population. Females produce only one litter per year. Unlike almost all other mammals, all males, and in the smaller species, most females are semelparous. We show that increased allocation to current reproduction reduces maternal survival, and that offspring growth and survival in the first breeding season is traded-off with performance of the second litter in iteroparous females. In iteroparous females, increased allocation to second litters is associated with severe weight loss in late lactation and post-lactation death of mothers, but increased offspring growth in late lactation and survival to weaning. These findings are consistent with terminal investment. Iteroparity did not increase lifetime reproductive success, indicating that terminal investment in the first breeding season at the expense of maternal survival (i.e. semelparity) is likely to be advantageous for females.
In contrast to most mammalian species, female sexual activity is not limited to the fertile phase of the ovarian cycle in anthropoid primates, which has long been proposed to conceal the timing of ovulation to males. It is now generally believed that females are still most attractive during the fertile phase, leading to high-ranking males successfully mate-guarding them specifically during this period. While studies conducted in species exhibiting exaggerated sexual swellings (probabilistic signal of the fertile phase) have generally supported this hypothesis, mixed support comes from others. Here, we investigated whether high-ranking males timed mate-guarding effort towards female fertile phases in rhesus macaques (Macaca mulatta). In this species, adult females do not exhibit sexual swellings, but undergo facial skin colour variation, an alternative oestrogens-dependent graded-signal of female reproductive status. We collected behavioural, hormonal and genetic paternity data during two mating seasons for one group of the free-ranging population of Cayo Santiago. Our results show that mate-guarding by top-ranking males did not completely cover the entire female fertile phase and that this tactic accounted for only 30-40% of all fertilisations observed. Males tended to prolong mate-guarding into the luteal phase (null probability of fertilisation), which mirrors the pattern of male attraction to female facial colour reported in an earlier study. These findings suggest that males may have limited knowledge regarding the exact timing of females’ fertile phase in rhesus macaques, which presumably allows females to gain more control over reproduction relative to other anthropoid primate species.
Prolonged sexual receptivity; concealed ovulation; mate-guarding; fecal steroids; reproductive strategies; fertile phase; genetic paternity analysis; primates; rhesus macaques
Studies of mother–infant relationships in nonhuman primates have increasingly attempted to understand the neuroendocrine bases of interindividual variation in mothering styles and the mechanisms through which early exposure to variable mothering styles affects infant behavioral development. In this study of free-ranging rhesus macaques on Cayo Santiago, Puerto Rico, we aimed to: 1) compare lactating and nonlactating females to investigate whether lactation is associated with changes in plasma cortisol, prolactin and oxytocin, as well as changes in CSF levels of serotonin and dopamine metabolites (5-HIAA and HVA); 2) examine the extent to which interindividual variation in maternal physiology is associated with variation in maternal behavior; 3) examine the extent to which interindividual variation in infant physiology and behavior is accounted for by variation in maternal physiology and behavior. Lactating females had higher plasma concentrations of cortisol, prolactin, and oxytocin but lower CSF concentrations of HVA than nonlactating females. Variation in maternal rejection behavior was positively correlated with variation in maternal plasma cortisol levels and in CSF 5-HIAA levels while variation in the time spent nursing and grooming was associated with maternal plasma oxytocin levels. Infants who were protected more by their mothers had higher cortisol levels than those who were protected less, while infants who were rejected more had lower CSF 5-HIAA than infants who were rejected less. Since exposure to high levels of maternal protectiveness and rejection is known to affect the offspring’s behavior and responsiveness to the environment later in life, our results are consistent with the hypothesis that these effects are mediated by long-term changes in the activity of the offspring’s HPA axis and brain serotonergic system.
Maternal behavior; Infant behavior; Cortisol; Monoamine neurotransmitters; Oxytocin; Prolactin; Rhesus monkeys
A fundamental life history question is how individuals should allocate resources to reproduction optimally over time (reproductive allocation). The reproductive restraint hypothesis predicts that reproductive effort (RE; the allocation of resources to current reproduction) should peak at prime-age, whilst the terminal investment hypothesis predicts that individuals should continue to invest more resources in reproduction throughout life, owing to an ever-decreasing residual reproductive value. There is evidence supporting both hypotheses in the scientific literature.
We used an uncommonly large, 38 year dataset on Alpine chamois (Rupicapra rupicapra) shot at various times during the rutting period to test these two hypotheses. We assumed that body mass loss in rutting males was strongly related to RE and, using a process-based approach, modelled how male relative mass loss rates varied with age. For different regions of our study area, we provide evidence consistent with different hypotheses for reproductive allocation. In sites where RE declined in older age, this appears to be strongly linked to declining body condition in old males. In this species, terminal investment may only occur in areas with lower rates of body mass senescence.
Our results show that patterns of reproductive allocation may be more plastic than previously thought. It appears that there is a continuum from downturns in RE at old age to terminal investment that can be manifest, even across adjacent populations. Our work identifies uncertainty in the relationship between reproductive restraint and a lack of competitive ability in older life (driven by body mass senescence); both could explain a decline in RE in old age and may be hard to disentangle in empirical data. We discuss a number of environmental and anthropogenic factors which could influence reproductive life histories, underlining that life history patterns should not be generalised across different populations.
To better understand the potential trade-off between female reproductive investment and longevity in an emerging model of human healthspan, we studied pet dogs to determine whether intensity of reproduction (total number of offspring) encumbered the likelihood of exceptional longevity. This hypothesis was tested by collecting and analyzing lifetime medical histories, including complete reproductive histories, for a cohort of canine “centenarians” — exceptionally long-lived Rottweiler dogs that lived more than 30 % longer than the breed’s average life expectancy. Reproductive intensity (number of litters, total number of pups) and tempo of reproductive effort (age at first reproduction, mean interbirth interval, age at last reproduction) in 78 exceptionally long-lived female Rottweilers (>13 years old) were compared to a cohort of 97 female Rottweilers that had usual longevity (age at death 8.0–10.75 years). We found no evidence that a mother’s physiological investment in offspring was associated with disadvantaged longevity. Instead, similar to some studies in women, our data showed an inverted U-shaped trend, suggesting that moderate investment in reproduction may promote longevity. Late reproductive success, a much-studied surrogate of maternal fitness in women, was not a strong predictor of longevity in this canine cohort. Instead, independent of reproductive investment, the duration of lifetime ovary exposure was significantly associated with highly successful aging. Our results from exceptionally long-lived pet dogs provide rationale for further investigative efforts to understand the ovary-sensitive biological factors that promote healthy longevity in women and pet dogs.
Aging; Trade-off; Parity; Comparative biology; Cost of reproductive effort; Ovarian conservation
In group living animals, especially among primates, there is consistent evidence that high-ranking males gain a higher reproductive output than low-ranking males. Primate studies have shown that male coalitions and sociality can impact male fitness; however, it remains unclear whether males could potentially increase their fitness by preferentially supporting and socializing with females. Here we investigate patterns of male interventions and the effect of coalitions and sociality on male fitness in rhesus macaques (Macaca mulatta) with particular focus on male-female interactions. We combined behavioral collected on Cayo Santiago with genetic data analyzed for male reproductive output and relatedness. Our results revealed that the ten top-ranking males provided the majority of all male support observed. In contrast to other primates, male rhesus macaques mainly formed all-down coalitions suggesting that coalitions are less likely used to enhance male dominance. Males supporting females during and before their likely conception were not more likely to fertilize those females. We also found no evidence that males preferably support their offspring or other close kin. Interestingly, the most important predictor of male support was sociality, since opponents sharing a higher sociality index with a given male were more likely to be supported. Furthermore, a high sociality index of a given male-female dyad resulted in a higher probability of paternity. Overall, our results strengthen the evidence that sociality affects fitness in male primates, but also suggest that in species in which males queue for dominance, it is less likely that males derive fitness benefits from coalitions.
male rhesus macaques; interventions; coalitions; fitness; sociality
The reproductive costs associated with the upregulation of immunity have been well-documented and constitute a fundamental trade-off between reproduction and self-maintenance. However, recent experimental work suggests that parents may increase their reproductive effort following immunostimulation as a form of terminal parental investment as prospects for future reproduction decline. We tested the trade-off and terminal investment hypotheses in a wild population of house wrens (Troglodytes aedon) by challenging the immune system of breeding females with lipopolysaccharide, a potent but non-lethal antigen. Immunized females showed no evidence of reproductive costs; instead, they produced offspring of higher phenotypic quality, but in a sex-specific manner. Relative to control offspring, sons of immunized females had increased body mass and their sisters exhibited higher cutaneous immune responsiveness to phytohaemagglutinin injection, constituting an adaptive strategy of sex-biased allocation by immune-challenged females to enhance the reproductive value of their offspring. Thus, our results are consistent with the terminal investment hypothesis, and suggest that maternal immunization can induce pronounced transgenerational effects on offspring phenotypes.
immune challenge; life-history trade-off; maternal effect; sex allocation; terminal investment
The genetics of primate life histories are poorly understood, but quantitative genetic patterns in other mammals suggest phenotypic differences among individuals early in life can be strongly affected by interactions with mothers or other caretakers. I used generalized linear mixed model extensions of complex pedigree quantitative genetic techniques to explore regression coefficients and variance components for infant and juvenile mortality rates across pre-reproductive age classes in the semi-free ranging Cayo Santiago rhesus macaques. Using a large set of records (max. n=977 mothers, 6240 offspring), strong maternal effects can be identified early in development but they rapidly “burn off” as offspring age and mothers become less consistent buffers from increasingly prominent environmental variation. The different ways behavioral ecologists and animal breeders have defined and studied maternal effects can be subsumed, and even blended, within the quantitative genetic framework. Regression coefficients identify loss of the mother, maternal age, and offspring age within their birth cohort as having significant maternal effects on offspring mortality, while variance components for maternal identity record significant maternal influence in the first month of life.
quantitative genetics; infant mortality; juveniles; reproductive ecology; Cayo Santiago
Understanding the genetic and neuroendocrine basis of the mother-infant bond is critical to understanding mammalian affiliation and attachment. Functionally similar non-synonymous mu-opioid receptor (OPRM1) SNPs have arisen and been maintained in humans (A118G) and rhesus macaques (C77G). In rhesus macaques, variation in OPRM1 predicts individual differences in infant affiliation for mothers. Specifically, infants carrying the G allele show increased distress on separation from their mothers, and spend more time with them upon reunion, than individuals homozygous for the C allele. In humans, individuals possessing the G allele report higher perceptions of emotional pain on receiving rejection by social partners. We studied maternal behavior over the course of a year among free-ranging female rhesus macaques on Cayo Santiago, Puerto Rico. We then trapped females and collected blood samples, from which we assessed OPRM1 genotype; we also collected CSF samples from which we measured oxytocin (OT) levels. We show that females possessing the G allele restrain their infants more (i.e. prevent infants from separating from them by pulling them back) than females homozygous for the C allele. Females possessing the G allele also show higher OT levels when lactating, and lower OT levels when neither lactating nor pregnant, than females homozygous for the C allele. This is the first study to demonstrate an association between OPRM1 genotype and maternal attachment for infants, and is one of the first studies of any free-ranging primate population to link functional genetic variation to behavior via potentially related neuroendocrine mechanisms.
mother-infant bond; opioids; oxytocin; functional genetics; attachment
Food-sharing among non-kin— one of the most fascinating cooperative behaviors in humans— is not widespread in non-human primates. Over the past few years, a large body of work has investigated the contexts in which primates cooperate and share food with unrelated individuals. This work has successfully demonstrated that species-specific differences in temperament constrain the extent to which food-sharing emerges in experimental situations, with despotic species being less likely to share food than tolerant ones. However, little experimental work has examined the contexts that promote food-sharing and cooperation within a species. Here, we examine whether one salient reproductive context—the consortship dyad— can allow the necessary social tolerance for co-feeding to emerge in an extremely despotic species, the rhesus macaque (Macaca mulatta). We gave naturally formed male-female rhesus macaque pairs access to a monopolizable food site in the free-ranging population at Cayo Santiago, Puerto Rico. Using this method, we were able to show that tolerated co-feeding between unrelated adults can take place in this despotic species. Specifically, our results show that consort pairs co-fed at the experimental food site more than non-consort control pairs, leading females to obtain more food in this context. These results suggest that co-feeding is possible even in the most despotic of primate species, but perhaps only in contexts that specifically promote the necessary social tolerance. Researchers might profit from exploring whether other kinds of within-species contexts could also generate cooperative behaviors.
Social tolerance; food-sharing; sexual consortships; non-human primates
In most primate species, females remain in the natal group with kin while males disperse away from kin around the time of puberty. Philopatric females bias their social behavior toward familiar maternal and paternal kin in several species, but little is known about kin bias in the dispersing sex. Male dispersal is likely to be costly because males encounter an increased risk of predation and death, which might be reduced by dispersing together with kin and/or familiar males (individuals that were born and grew up in same natal group) or into a group containing kin and/or familiar males. Here we studied the influence of kinship on familiar natal migrant rhesus macaques (Macaca mulatta) on Cayo Santiago, Puerto Rico, by combining demographic, behavioral, and genetic data. Our data suggest that kinship influences spatial proximity between recent natal immigrants and males familiar to them. Immigrants were significantly nearer to more closely related familiar males than to more distantly related individuals. Within a familiar subgroup, natal migrants were significantly closer to maternal kin, followed by paternal kin, then non-kin, and finally to males related via both the maternal and paternal line. Spatial proximity between natal immigrants and familiar males did not decrease over time in the new group, suggesting that there is no decline in associations between these individuals within the first months of immigration. Overall, our results might indicate that kinship is important for the dispersing sex, at least during natal dispersal when kin are still available.
Familiarity; Kinship; Male rhesus macaques; Natal dispersal; Spatial proximity
Menopause is a seemingly maladaptive life-history trait that is found in many long-lived mammals. There are two competing evolutionary hypotheses for this phenomenon; in the adaptive view of menopause, the cessation of reproduction may increase the fitness of older females; in the non-adaptive view, menopause may be explained by physiological deterioration with age. The decline and eventual cessation of reproduction has been documented in a number of mammalian species, however the evolutionary cause of this trait is unknown.
We examined a unique 30-year time series of killer whales, tracking the reproductive performance of individuals through time. Killer whales are extremely long-lived, and may have the longest documented post-reproductive lifespan of any mammal, including humans. We found no strong support for either of the adaptive hypotheses of menopause; there was little support for the presence of post-reproductive females benefitting their daughter's reproductive performance (interbirth interval and reproductive lifespan of daughters), or the number of mature recruits to the population. Oldest mothers (> 35) did appear to have a small positive impact on calf survival, suggesting that females may gain experience with age. There was mixed support for the grandmother hypothesis – grandoffspring survival probabilities were not influenced by living grandmothers, but grandmothers may positively influence survival of juveniles at a critical life stage.
Although existing data do not allow us to examine evolutionary tradeoffs between survival and reproduction for this species, we were able to examine the effect of maternal age on offspring survival. Our results are consistent with similar studies of other mammals – oldest mothers appear to be better mothers, producing calves with higher survival rates. Studies of juvenile survival in humans have reported positive benefits of grandmothers on newly weaned infants; our results indicate that 3-year old killer whales may experience a positive benefit from helpful grandmothers. While our research provides little support for menopause evolving to provide fitness benefits to mothers or grandmothers, our work supports previous research showing that menopause and long post-reproductive lifespans are not a human phenomenon.
Reproductive investment decisions form an integral part of life-history biology. Selection frequently favours plasticity in investment that can generate maternal effects on offspring development. For example, if females differentially allocate resources based on mate attractiveness or quality, this can create a non-genetic link between mate attractiveness and offspring fitness with potential consequences for ecological and evolutionary dynamics. It is therefore important to understand under what conditions differential investment into offspring in relation to male quality is expected to occur and the direction of the effect. Two opposite predictions, increased investment into offspring produced with high-quality mates (differential allocation (DA)) and increased investment with low-quality males (reproductive compensation (RC)) have been suggested but no formal theoretical treatment justifying the assumptions underlying these two hypotheses has been conducted to date. Here, we used a state-based approach to investigate the circumstances under which the variation in mate quality results in differential female investment into offspring and how this interacts with female energetic resource levels. We found that a pattern of increased investment when mating with high-quality mates (i.e. DA) was the most common optimal investment strategy for females in our model. By contrast, increased investment when mating with low-quality mates (i.e. RC) was predicted only when the relative impact of parental investment on offspring quality was low. Finally, we found that the specific pattern of investment in relation to male quality depends on female energetic state, the likelihood for future mating opportunities and the expected future distribution of mate quality. Thus, the female's age and body condition should be important factors mediating DA and RC, which may help to explain the equivocal results of empirical studies.
dynamic program; differential allocation; compensation hypothesis; sexual selection; mate choice; maternal effects
In mammals, when females are clumped in space, male access to receptive females is usually determined by a dominance hierarchy based on fighting ability. In polygynandrous primates, as opposed to most mammalian species, the strength of the relationship between male social status and reproductive success varies greatly. It has been proposed that the degree to which paternity is determined by male rank decreases with increasing female reproductive synchrony. The priority-of-access model (PoA) predicts male reproductive success based on female synchrony and male dominance rank. To date, most tests of the PoA using paternity data involved nonseasonally breeding species. Here, we examine whether the PoA explains the relatively low reproductive skew in relation to dominance rank reported in the rhesus macaque, a strictly seasonal species. We collected behavioral, genetic, and hormonal data on one group of the free-ranging population on Cayo Santiago (Puerto Rico) for 2 years. The PoA correctly predicted the steepness of male reproductive skew, but not its relationship to male dominance: the most successful sire, fathering one third of the infants, was high but not top ranking. In contrast, mating success was not significantly skewed, suggesting that other mechanisms than social status contributed to male reproductive success. Dominance may be less important for paternity in rhesus macaques than in other primate species because it is reached through queuing rather than contest, leading to alpha males not necessarily being the strongest or most attractive male. More work is needed to fully elucidate the mechanisms determining paternity in rhesus macaques.
Dominance; Reproductive skew; Mating skew; Priority-of-access model; Genetic paternity analysis; Primates; Rhesus macaques
The differential allocation hypothesis predicts that reproductive investment will be influenced by mate attractiveness, given a cost to reproduction and a tradeoff between current and future reproduction. We tested the differential allocation hypothesis in the swordtail fish Xiphophorus multilineatus, where males have genetically influenced (patroclinous inheritance) alternative mating tactics (ARTs) maintained by a tradeoff between being more attractive to females (mature later as larger courting males) and a higher probability of reaching sexual maturity (mature earlier as smaller sneaker males). Males in X. multilineatus do not provide parental care or other resources to the offspring. Allelic variation and copy number of the Mc4R gene on the Y-chromosome influences the size differences between males, however there is no variation in this gene on the X-chromosome. Therefore, to determine if mothers invested more in offspring of the larger courter males, we examined age to sexual maturity for daughters. We confirmed a tradeoff between number of offspring and female offspring’s age to sexual maturity, corroborating that there is a cost to reproduction. In addition, the ART of their fathers significantly influenced the age at which daughters reached sexual maturity, suggesting increased maternal investment to daughters of courter males. The differential allocation we detected was influenced by how long the wild-caught mother had been in the laboratory, as there was a brood order by father genotype (ART) interaction. These results suggest that females can adjust their reproductive investment strategy, and that differential allocation is context specific. We hypothesize that one of two aspects of laboratory conditions produced this shift: increased female condition due to higher quality diet, and/or assessment of future mating opportunities due to isolation from males.
We investigated the effects of population fluctuation on the offspring’s sex allocation by a weakly polygynous mouse, Apodemus argenteus, for 3 years. In acorn-poor seasons, heavier mothers invested more in sons, and lighter mothers invested more in daughters. In acorn-rich seasons, heavier mothers invested more in daughters, and lighter mothers invested more in sons. Maternal body condition and litter size affected the sex allocation. Furthermore, there was a maternal investment trade-off between a son’s birth mass and the number of daughters. Based upon the effect of population fluctuation on the lifetime reproductive success of each sex, we proposed the new “safe bet hypothesis”. This hypothesis predicts that frequent and unpredictable change in female distribution, which is often caused by abrupt fall in food condition, favors female-biased maternal investment to offspring by polygynous mammals and is applicable to many small mammals inhabiting in unstable environments.
Apodemus argenteus; Trivers and Willard’s hypothesis; Sex allocation; Maternal investment trade-off; Safe bet hypothesis
Although the proximate mechanisms behind the formation of copulatory plugs are well understood, their distribution and function among primates remain largely unstudied. During a study of female rhesus macaque (Macaca mulatta) mating behaviour on Cayo Santiago, we examined the pattern of the distribution of visible copulatory plugs among females and their potential relationship to female reproductive traits and sexual skin colouration. Our results indicate that non-pregnant females were observed more frequently with visible copulatory plugs than were pregnant females. Female parity status, age and rank were unrelated to copulatory plug observation. Genital colouration components were higher among females observed with copulatory plugs. We suggest that although copulatory plugs are generally thought of in terms of male physiology, plug observation may also be affected by female reproductive status.
Rhesus macaques; Copulatory plugs; Colouration; Sperm competition; Sexual skin
Male sexually-selected traits can evolve through different mechanisms: conspicuous and colorful ornaments usually evolve through inter-sexual selection, while weapons usually evolve through intra-sexual selection. Male ornaments are rare among mammals in comparison to birds, leading to the notion that female mate choice generally plays little role in trait evolution in this taxon. Supporting this view, when ornaments are present in mammals they typically indicate social status and are products of male-male competition. This general mammalian pattern, however, may not apply to rhesus macaques (Macaca mulatta). Males of this species display conspicuous skin coloration, but this expression is not correlated to dominance rank, and is therefore unlikely to have evolved due to male-male competition. Here, we investigate whether male color expression influences female proceptivity towards males in the Cayo Santiago free-ranging rhesus macaque population. We collected face images of 24 adult males varying in dominance rank and age at the peak of the mating season, and modeled these to rhesus macaque visual perception. We also recorded female socio-sexual behaviors towards these males. Results show that dark red males received more sexual solicitations, by more females, than pale pink ones. Together with previous results, our study suggests that male color ornaments are more likely to be a product of inter- rather than intra-sexual selection. This may especially be the case in rhesus macaques due to the particular characteristics of male-male competition in this species.
Ornaments; sexual selection; female mate choice; sexual skin; color; anthropoid primates
Life history variables such as the age at first reproduction and the interval between consecutive births are measures of investment in growth and reproduction in a particular population or species. As such they allow for meaningful comparisons of the speed of growth and reproduction between species and between larger taxa. Especially in primates such life history research has far reaching implications and has led for instance to the “grandmother hypothesis”. Other links have been proposed with respect to dietary adaptations: Because protein is essential for growth and one of the primary sources of protein, leaves, occurs much less seasonally than fruits, it has been predicted that folivorous primates should grow faster compared to frugivorous ones. However, when comparing folivorous Asian colobines with frugivorous Asian macaques we recently documented a longer, instead of a shorter gestation length in folivores while age at first reproduction and interbirth interval did not differ. This supports earlier findings for Malagasy lemurs in which all life history variables tested were significantly longer in folivores compared to frugivores. Wondering why these trends were not apparent sooner, we tried to reconstruct our results for Asian primates with data from four popular life history compilations. However, this attempt failed; even the basic, allometric relationship with adult female body mass that is typical for life history variables could not be recovered. This negative result hints at severe problems with data quality. Here we show that data quality can be improved significantly by standardizing the variables and by controlling for factors such as nutritional conditions or infant mortality. Ideally, in the future, revised primate life history data should be collated in a central database accessible to everybody. In the long run such an initiative should be expanded to include all mammalian species.
Recent research has found empirical evidence in support of the Trivers-Willard Hypothesis that offspring sex allocation is correlated with maternal investment. Tammar wallabies birthing sons have higher investment ability; however a mechanism for sex specific differential allocation of maternal resources in wallabies remains elusive. In metatherians the majority of maternal investment is during lactation. To examine if differential allocation occurs during lactation, we measured total milk protein, lipid and carbohydrates, from mothers with male and female pouch young, during phase 2B (100–215 days post partum) and phase 3 (215–360 days post partum) of lactation. Mothers of sons allocated significantly higher levels of protein than mothers of daughters during phase 2B of lactation, however no sex specific difference in maternal allocation was found for lipids, carbohydrates, or any milk component during phase 3 of lactation. We were unable to measure milk production to establish any differences in the amount of milk allocated. However, with the production of more milk comes a dilution effect on milk components. Given that we find no apparent dilution of milk components may suggest equality in milk production. Offspring body weight at 14 months of age was related to protein allocation during phase 2B of lactation, providing a maternal mechanism for differential allocation with fitness consequences. We believe collection of earlier phase 2A (0–100 days post partum) milk may yield important results given that differential investment in metatherians may be most apparent early in lactation, prior to any significant maternal investment, when a decision on termination of investment can be made with very little energetic loss to the mother. Interestingly, small mothers did not birth sons and better maternal condition was associated with raising sons. These data are in support of TWH and demonstrate a potential mechanism through which condition dependent and sex specific maternal investment may occur.
Optimal parental sex allocation depends on the balance between the costs of investing into sons vs. daughters and the benefits calculated as fitness returns. The outcome of this equation varies with the life history of the species, as well as the state of the individual and the quality of the environment.We studied maternal allocation and subsequent fecundity costs of bank voles, Myodes glareolus, by manipulating both the postnatal sex ratio (all-male/all-female litters) and the quality of rearing environment (through manipulation of litter size by −2/+2 pups) of their offspring in a laboratory setting.We found that mothers clearly biased their allocation to female rather than male offspring regardless of their own body condition. Male pups had a significantly lower growth rate than female pups, so that at weaning, males from enlarged litters were the smallest. Mothers produced more milk for female litters and also defended them more intensively than male offspring.The results agree with the predictions based on the bank vole life history: there will be selection for greater investment in daughters rather than sons, as a larger size seems to be more influencial for female reproductive success in this species. Our finding could be a general rule in highly polygynous, but weakly dimorphic small mammals where females are territorial.The results disagree with the narrow sense Trivers & Willard hypothesis, which states that in polygynous mammals that show higher variation in male than in female reproductive success, high-quality mothers are expected to invest more in sons than in daughters.
cost of reproduction; litter size manipulation; nest defence; polygynous mating system; sexual size dimorphism
Animal communication involves the transfer of information between a sender and one or more receivers. However, such interactions do not happen in a social vacuum; third parties are typically present, who can potentially eavesdrop upon or intervene in the interaction. The importance of such bystanders in shaping the outcome of communicative interactions has been widely studied in humans, but has only recently received attention in other animal species. Here, we studied bouts of infant crying among rhesus macaques (Macaca mulatta) in order to investigate how the presence of bystanders may affect the outcome of this signalling interaction between infants and mothers. It was hypothesized that, as crying is acoustically aversive, bystanders may be aggressive to the mother or the infant in order to bring the crying bout to a close. Consequently, it was predicted that mothers should acquiesce more often to crying if in the presence of potentially aggressive animals. In line with this prediction, it was found that mothers gave infants access to the nipple significantly more often when crying occurred in the presence of animals that posed a high risk of aggression towards them. Both mothers and infants tended to receive more aggression from bystanders during crying bouts than outside of this time, although such aggression was extremely rare and was received by less than half of the mothers and infants in the study. Mothers were also found to be significantly more aggressive to their infants while the latter were crying than outside of crying bouts. These results provide new insight into the complex dynamics of mother–offspring conflict, and indicate that bystanders may play an important role in shaping the outcome of signalling interactions between infants and their mothers.
bystanders; audience; communication; crying; mother–offspring conflict; rhesus macaque
Information on basic reproductive parameters and life-history traits is crucial for the understanding of primate evolution, ecology, social behavior, and reproductive strategies. Here, we report 4 yr of data on reproductive and life-history traits for wild female Assamese macaques (Macaca assamensis) at Phu Khieo Wildlife Sanctuary, northeastern Thailand. During 2 consecutive reproductive seasons, we investigated reproductive behavior and sexual swelling size in 16 females and collected 1832 fecal samples. Using enzyme immunoassays, we measured fecal estrogen and progesterone metabolites to assess ovarian activity and timing of ovulation and to ascertain conceptions and pregnancies. Timing of reproduction was strictly seasonal (births: April–July, 86% in April–June, 4 yr, n = 29; conceptions: October–February, 65% in December–January, 2 yr, n = 17). Females showed no cyclic ovarian activity outside the mating season and conceived in their first or second cycle (mean: 1.2 cycles to conception, n = 13). Gestation length was on average 164.2 d (range: 158–170, n = 10), and females had their first infant at an age of 5 yr (n = 4). Interbirth intervals were bimodally distributed, with females giving birth on average every 13.9 or 23.2 mo. Shorter interbirth intervals were linked to early parturition within the birth season. Most females displayed subcaudal sexual swellings which, however, did not reliably indicate female reproductive status or fertility. Overall, our results fall within the range of findings reported for other macaque species. These results thus add to the growing body of information available for wild macaques, facilitating comparative studies for a better understanding of interspecific differences in social and reproductive patterns.
Assamese macaques; fecal hormone analysis; gestation length; interbirth interval; seasonality; sexual swellings