On the evolutionary stability of female infanticide

Territoriality among female rodents may have evolved as an adaptation to intraspecific competition for resources or, alternatively, to defend pups against infanticide. In order to evaluate the latter, we analyse the conditions that allow an infanticidal strategy to invade a population of non-infanticidal females, and the circumstances under which infanticide may become an evolutionarily stable strategy (ESS). Our game theoretical analyses indicate that infanticide has to be associated with some direct (cannibalism) or indirect (reduced competition) resource benefits in order to invade a non-infanticidal population. We also expect that females will primarily kill litters of nearby neighbors, thereby removing the closest competitors while keeping costs at a low level. However, once established in a population, infanticide may be an ESS, even if females do not gain any resource benefits. This is theoretically possible if a female through infanticide can reduce the possibility that other, potentially infanticidal, females establish and/or stay close to her nest. While behavioral data indicate that these special circumstances sometimes occur, they may be too specific to apply generally to small rodents. Therefore, we expect that the evolutionary stability of infanticide often requires resource benefits, and that female infanticide in small rodents may, in fact, be a consequence rather than a cause of territoriality. Received: 27 June 1996 / Accepted after revision: 28 December 1996     

Adoption or infanticide: options of replacement males in the European starling

The behaviour of a male bird towards a potential mate and her clutch may depend both on his expected paternity and on the likelihood that she will produce a replacement clutch if he commits infanticide. In this study we evaluate the choices made by replacement male European starlings Sturnus vulgaris. By removing males before and during laying, we induced other males, mainly neighbours, to mate with the reproductively active females. When the original male was removed before laying, a new male adopted the subsequent clutch in 14 out of 15 cases. When ten females were widowed during their laying period, replacement males never adopted their clutches. The paternity of replacement males was a function of when they replaced the former male. When replacement occurred more than 3 days before egglaying, the new male fathered nearly all offspring; when it occurred the day before laying, the new male still fathered more than every second young. When the original male was removed during his mate’s laying period, in five out of ten cases a replacement male committed infanticide by throwing out the eggs, but this only occurred in one out of 15 cases when removal took place before laying. The evidence for infanticide actually being committed by the replacement male was circumstantial. Four out of six of the females affected by apparent infanticide produced replacement clutches in which the male presumably had higher paternity than in the original clutch. In all cases, the male adopted the replacement clutch. In five cases when the original male was removed during laying, the neighbours neither adopted the brood nor committed infanticide, although they sometimes were seen courting the widowed female and copulating with her. These cases occurred later during laying than those were males comitted infanticide. The time from infanticide to the laying of the replacement clutch tended to increase as infanticide was committed later in the laying sequence. We conclude that strategies of potential replacement males are influenced by their expected paternity in the current brood and the probability that the female will produce an early replacement clutch. Received: 10 March 1995 / Accepted after revision: 28 October 1995     

Pup guarding by greater spear-nosed bats

Alloparental care poses an evolutionary dilemma because effort is expended on non-filial offspring. Thus, instances of alloparental care have been attributed to either mistaken identity, (i.e., recognition errors) or active cooperation. In greater spear-nosed bats (Phyllostomus hastatus), reproductive females roost together in stable long-term social groups in cave ceilings. Non-volant pups frequently fall from roost sites to the cave floor where they can die unless retrieved by an adult. In this study, we examined the function of adult female visits to non-filial young and tested whether visits were attributable to recognition errors or to cooperation. We found that females visited non-filial pups from their own social group more than expected. Females from different social groups attacked and sometimes killed pups, and male pups were attacked more frequently than female pups. Visits by group mates benefited fallen pups by reducing the likelihood of attack by females from other groups. In contrast to the mistaken identity hypothesis, we found that some females leave their own pups to approach and remain with group mates’ pups. We used microsatellite markers to estimate relatedness and test whether kinship could explain this alloparental care and found that females were unrelated to the pups they guard. We conclude that females who reside in highly stable social groups exhibit cooperative behavior that cannot be explained by kinship and is unlikely to be due to direct or generalized reciprocity. Instead, our data suggest that alloparental care likely involves a complex interplay between group membership and cooperative foraging.     

The effects of mating on infanticide and pup discrimination in white-footed mice

Summary A laboratory study was conducted with white-footed mice (Peromyscus leucopus) to determine (1) if cohabitation and/or successful mating inhibits infanticide, and if so for how long, and (2) if males and females can discriminate between their own (familiar) and strange (unfamiliar) pups. Infanticidal males and females that were paired for 15 days and did not reproduce were not inhibited from killing pups, whereas infanticidal males and females that were paired until they reproduced were inhibited from committing infanticide at 4 days postpartum. Neither males nor females discriminated between familiar and unfamiliar pups with regard to engaging in infanticide. Approximately 50% of wild-caught males and females killed pups in the laboratory. Infanticidal pairs bred significantly sooner than did noninfanticidal pairs. The results are consistent with the promiscuous nature of P. leucopus males and territoriality of P. leucopus females. Offprint requests to: Donna M. Cicirello, Kellogg Biological Station, 3700 E. Gull Lake Drive, Hickory Corners, MI 49060, USA     

Time constraints and multiple choice criteria in the sampling behaviour and mate choice of the fiddler crab,Uca annulipes

Active female sampling occurs in the fiddler crab Uca annulipes. Females sample the burrows of several males before remaining to mate in the burrow of the chosen partner. Females time larval release to coincide with the following nocturnal spring tide and must therefore leave sufficient time for embryonic development after mating. Here we show how this temporal constraint on search time affects female choosiness. We found that, at the start of the sampling period (when time constraints are minimal), females selectively sample the larger males in the population. Towards the end of the sampling period (when the temporal constraints increase the costs of sampling), females are less selective. Furthermore, we suggest that the number of males sampled (and other indices of ‘‘sampling effort’’) may not be reliable indicators of female choosiness and may not reflect the strength of female mating preferences under certain conditions. Burrow quality also emerged as an important criterion in final mate choice. Burrow structure potentially influences reproductive success, and mate acceptance based on burrow structure appears to involve a relatively invariant threshold criterion. Since there is no relationship between male size and burrow quality, females are using at least two independent criteria when choosing potential mates. We envisage mate choice as a two-stage process. First, females select which males to sample based on male size. They then decide whether or not to mate with a male based on burrow features. This sampling process explains how two unrelated variables can both predict male mating success. Received: 23 March 1995/Accepted after revision: 14 January 1996     

Weight comparisons and litter size manipulation in Columbian ground squirrels (Spermophilus columbianus) show evidence of costs of reproduction

I studied reproductive costs in the female Columbian ground squirrel (Spermophilus columbianus) using individually marked animals. I compared weight changes during the active season and over winter, and mortality for females that did and did not wean young. Females raising young were heavier at emergence in that spring than unsuccessful ones. Females that did not raise young gained more weight during summer, were heavier than successful females at the time of entry into hibernation, and were heavier emerging from hibernation the following spring. Over-winter mortality was higher for females that reared young compared to reproductively unsuccessful females. A food supplementation experiment showed that energy-rich food can accelerate individuals’ weight gain. Interactions between litter size, birth weight, weight at emergence from the natal burrow, survival of young to yearling age, and maternal fitness were also studied. Litter sizes were experimentally manipulated to evaluate how females cope with costs of rearing one additional young. Birth weight of juveniles was positively correlated with survival to emergence from the natal burrow and with survival to yearling age. Partial litter loss was higher in experimentally enlarged litters than in either experimentally decreased or control litters. Total litter loss, survival of adult females or the probability of weaning young the following year were not affected by the litter size manipulation. Females appear to adjust the size of their litter before birth, and to some extent during lactation, to their ability to wean young. Received: 20 January 2000 / Received in revised form: 12 March 2000 / Accepted: 18 March 2000     

The association of female kin in the arctic ground squirrel Spermophilus parryii

Summary The association of kin in arctic ground squirrels (Spermophilus parryii) was studied near Haines Junction, Yukon Territory, during the summers of 1977, 1978, and 1979. Males dispersed in this population, thus only females were likely to live near relatives.Close female kin (sisters, mothers/daughters) had greater overlap of home ranges and interacted more amicably and less agonistically than did less closely related females. Closely related females clumped their young at emergence, whereas more distantly related females did not; there was little indication of clumping of pre-emerged young. The overlap of home ranges of distant relatives (known genetic relatives that had not associated in a natal burrow) was intermediate to that of close relatives and non-relatives. The types of interactions between distant relatives were more similar to those between non-relatives than between close relatives.I conclude that female arctic ground squirrels exhibit nepotism. Females may benefit from associations with relatives by sharing watching for predators once juveniles become conspicuous. Male arctic ground squirrels commit infanticide and several females may be more effective at protecting their young from infanticidal males than females living alone. I suggest that clumping of young by close relatives may provide a mechanism allowing distantly related females to identify each other.Address for offprint requests     

Reproductive benefits of infanticide in a biparental burying beetle Nicrophorus orbicollis

Summary Single females and pairs of Nicrophorus orbicollis Say were established as residents on carcasses to investigate interactions with intruders under natural conditions. When carcasses were exhumed 10–12 days later, one or more intruders were found on 19% of nests. Intruders expelled one or both of the residents and sometimes paired with a resident or another intruder of the opposite sex. An intruder tended to be larger than the same-sex adult it replaced on a carcass. A male intruder that joined a single female, on the other hand, was not necessarily larger than the resident female. There was indirect evident that intruders committed infanticide and then started their own reproductive attempt on the carcass. Follow-up experiments in the laboratory indicated that interactions between intruders and residents were agonistic and that infanticide regularly occurred as a consequence of a takeover. Use of a genetic marker demonstrated that male and female intruders obtained reproductive benefits from infanticide. However, intruders sometimes cared for mixed broods consisting of their own young and young of a prior resident.     

Defending young biparentally: female risk-taking with and without a male in the burying beetle, <Emphasis Type="Italic">Nicrophorus pustulatus</Emphasis>

A positive correlation between the parental effort of a male and female should promote stable biparental care. Risk-taking (as assessed by injuries) against infanticidal intruders by Nicrophorus pustulatus females was expected to be low when females had a low probability of successful defense of the young. I tested the hypothesis that when the presence of a male partner increased the probability of successful defense from low to moderate that female risk-taking would increase. Single females and pairs with first instar larvae were confronted by potentially infanticidal male and female conspecific intruders. Male intruders routinely took over nests from unpaired females (30 of 36 trials). Unpaired females and male intruders were injured infrequently, indicating less intense fights despite the high probability of infanticide. A resident female defending against a male intruder was injured more often when paired than unpaired, suggesting greater risk-taking. A male parent that delays desertion, therefore, receives fitness benefits not only from his own defense of the young, but from greater female defense against male intruders as well. It is hypothesized that the threat of infanticidal takeovers by males promotes extended biparental care in burying beetles. When the intruder was female, on the other hand, a female parent on her own had a moderate probability of successfully defending the brood (22 of 36 trials). The presence of a male partner against female intruders almost guaranteed successful defense (35 of 36 trials) and female intruders did not appear to contest pairs vigorously. Against female intruders the presence of a male partner did not significantly change injury rates of the defending female.     

Infant handling by female Barbary macaques (Macaca sylvanus) at Affenberg Salem: testing functional and evolutionary hypotheses

Assisting the genetic parents in the rearing of young, a widespread phenomenon in many birds and mammals, is usually regarded as an altruistic or mutualistic behavior. Infant handling by females other than the mother is also common in many primates, but due to high within- and between-species variation and limited knowledge about fitness consequences there is no consensus about its evolutionary and functional significance. Analysis of female infant-handling patterns and its reproductive consequences in three groups of semifree-ranging Barbary macaques revealed that nulliparous females significantly more often handled infants than parous females, but infant handling experience did not affect survival of their own first live-born offspring. Females interacted preferentially with closely related infants, but infant handling frequency improved neither infant survival nor maternal fecundity. Reciprocation of infant handling by mothers was rare. Although “aunting to death” occurred in the population, the hypothesis that infant handling serves to reduce the fitness of competitors was not supported. Limited evidence suggests that females at least sometimes use infants as strategic tools in the course of alliance formation. In concert with this poor evidence for a functional basis of the behavior, several lines of evidence support the hypothesis that infant handling evolved as a non-adaptive by-product of a strong selection for mother-offspring bonding. (1) Rates of infant handling were highest among females that experienced early infant loss. (2) Females caring for infants or yearlings of their own handled other infants significantly less often than females without dependent offspring. (3) Infant handling by females was most prevalent during the infants’ first month of life. (4) Both “aunting to death” and a successful adoption occurred irrespective of kinship relations. Although the by-product hypothesis appears to be the only one able to explain all results of this study, the apparent rarity of infant handling in non-female-bonded species suggest that kin selection is a possible alternative explanation for the evolution of female infant-handling in primates. Received: 30 December 1995/Accepted after revision: 10 May 1996     

Communal nursing in the evening bat,Nycticeius humeralis

Summary Nursing observations over two summers involving 76 lactating female evening bats, Nycticeius humeralis, and 128 pups in an attic in northern Missouri indicate that communal nursing occurs rarely until 2 weeks before weaning during which time over 18% of nursing bouts involve nondescendant offspring. The average relatedness among female pairs nursing non-descendant offspring, based on identity-by-descent estimates using allozyme data, was 0.04 (SE=0.12). Mitochondrial DNA d-loop sequence comparisons confirm that at most only 2 of 20 female pairs nursing non-descendant offspring came from the same matriline. Thus, females do not nurse matrilineal kin preferentially despite female natal philopatry. In addition, the average degree of relatedness within a colony (r=0.01, SE =0.03) is too low to provide any indirect benefits from communal nursing. Female error alone is insufficient to explain these observations because females tended to allow female nondescendant young to nurse but excluded nondescendant males, particularly when they had all-male litters. Furthermore, communal nursing bouts did not differ in duration from parental nursing bouts and involved 31 % of all banded females and 24% of all banded pups observed nursing. Communal nursing occurred most frequently when pups began hunting on their own and when lactating females attained their lowest average pre-fed body weight. Mortality during this period was higher for male than female pups, and relative weights implicate starvation as the cause. Time-lapse video records of four families of bats in captivity showed that the number of nursing bouts was proportional to daily weight change. I propose that these results are consistent with both immediate and delayed benefits accruing to females which experience variable hunting success. If a female with extra milk reduced her weight by dumping milk prior to her next foraging trip, she could obtain an immediate energetic benefit and maintain maximum milk production. By restricting such milk donations to nondescendant females she may also increase colony size and thereby enhance her future acquisition of information about foraging and roosting sites.     

Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (<Emphasis Type="Italic">Mirounga leonina</Emphasis>) breeding at the Kerguelen Islands

Individual specialisation is widespread and can affect a population’s ecological and evolutionary dynamics. Whether intra-specific niche differences can influence reproductive investment was examined in a marine mammal, the southern elephant seal (Mirounga leonina), whose females were known to forage in two different areas during the austral winter. The study was conducted at Kerguelen Islands (49°21′S, 70°18′E), southern Indian Ocean, in late winter–early spring 2006. Pups were used as proxies of their mothers’ biology and combined information on their weaning mass (a proxy of females’ foraging success and short-term fitness) together with their blood δ¹³C value (a proxy of female foraging zone). First, the use of isotopic signature of pups was validated to study the female foraging ecology during their pre-breeding trip by demonstrating that δ¹³C and δ¹⁵N values of pups and their mothers were positively and linearly correlated. Then, blood samples were taken from a large number of newly-weaned pups, which were also weighed, to provide information at the population level. Estimated δ¹³C values of female seals encompassed a large range of values (from −23.7 to −19.1‰) with an unimodal frequency distribution, suggesting no contrasted foraging areas within the population. No significant relationship was found between pup weaning mass and their carbon signature, indicating no link between female foraging areas and maternal foraging success and investment. Finally, blood δ¹³C and δ¹⁵N values gave new insights into southern elephant seal ecology, suggesting that females mainly foraged north of the Polar Front where they preyed upon myctophid fish in late winter.     

Communal suckling in the cavy Galea musteloides

We quantified the extent of communal suckling in the cavy Galea musteloides. Six groups of animals were held in large indoor enclosures and suckling behavior was recorded over 113 h of observation. The groups contained 2–6 lactating females and 3–14 sucking pups. Due to the relative synchronization of births, 73% of the pups present in each group during lactation were non-offspring. Each of the 22 lactating females in the six groups suckled non-offspring in addition to her own offspring. On average, females suckled 86% of non-offspring present in their groups. Thus, 98% of all pups (n = 47) received milk from non-mothers. Although suckling frequencies were significantly higher for mothers with their own individual offspring than with non-offspring individuals, females invested more total time suckling all non-offspring than did suckling just their own; this was possible because for each mother many more non-offspring than offspring pups were present during lactation. Suckling bouts were significantly longer for mothers with their own individual offspring than with non-offspring individuals. The proportion of non-offspring suckling of mothers correlated negatively with the proportion of own young among the pups of a group. Non-offspring suckling did not affect future reproduction of females. Our observations demonstrate extensive practice of communal suckling in G. musteloides under laboratory conditions. Probably because all mothers of a group participated more or less equally in communal suckling behavior, the obvious cost of giving energetically expensive milk to non-offspring did not result in reduced (future) reproductive success. Potential benefits directly involved with communal suckling are unclear. More indirectly, communal suckling as well as birth synchrony might contribute to the formation of advantageous multi-litter kindergardens.     

Individual differences in the mechanisms underlying the onset and maintenance of paternal behavior and the inhibition of infanticide in the monogamous biparental California mouse,Peromyscus californicus

Virgin male Peromyscus californicus tend to behave infanticidally or nonparentally towards pups, whereas virtually all males exhibit parental behavior following birth of their own young. Most males (65–75%) living with their pregnant partner attacked or ignored unfamiliar pups and did not become parental (behave parentally) prior to birth of their young. However, a significant minority of males (34%) living with their partner became parental after just 24 h of postcopulatory cohabitation with the female and remained parental throughout their mate's pregnancy. Males that were infanticidal before the birth of their young became parental only after their young were born. The presence of the mother was necessary for the postpartum maintenance of paternal behavior and the inhibition of infanticide in males that were infanticidal prepartum. In contrast, males that were paternal prior to birth of their young continued to be paternal after birth, even in the absence of postpartum contact with the mother. Thus, different mechanisms are involved in the inhibition of infanticide and the onset and maintenance of paternal behavior. Correspondence to: D.J. Gubernick     

Reproductive success in a low skew, communal breeding mammal: the banded mongoose, Mungos mungo

In most cooperatively breeding species, reproduction is monopolised by a subset of group members. However, in some species most or all individuals breed. The factors that affect reproductive success in such species are vital to understanding why multiple females breed. A key issue is whether or not the presence of other breeders is costly to an individual’s reproductive success. This study examines the factors that affect the post-parturition component of reproductive success in groups of communal-breeding banded mongoose (Mungos mungo), where up to ten females breed together. Per-litter reproductive success was low (only 18% of pups survived from birth to independence). Whilst singular breeding was wholly unsuccessful, there were costs associated with breeding in the presence of increasing numbers of other females and in large groups. Synchronisation of parturition increased litter success, probably because it minimises the opportunity for infanticide or decreases competitive asymmetry between pups born to different females. There was no evidence of inbreeding depression, and reproductive success was generally higher in litters where females only had access to related males within their group. I conclude that communal breeding in female banded mongooses represents a compromise between the benefits of group-living and communal pup care on the one hand, and competition between females to maximise their personal reproductive success on the other. Such conflicts are likely to occur in most communal breeding species. Whilst communal breeding systems are generally considered egalitarian, negative effects of co-breeders on individual reproductive success is still an issue.     

Competition and group size in Thomas’s langurs (Presbytis thomasi): the folivore paradox revisited

Among primates, group size is highly variable. The standard ecological model assumes that better predation avoidance as group size increases favours living in larger groups, whereas increased travel costs and reduced net food intake due to within-group competition for resources set the upper limit. Folivorous primates, however, tend to defy this generalisation in that some live in small groups despite low costs of feeding competition. To resolve this ’folivore paradox’, it has been suggested that folivore group size is limited by social factors such as male harassment or infanticide, or that females can disperse more easily and thus maintain group size near optimum levels. In this paper, we examine the effects of group size on home range size, day-journey length, activity budget and diet in wild Thomas’s langurs (Presbytis thomasi), which live in one-male multi-female groups with a limited life cycle. We examined only data from the stable middle tenure phase when factors such as the strength of the breeding male or the way in which groups were formed did not influence ranging and activities. During this phase, group size affected day-journey length and home range size, and had a minor effect on diet, but did not influence time spent feeding or resting, allogrooming or birth rates. Hence the upper limit to group size during the middle tenure phase in Thomas’s langurs is not set by feeding competition. The folivore paradox is not due to frequent female dispersal in Thomas’s langurs. The timing of female dispersal is not as expected if it serves to keep group sizes near the ecological optimum, and groups seem to be below this optimum. Instead, female reproductive success is presumably maximised in small to mid-sized groups because larger groups show a clear trend to experience higher risk of take-over, often accompanied by infanticide. Because females can redistribute themselves among nearby groups when groups reorganise each time a new male starts up a new group, females can keep the group small. Thus, a social factor, risk of infanticide, seems to provide the selective advantage to small group size in Thomas’s langurs. Received: 29 July 1999 / Revised: 17 November 1999 / Accepted: 15 October 2000     

Mother-pup separation and adoption in northern elephant seals

Summary The evolution of fostering behavior, parental care directed toward another's young, has been the focus of much recent interest. During a five-year study of northern elephant seals (Mirounga angustirostris) at Año Nuevo, California, we recorded the frequency of mother-pup separation, reunion, and adoption of orphaned pups in crowded and low-density breeding areas. While most females nursed their own pup exclusively until it was weaned, many females, especially young mothers (age 3–5 years), were unable to raise a pup successfully. In the crowded main breeding harem on Año Nuevo Island, 24 to 47 percent of the pups born each year were separated from their mothers from 1977 to 1980. Mother-pup separation and pup mortality were associated with the following inter-related factors: 1) female density; 2) weather and tidal conditions; 3) topographical features of the breeding areas (i.e., degree of exposure to high tides and surf); and 4) the proportion of young, maternally inexperienced females pupping in a particular area. Most motherpup separations were caused directly by 1) adult males moving through the harem; 2) pups wandering from their mothers; 3) female aggression; and 4) inclement weather. Most of the separations, as well as adoptions, occurred when pups were quite young. Mother-pup recognition appeared to be based on a combination of acoustic, visual, and olfactory cues, and most mother-pup reunions were effected by the female rather than her pup. On the main island breeding area, 572 orphans were marked. Of these, five percent relocated their mother, 27 percent were adopted or frequently cared for by foster mothers, and 68 percent were not adopted, or rarely fostered. The survival of an orphan was clearly contingent on the amount of care it received; most orphans which were not nursed or protected by females died before reaching 6 weeks of age. Frequently, an adopted orphan's foster mother was in the stage of lactation which corresponded closely to that of its own mother. The most common fostering event involved females that had lost their own pup and adopted a single orphan. Other pupless females attempted to steal a pup, cared for a pup while it was still with its mother, adopted a weaned pup, adopted two pups, or indiscriminately nursed any orphaned pup that approached. Some females kept their own pup in addition to fostering in alien pup. Most foster mothers were young and had little or no previous maternal experience. The formation of large, high-density breeding rookeries, due to a scarcity of suitable breeding sites, results in frequent mother-pup separations, especially during inclement weather and tidal conditions. Many opportunities for adoptive behavior are therefore presented, because of the great number of orphans and pupless females. Increased maternal experience appears to be a benefit associated with adoption. Some instances of fostering behavior may also be based on reproductive errors on the part of the foster mother.     

A conspicuous courtship signal in the fiddler crab Uca perplexa: female choice based on display structure

The fiddler crab Uca perplexa has a conspicuous male courtship signal that is directed at females to attract them to the male’s burrow for mating. The signal involves waving the unflexed large claw up and down. To determine whether the spatiotemporal structure of the wave is under selection by female choice, we examined whether females had a preference for any particular features of the wave. Females respond to a waving display by either visiting the male’s burrow entrance or by electing to pass without visiting the burrow. We filmed mate-searching females and the waving males that they visited or passed. We documented the wave structure of these males using frame-by-frame analysis. Males produce a two-part wave with component A preceding component B. Both components have an upstroke, a pause at the apex and a downstroke. The tip of the claw was raised much higher in B than in A. Visited males had a shorter delay between the two wave components than did males that the females passed without visiting. Visited males also produced component B waves that had a slower upstroke than those of passed males. There was a significant correlation between the relative height of the raised claw and the duration of the upstroke of component B. Females were selecting males that raised their major claw to the highest position (two to three times as high as the carapace width). Passed males brought down their major claw earlier and from a lower position than did visited males. The data suggests that wave structure has evolved through female choice. Male display rate and body size were not female choice cues. An earlier study showed that display duration was also not used by females in selecting mates.     

More than friends? Behavioural and genetic aspects of heterosexual associations in wild chacma baboons

In mammals, fathers are facultative caretakers, and male care is expected to evolve only if it is directed towards related young. Yet, in several promiscuous primate societies, males seem to care for infants despite a presumably low paternity confidence. In cercopithecines, cohesive associations (‘friendships’) between a lactating female and an adult male are frequent and provide the female and her infant with protection against various sources of aggression, including infanticide. However, the benefits gained by males through such relationships remain unclear, in part, because the relatedness between males and their protected infants has rarely been examined. Moreover, little is known about the nature of the cues underlying kin discrimination by males in societies where females mate polyandrously. In this study, we combine behavioural and genetic data from wild chacma baboons (Papio ursinus) in Namibia to investigate (1) whether males are related to their friend’s infant and (2) whether similarity between the major histocompatibility complex (MHC) genotype of males and infants (potentially perceived through odour phenotype) favours the establishment of friendships. We first show that males share close genetic ties with their friend’s infants, most often by having sired the infant. Secondly, we find that male–infant MHC (Class II–DRB) similarity, in contrast to paternity, does not predict male–infant associations. Overall, our results clarify the nature of the evolutionary benefits gained by males in these heterosexual associations, which can be considered as true paternal care. However, the proximate mechanisms underlying paternity recognition remain to be identified.     

Asymmetric sexual conflict over parental care in a biparental cichlid

Theoretical models predict that parents should adjust the amount of care both to their own and their partner’s body condition. In most biparental species, parental duties are switched repeatedly allowing for repeated mutual adjustment of the amount of care. In the mouthbrooding cichlid Eretmodus cyanostictus, terms are switched only once with females taking the first share. The timing of the shift of the clutch between mates strongly determines both partners’ brooding period and thereby their parental investment. Females signal their readiness to transfer the young several days before the male finally takes them, suggesting sexual conflict over the timing of the shift. In a lab experiment, we reduced the body condition of either the female or the male of a pair to test whether energy reserves affect the timing of the shift and whether female signalling behaviour depends on energetic state. Males with a lowered condition took the young later and incubated for a shorter period, which prolonged the incubation time of their female partners. When female condition was lowered, female and male incubation durations remained unchanged, although females signalled their readiness to shift more intensely. Our results suggest that males adjust their parental investment to own energy reserves but are unresponsive to their mate’s condition. Females appear to carry the entire costs for the male’s adjustment of care. We propose that intrinsic asymmetries in the scope for mutual adjustment of parental investment and the costs of negotiation crucially influence solutions of the conflict between sexes over care.