Androgen and glucocorticoid levels reflect seasonally occurring social challenges in male redfronted lemurs (Eulemur fulvus rufus)

Intense reproductive competition and social instability are assumed to increase concentrations of glucocorticoids and androgens in vertebrates, as a means of coping with these challenges. In seasonally breeding redfronted lemurs (Eulemur fulvus rufus), the mating and the birth season and the associated increased male competition are predicted to pose such reproductive challenges. In this paper, we investigate seasonal variation in hormone excretion in male redfronted lemurs, and examine whether this variation is associated with social or ecological factors. Although dominance status has been shown to affect individual stress levels across many taxa, we predicted no rank-related differences in glucocorticoids for redfronted lemurs because relatively equal costs are associated with both high and low rank positions (based on patterns of rank acquisition/maintenance and threats toward subordinates). Over a 14-month period, we collected behavioral data (1843 focal hours) and 617 fecal samples from 13 redfronted lemur males in Kirindy Forest/Madagascar. We found no general rank-related pattern of testosterone or glucocorticoid excretion in this species. Both hormones were excreted at significantly higher levels during the mating and the birth season, despite social stability during both periods. The elevated mating season levels may be explained by increased within-group reproductive competition during this time and are in line with previous studies of other seasonally reproducing primates. For the birth season increase, we propose that the predictable risk of infanticide in this highly seasonal species affects male gonadal and adrenal endocrine activity. We evaluate alternative social and ecological factors influencing the production of both hormone classes and conclude based on our preliminary investigations that none of them can account for the observed pattern.     

The endocrinology of male rhesus macaque social and reproductive status: a test of the challenge and social stress hypotheses

Social status primarily determines male mammalian reproductive success, and hypotheses on the endocrinology of dominance have stimulated unprecedented investigation of its costs and benefits. Under the challenge hypothesis, male testosterone levels rise according to competitive need, while the social stress hypothesis predicts glucocorticoid (GC) rises in high-ranking individuals during social unrest. Periods of social instability in group-living primates, primarily in baboons, provide evidence for both hypotheses, but data on social instability in seasonally breeding species with marked social despotism but lower reproductive skew are lacking. We tested these hypotheses in seasonally breeding rhesus macaques on Cayo Santiago, Puerto Rico. We documented male fecal GC and androgen levels over a 10-month period in relation to rank, age, natal status, and group tenure length, including during a socially unstable period in which coalitions of lower ranked males attacked higher ranked males. Androgen, but not GC, levels rose during the mating season; older males had lower birth season levels but underwent a greater inter-season rise than younger males. Neither endocrine measure was related to rank except during social instability, when higher ranked individuals had higher and more variable levels of both. High-ranking male targets had the highest GC levels of all males when targeted and also had high and variable GC and androgen levels across the instability period. Our results provide evidence for both the challenge and social stress hypotheses.     

Sex differences in survival costs of reproduction in a promiscuous primate

In sexually promiscuous mammals, female reproductive effort is mainly expressed through gestation, lactation, and maternal care, whereas male reproductive effort is mainly manifested as mating effort. In this study, we investigated whether reproduction has significant survival costs for a seasonally breeding, sexually promiscuous species, the rhesus macaque, and whether these costs occur at different times of the year for females and males, namely in the birth and the mating season, respectively. The study was conducted with the rhesus macaque population on Cayo Santiago, Puerto Rico. Data on 7,402 births and 922 deaths over a 45-year period were analyzed. Births were concentrated between November and April, while conceptions occurred between May and October. As predicted, female mortality probability peaked in the birth season whereas male mortality probability peaked in the mating season. Furthermore, as the onset of the birth season gradually shifted over the years in relation to climatic changes, there was a concomitant shift in the seasonal peaks of male and female mortality. Taken together, our findings provide the first evidence of sex differences in the survival costs of reproduction in nonhuman primates and suggest that reproduction has significant fitness costs even in environments with abundant food and absence of predation.     

Experimentally elevated plasma levels of testosterone do not increase male reproductive success in blue tits

In temperate passerines, increased testosterone (T) levels during breeding mediate male aggressive and mating behaviour. If individual variability in T levels is reflected in behavioural differences during mating, males with higher T might gain higher reproductive success. This can be tested experimentally by elevating T levels. However, high exogenous T levels are known to have negative effects on male sperm production. This may reduce male fitness, particularly if sperm competition is intense. We experimentally elevated T levels in breeding blue tit males to investigate how T levels above the natural mean influence male reproductive success. Contrary to most—if not all—previous experimental manipulations of T levels in birds, we restricted the treatment with exogenous T to the time when females were fertile and T levels were naturally high in males. In blue tits, extra-pair paternity is an important component of male reproductive success, and its frequency is likely influenced by androgen-mediated behaviours such as mate attraction and aggression towards other males. Here we show that T-males were equally likely to become cuckolded and did not gain more extra-pair paternity than control males. Cuckolded T-males, however, lost more paternity than control males. We discuss the possibility that this is caused by negative effects of T treatment on sperm production.Communicated by M. Webster     

Mouse lemurs in space and time: a test of the socioecological model

Socioecological theory predicts that the distribution of fertile females in space and time is the major determinant of male spacing behavior and mating strategies. Using a small nocturnal Malagasy primate, the gray mouse lemur (Microcebus murinus), we determined the spatiotemporal distribution of estrous females during the brief annual mating season to examine the predictive power of the socioecological model for male mating strategies. Mouse lemurs are particularly interesting in this respect because this polygynous species is characterized by seasonal reproduction, seasonally reversed sexual dimorphism, and relatively large testes. All resident animals in our 8-ha study area, a total of 30 adult males and 27 adult females, were individually marked and regularly recaptured to determine female reproductive status and to obtain home range data. We found that the mating season is limited to 4 weeks following female emergence from hibernation. Only 3-9 females could have synchronized estruses during a given week, indicating a moderately high male monopolization potential. However, receptive females were not spatially clumped and male ranges overlapped with those of many other rivals. Therefore, we suggest that individual powerful males may be unable to defend exclusive permanent access to receptive females because of prohibitive costs of range defense resulting from the strongly male-biased operational sex ratio and the corresponding intruder pressure. Our general conclusions are (1) that the socioecological model provides a useful heuristic framework for the study of mating systems, but that (2) it does not specify the degree of spatiotemporal clumping of receptive females at which male mating strategies switch among mate guarding, spatial exclusion of rivals, and roaming, and that (3) the operational sex ratio can have profound effects on male mating strategies as well.     

Female mate choice,male migration,and the threat of infanticide in ringtailed lemurs

Summary Paternity determination by DNA fingerprinting is reported for a long-term study group of semi-free-ranging ringtailed lemurs (Lemur catta), together with behavioral data collected independently. In 1985, fraternal twin males unfamiliar and unrelated to the resident ringtailed lemurs were introduced to the forest enclosure. Every mature resident male attacked the immigrants frequently across the next 5 months, whereas no female ever did. All observed estrous females showed sexual proceptivity toward the' immigrant males; three solicited copulation exclusively from them. Each female repelled sons, matrilineal brothers, and other resident males from attempting to copulate. Over a 5-year period, four of five females always reproduced with distantly related or unrelated males (Fig. 3). Despite low dominance status throughout the case study, an immigrant sired the off-spring of each female that was proceptive toward only the immigrants, demonstrating that female choice can override male dominance relations to determine reproductive success among male ringtailed lemurs. In the birth season following the 1985–1986 immigration, each of four females targeted one or two particular adult males for consistent attack across the period of infant dependency, beginning days after parturition. Paternity determinations, colony records, and subsequent study of two groups allowed 66 cases of this mode of maternal aggression to be documented. In each, the targeted male had not fathered the protected infant, and almost invariably, he was unrelated to the infant's mother. New mothers attacked every male that immigrated following their infants' conceptions and a few familiar males with whom they had not been seen to copulate during the previous breeding season. Recent attempts by immigrant males to kill infants confirmed the anti-infanticidal function of maternal targeting of males. All results were interpreted together to advance a prospective model of the mating system of ringtailed lemurs. Female avoidance of incest has led to the evolution of natal male dispersal. Subsequently, males should prefer to transfer into groups containing few and/or status-vulnerable males. We predict that, by killing others' infants, males simultaneously increase chances for success in females' next reproductive efforts and terminate current fathers' reproductive eligibility in a group. Basic hypotheses that await testing are that (a) raising an infant through weaning reduces a female's chances for reproductive success the following year and (b) males that demonstrate the capacity to promote the survival of infant offspring are most attractive to females as mates.     

Infanticide by resident males and female counter-strategies in wild Japanese macaques (Macaca fuscata)

Infanticide was observed for the first time in a wild, non-provisioned troop of Japanese macaques on Yakushima Island, Japan. Eight adult resident males attacked unweaned infants in the pre- and early mating season, and one infanticide was observed directly. These attacks were not consistent with the social pathology, side effect of male aggression, cannibalism, or the resource defense hypothesis, but were generally consistent with the sexual-selection hypothesis. First, most male attackers had risen in dominance rank because several high-ranking males had left the troop. Second, in 78% of cases, male attackers had not previously been observed to mate with the mothers of victims. Moreover, analysis of subject animal DNA showed that males did not attack their own offspring. The two mothers who lost their unweaned infants, however, were not subsequently observed to mate. In fact, almost no mating behavior was observed in the troop. This was most likely due to a poor fruiting year. Resumption of mating by females who lost their infants may have been inhibited by an intervening environmental variable which suppressed female reproductive function. These observations contribute to a growing body of evidence which suggests that sexually selected infanticide can occur in seasonally breeding, multi-male, multi-female primate groups. Female Japanese macaques are known to mate with multiple males. We found evidence that female mating with multiple males inhibits contact aggression towards their infants. Adult males attacked infants eight times more often when they had not previously mated with the mother. Received: 2 September 1999 / Received in revised form: 27 April 2000 / Accepted: 3 May 2000     

Seasonal variation in male-female competition,cooperation and selfish hoarding in a monogamous songbird

Both cooperation and conflict between the sexes are commonplace in monogamous mating systems. However, little is known about how cooperation and competition varies seasonally in monogamous species that maintain permanent territories. We presented territorial pairs of male and female New Zealand robins (Petroica australis) with a large supply of insect prey at monthly intervals for 2 years. Behavioural observations after food presentation were then made to quantify seasonal and sexual differences in aggressive interactions over prey, prey acquisition rates, mate provisioning, offspring provisioning, selfish food hoarding and cache retrieval. Data were used to evaluate sex-specific behavioural strategies of mediating competition for food. Results showed that males aggressively excluded females from experimental food sources year-round. Females only accessed food sources when males left them unattended. Consequently, females acquired fewer prey than males. After controlling for differences in prey acquisition, both sexes consumed similar amounts of prey in the non-breeding season. Even though males aggressively excluded females from accessing food sources directly, males fed large amounts of prey to females during the breeding season. Both sexes provisioned young at similar rates. Males cached less prey than females in the breeding season but more prey than females in the non-breeding season. Females showed similar caching intensities year-round. Although males tried to defend their hoards, females frequently retrieved male-made caches. Overall, results showed that although New Zealand robins cooperate to raise offspring during the breeding season, conflict between the sexes occurs year-round. Males and females display different behavioural strategies to gain access to experimental food sources, which appear to lessen male–female competition for food and evenly distribute food resources between the sexes.     

Sexual selection and the function of a melanin-based plumage ornament in polygamous penduline tits <Emphasis Type="Italic">Remiz pendulinus</Emphasis>

Melanin-based ornaments are often involved in signaling aggression and dominance, and their role in sexual selection is increasingly recognized. We investigated the functions of a melanin-based plumage ornament (facial ‘mask’) in male Eurasian penduline tits Remiz pendulinus in the contexts of male–male aggression, mating success, and parental care. The penduline tit is a passerine bird with a unique mating system in which both sexes may mate with several mates in a breeding season, and one (or both) parent deserts the clutch. Our study revealed that mask size of males is more likely an honest signal used by females in their mate choice decisions than a trait involved in male–male competition. First, mask size increased with both age and body condition, indicating that the mask may signal male quality. Second, males with larger masks paired more quickly and had more mates over the breeding season than males with smaller masks. Third, we found no evidence that male mask size signals male–male aggression or dominance during competitive encounters. The increased mating success of large-masked males, however, did not translate into higher reproductive success, as nestling survival decreased with mask size. Therefore, we conclude that there is either no directional selection on male mask size or males with larger masks receive indirect, long-term benefits.     

Vocal interactions in common nightingales (<Emphasis Type="Italic">Luscinia megarhynchos</Emphasis>): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.     

Influence of social factors on sex ratio at birth,maternal investment and young survival in a prosimian primate

Summary In order to determine whether social factors influence sex ratio at birth in lesser mouse lemurs, experiments were conducted during 5 successive breeding periods on 51 females. At the beginning of the breeding season, females were either isolated (I) or grouped (G) in heterosexual groups with an increasing number of females (2, 3 or 4). To ensure mating, I females were introduced in a group only during the oestrous period. After mating, both I and G females were isolated during pregnancy and lactation. Reproductive capacities of females in terms of oestrus occurrences (n = 324), impregnations (n–210), pregnancies (n = 136) or abortions (n = 38) or litter sizes (1–3 young) were affected neither by age and parity of females nor by group housing prior to conception. G females produced significantly more sons than daughters (67% males for 189 newborn) while females living alone except during the mating period demonstrated a significant inverse tendency (39.6% males for 96 newborn). Distribution of sexes in litters was statistically different from random and varied according to the shift of sex ratio at birth. In G females, the shift in the sex ratio towards males was consistent across the different groups, independent of the number of females living together, suggesting that the presence of only 1 female is sufficient to induce a bias in the sex ratio. No correlation was found between infant survival at weaning and age, parity or group housing of the mother. The maternal investment allocated to male or female newborn was similar provided the litter contained at least 1 male. In litters without males, growth and survival of female infants were significantly less. These results on sex ratio bias in captive female mouse lemurs agree with directions of bias predicted by the local resource competition model for facultative sex ratio adjustment (Clark 1978). Nevertheless, the pattern observed in mouse lemurs appears to be independent of the nutritional state of the female and of differential maternal investment.     

Birth season testosterone levels in male Verreaux’s sifaka, Propithecus verreauxi: insights into socio-demographic factors mediating seasonal testicular function

Previous research suggests that although reproduction and testicular function in wild sifaka (Propithecus verreauxi) are highly seasonal, birth season elevations in fecal testosterone (T) in transferring males indicate that social factors may be as important as climatic factors in regulating reproductive function in sifaka. This paper examines the relationship of male dispersal and social status to the patterning of birth season aggression and testicular function in P. verreauxi at Beza Mahafaly Special Reserve, Madagascar. Behavioral and hormonal data were collected from a total of 38 adult males, 15 residing in seven stable groups and 23 living in eight unstable groups, yielding 186 fecal samples and 493 focal animal hours of observation. The results suggest that birth season elevations in fecal testosterone are the consequence of social disruption resulting from male movements between groups and the particular responses of individual males to dispersal events. Hormonal responses to dispersal events appear to precede behavioral responses and occasionally register reactions not concomitantly evident in the behavioral response, and may be predictive of future events. Hormonal reactions occurred primarily in resident alpha males, suggesting that they differ in their assessment of destabilizing influences, perhaps due to different reproductive opportunities and/or investment. These findings are important for the new insights they provide into the role of androgens in mediating male dispersal, life history, and reproductive strategies, and suggest that investigations of androgen-behavior interactions in free-ranging populations can be a powerful new tool for assessing the contextual and motivational basis of social behavior. Received: 22 November 1999 / Revised: 27 April 2000 / Accepted: 15 October 2000     

Exogenous testosterone increases female aggression in the European starling (<Emphasis Type="Italic">Sturnus vulgaris</Emphasis>)

In the European starling, Sturnus vulgaris, optimal mating systems differ between males and females. Males gain from polygyny, whereas monogamy increases female fitness. The cost of polygyny to females lead to intense female–female competition, and it has previously been shown that the intensity of female aggression during the pre-breeding period can predict the realised mating system. The physiological regulation of such female aggression in starlings is not yet known. This study examines the role of testosterone in mediating aggressive behaviours involved in intra-specific reproductive competition in female starlings. Testosterone levels were experimentally elevated with testosterone implants in females during the pre-laying period. To simulate a situation in which an additional female tried to mate with the focal female’s mate, a caged female was presented close to a nest-site to which the male could attract a secondary female. Testosterone was significantly related to several behaviours involved in female–female interactions. Females with testosterone implants spent significantly more time close to the caged female and produced more song bouts than control females. In contrast, male behaviour was unrelated to the experimental status of the mate. Females mated to males that attracted a secondary female were less aggressive towards the caged female than those that remained monogamously mated. The effect of exogenous testosterone in this study indicates that androgens may mediate social behaviours in female starlings during the breeding season.     

Intrasexual selection in Mirza coquereli : evidence for scramble competition polygyny in a solitary primate

The primates of Madagascar (Lemuriformes) deviate from fundamental predictions of sexual selection theory in that polygynous species lack sexual dimorphism, have even adult sex ratios and often live in female-dominated societies. It has been hypothesized that intrasexual selection in these species is either reduced or primarily focused on traits related to scramble competition. The goal of this study was to examine these hypotheses by studying the mating system of a solitary nocturnal species, Mirzacoquereli. During a 4-year field study in western Madagascar, I captured and followed 88 individually marked animals. I found that adult males were significantly larger than females, providing the first evidence for sexual size dimorphism in lemurs. In addition, the adult sex ratio was biased in favour of females in 3 out of 4 years. There was no significant sex difference in canine size, however. Males showed pronounced seasonal variation in testis size with a 5-fold increase before and during the short annual mating season. During the mating season, males had more injuries than females and more than quadrupled their home ranges, overlapping with those of more than ten females, but also with about the same number of rivals. Only about one social interaction per 10 h of observation was recorded, but none of them were matings. Together, these results indicate that these solitary lemurs are clearly subject to intrasexual selection and that male-male competition is primarily, but not exclusively, of the scramble type. In addition, they suggest that the above-mentioned idiosyncracies may be limited to group-living lemurs, that social systems of solitary primates are more diverse than previously thought, and that the temporal distribution of receptive females is responsible for this particular male mating strategy. Received: 11 January 1997 / Accepted after revision: 18 April 1997     

Paternity analysis shows experience,not age,enhances mating success in an aquatically mating pinniped,the Weddell seal (<Emphasis Type="Italic">Leptonychotes weddellii</Emphasis>)

For polygynous mammals with no paternal care, the number of offspring sired is often the sole measure of male reproductive success. The potential for polygyny is highest when resources or other environmental factors such as restricted breeding sites force females to aggregate. In these circumstances, males compete intensely for females and mating success may vary greatly among males, further intensifying selection for those traits that confer an advantage in reproduction. Hence, determinants of male success in competition for females are likely to be under strong sexual selection. Paternity analysis was used in conjunction with measures of age, site fidelity, and behavior during the breeding season to assess variance in male breeding success in Weddell seals (Leptonychotes weddellii) breeding at Turtle Rock, McMurdo Sound (77.727S, 166.85E) between 1997 and 2000. Paternity could be assigned to 177 pups at relaxed or 80% confidence level or 111 pups at strict or 95% confidence levels. Weddell seals at Turtle Rock show a modest degree of polygyny with the greatest number of pups sired by any individual male in a single season equalling 5 or ∼10% of the pups born. Over four consecutive years, most (89.2%) males sired at least one pup. In a generalized linear model (GLM), age and the age first seen at the study site as an adult were unrelated to mating success, but adult experience, either site-specific or elsewhere in McMurdo Sound, over the reproductive life span of males explained nearly 40% of variance in total mating success with 80% confidence and 24% of variance at 95% confidence. While learning where females are likely to be may enhance male reproductive success, aquatic mating reduces the ability of males to monopolize females, and thereby increases equity in mating success.     

Genetic evidence for mixed parentage in nests of the emu (Dromaius novaehollandiae)

Parentage in emus (Dromaius novaehollandiae) was examined by microsatellite analysis using four independent loci. Of 106 chicks sampled in one breeding season from 18 nests, 54 (51%) were not fathered by the nesting male, 12 (11%) were not from the observed mate of the sitting male, and 9 (8%) represented intra-specific brood parasitism, having no alleles in common with either nest parent. Some males (11%) fathered all chicks in their nests, but the majority showed high levels of cuckoldry. Those males commencing incubation earliest in the season tended to have the highest levels of paternity in their own nests. These results reveal a high frequency of extra-pair fertilisations and resultant cuckoldry in a predominantly socially monogamous bird and support recent reports which have described the emu mating system as a complexity of polyandrous, promiscuous and monogamous behaviour. Parentage assignment of chicks resulting from extra-pair fertilisations revealed an evenly scattered pattern of paternity that did not show any particular male dominance in reproductive success. These results lead to a reassessment of behavioural observations of emus, the consequences of parentage distribution, and theories about mating systems and sexual selection. The frequency of extra-pair copulations and intra-specific brood parasitism suggests patterns of descent that differ greatly from those implied by social monogamy. Received: 27 November 1999 / Received in revised form: 8 January 2000 / Accepted: 8 February 2000     

Plasma levels of androgens and gonadal development of breeders and helpers in the bell miner (Manorina melanophrys)

Plasma concentration of androgens and gonad development were studied in wild cooperatively breeding bell miners (Manorina melanophrys) of different age, sex and social status. Plasma levels of androgens increased with age of birds. For sexually mature (SM) bell miners, androgen levels were higher in breeders than in nonbreeding (NB) helpers (P < 0.005), and they were marginally higher in females than in males (P < 0.10). Thus female breeders showed the highest plasma levels of androgens. This pattern may be explained by an asymmetry between the sexes in the levels of competition needed to reach and defend breeding status; the degree of intra-sexual competition seems to be higher among female bell miners than among males. In general, male breeders had larger gonads than male SMNB helpers; male breeder gonads produced spermatozoa whereas most helpers' gonads did not. However, old (i.e. 34 months) SMNB male helpers had large gonads producing spermatozoa in spite of having low plasma levels of androgens. We suggest that young male bell miners may be under a voluntary mechanism of reproductive suppression perhaps favoured by inbreeding avoidance; although some aggressive interactions observed between very young helpers (about 2 months) and adult members of the breeding unit suggests that a certain degree of imposition also occurs at young ages. Older male helpers may avoid aggression with the male breeder and imposed reproductive suppression, hence the low circulating levels of androgens, but they have the potential to sneak extra-pair copulations. Gonad size decreased during the non-breeding season in both sexes.     

Effects of testosterone on male-male competition and male-female interactions in blue tits

In temperate-zone birds, testosterone (T) influences male behavior during the breeding season. The elevation of plasma levels of T to a breeding baseline is necessary for basic reproductive behaviors, but it is still unclear whether variation in T levels above this critical threshold influences the intensity of these behaviors. Such a relationship between T and sexually selected traits is a critical assumption of the immunocompetence handicap hypothesis. We here experimentally elevated T levels in blue-tit males above the natural mean (T-males) during the period of nest building and egg laying, without manipulating hormone levels during chick feeding. T-males neither interacted more often with other males, nor did they respond more aggressively to a dummy intruder, compared to males with control implants. T-males did not guard their social mates more closely, but they were more likely to interact with potential extra-pair mates. Females mated to T-males did not change their behavior during egg laying and the treatment did not significantly affect male and female feeding rates. Despite this, nests of T-males produced larger and heavier fledglings in one study year. Our observations suggest that T levels above the natural mean during the mating period do not increase aggressive or territorial behavior in male blue tits. However, if females perceived T-males as high-quality mating partners, superior offspring development in nests of T-males might be caused by higher maternal investment. Hence, male behaviors involved in mate attraction may have been influenced by T levels above the natural mean.Communicated by R. Gibson     

Infanticide in seasonally breeding multimale groups of Hanuman langurs (Presbytis entellus) in Ramnagar (South Nepal)

In the seasonally breeding langur (Presbytis entellus) population of Ramnagar, South Nepal, where multimale groups prevail, 25 attacks on 11 infants (including one actual killing) by seven adult males were witnessed in five groups by six observers between 1990 and 1996. Circumstantial evidence also indicates three additional attempts at infanticide and in seven additional cases infanticide was presumed or likely. Infanticide presumably accounted for 30.8–62.5% of infant mortality in the first 2 years of life. Most attackers (91.4%) were residents of the infants' group and had immigrated after the infants had been born (75.0%) or conceived (25.0%). Thus, they were not related to the victims. The interbirth interval was shortened if an infant died either prior to September of its 1st year of life (mean = 1.2 years), or its 2nd year (mean = 2.0) and even its 3rd year (mean = 2.4). All attackers remained in the group at least until the next mating season; high-ranking males maintained their dominance rank and lower-ranking males rose in rank. Since rank and mating success were correlated and rank and reproductive success might be correlated, all attackers had a good chance of siring the next infant of the victims' mothers and could thus have benefited by their action. Infanticide seems to be a male reproductive strategy at Ramnagar. Infanticide has never before been reported among seasonally breeding langurs living at such low densities. This is also the first detailed report of infanticide as a male reproductive strategy in a seasonally breeding primate population. Received: 19 December 1996 / Accepted after revision: 7 June 1997     

High degree of paternity loss in a species with alternative reproductive tactics

In many mating systems, males adopt alternative reproductive tactics (ARTs) to maximize reproductive success. In fishes, guarding males often invest more energy into courtship, defense, and paternal care, whereas cuckolding males forego such costs and steal fertilizations by releasing their sperm in the nest of a guarding male. These two tactics have been documented in the plainfin midshipman fish (Porichthys notatus), yet the relative reproductive success of the guarding and cuckolding male tactics remains unknown. In this study, we used microsatellite markers to determine the level of paternity of the guarding type I males. We explored how paternity varied with male phenotype and across the breeding season. Our results revealed the lowest documented levels of paternity in a species with obligate paternal care. Although paternity remained consistently low, it did increase as the breeding season progressed. Male body size did not significantly predict paternity. The low paternity in this species may be explained, in part, by aspects of their reproductive ecology including the duration of parental care period, limited nest availability and competition for nests, as well as the occurrence of nest takeovers. Overall, our findings contribute to the understanding of the ultimate factors underlying ARTs in this species and highlight the importance of investigating reproductive success across the entire breeding season.