Aggression and submission reflect reproductive conflict between females in cooperatively breeding meerkats Suricata suricatta

In many cooperatively breeding species, dominant females suppress reproduction in subordinates. Although it is commonly assumed that aggression from dominant females plays a role in reproductive suppression, little is known about the distribution of aggressive interactions. Here, we investigate the distribution of aggressive and submissive interactions among female meerkats (Suricata suricatta). In this species, dominant females produce more than 80% of the litters, but older subordinates occasionally breed. Dominant females commonly kill the pups of subordinates and usually evict older female subordinates from the group 1–3 weeks before the birth of the dominant female's litter. The aggression frequency of the dominant female toward subordinates and the submission frequency that each subordinate female showed to the dominant female increased as the age of the subordinate female increased and as the birth of the dominant female's pups approached. Moreover, as birth approached, both of these behaviors intensified more quickly between the dominant female and older subordinates than between the dominant female and younger subordinates. The aggression frequency of the dominant female toward each subordinate female predicted whether that subordinate female was evicted from the group; the submission frequency by each subordinate female predicted the timing of their eviction during the pregnancy period of the dominant female. These results support the idea that conflict between dominant and subordinate females increases with the age of subordinates and, since older subordinate females are most likely to reproduce, suggest that dominant females may less easily control reproductive attempts by older subordinate females.     

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.     

Tug-of-war over reproduction in a cooperatively breeding cichlid

In group-living animals, dominants may suppress subordinate reproduction directly and indirectly, thereby skewing reproduction in their favour. In this study, we show experimentally that this ability (‘power’) is influenced by resource distribution and the body size difference between unrelated dominants and subordinates in the cichlid Neolamprologus pulcher. Reproduction was strongly skewed towards the dominant female, due to these females producing more and larger clutches and those clutches surviving egg eating better than those of subordinate females, but was not so when subordinates defended a patch. If breeding shelters were provided in two patches, subordinate females were more likely to exclusively defend a patch against the dominant female and breed, compared to when the same breeding resource was provided in one patch. Relatively large subordinate females were more likely to defend a patch and reproduce. Females also directly interfered with each other’s reproduction by eating the competitors’ eggs, at which dominants were more successful. Although dominant females benefited from subordinate females due to alloparental care and an increase in egg mass, they also showed costs due to reduced growth in the presence of subordinates. The results support the view that the dominant’s power to control subordinate reproduction determines reproductive partitioning, in agreement with the predictions from tug-of-war models of reproductive skew.     

Reproductive suppression and inbreeding avoidance in wild populations of co-operatively breeding meerkats (Suricata suricatta)

Meerkats live in co-operatively breeding familial groups in which reproduction is monopolised by a dominant pair of breeders. Offspring of the breeders are behaviourally subordinate, and typically remain in their natal group as sexually mature, non-breeding helpers. In this study, we investigated the proximate factors limiting subordinate reproduction. Evidence for reproductive suppression by dominants was investigated by comparing life history, behaviour and hormonal profiles of dominants and subordinates. Baseline levels of plasma luteinising hormone (LH) were significantly higher in dominant than in subordinate females. However, following an exogenous injection of gonadotrophin-releasing hormone (GnRH), both categories had comparable concentrations of circulating LH. There were no significant differences in pre- or post-GnRH challenge LH levels in dominant or subordinate males. Reproduction in both dominant and subordinate females rarely occurred in the absence of unrelated males. Given that groups typically comprise parents and offspring, lack of suitable mates emerged as the primary constraint on subordinate reproduction. When this constraint was removed, subordinates typically bred but at a lower rate than dominants. This difference in reproduction may be attributed to intrasexual competition manifested through direct interference by dominant females through subordinate evictions, infanticide and the abandoning of subordinate litters. We argue that differences in reproductive regulation within mammalian co-operative breeding systems may be explained by differences in the mating strategy (inbreeding versus outbreeding) and the probability that subordinates in obligate outbreeding species will encounter unrelated opposite-sex partners. Received: 19 April 2000 / Accepted: 17 July 2000     

Socioecological influences on the reproductive success of female mountain gorillas (<Emphasis Type="Italic">Gorilla beringei beringei</Emphasis>)

Over the past few decades, socioecological models have been developed to explain the relationships between the ecological conditions, social systems, and reproductive success of primates. Feeding competition, predation pressures, and risk of infanticide are predicted to influence how female reproductive success (FRS) depends upon their dominance rank, group size, and mate choices. This paper examines how those factors affected the reproductive success of female mountain gorillas (Gorilla beringei beringei) of the Virunga Volcanoes, Rwanda from 1967–2004. Reproductive success was measured through analyses of interbirth intervals, infant survival, and surviving infant birth rates using data from 214 infants born to 67 females. Mountain gorillas were predicted to have “within-group scramble” feeding competition, but we found no evidence of lower FRS in larger groups, even as those groups became two to five times larger than the population average. The gorillas are considered to have negligible “within-group contest” competition, yet higher ranked mothers had shorter interbirth intervals. Infant survival was higher in multimale groups, which was expected because infanticide occurs when the male dies in a one-male group. The combination of those results led to higher surviving birth rates for higher ranking females in multimale groups. Overall, however, the socioecological factors accounted for a relatively small portion of the variance in FRS, as expected for a species that feeds on abundant, evenly distributed foliage.     

The lemur syndrome unresolved: extreme male reproductive skew in sifakas (<Emphasis Type="Italic">Propithecus verreauxi</Emphasis>), a sexually monomorphic primate with female dominance

The primates of Madagascar (Lemuriformes) are unusual among mammals in that polygynous species lack sexual dimorphism, and females dominate males socially in most species. Moreover, lemur groups are relatively small and characterized by even adult sex ratios despite the fact that one male should be able to exclude other males from the group. One hypothesis to explain this combination of behavioral, morphological, and demographic traits (the “lemur syndrome”) postulates that male–male competition is relaxed and, hence, variance in male reproductive success is low. Reproductive skew theory provides a framework for testing this and several related predictions about lemur social evolution. Specifically, low reproductive skew is also predicted if dominant males or adult females make reproductive concessions to subordinates or if the latter group successfully pursues alternative reproductive tactics. However, suitable data on paternity, demography, and behavior for a conclusive test of these predictions have not been available in the past. In this paper, we show that male reproductive success in ten groups of Verreaux’s sifakas (Propithecus verreauxi) was extremely skewed in favor of the dominant male over 9 years. Our genetic analyses also revealed that more than a third of all groups are effectively harem groups because only one male was unrelated to the resident female(s). In groups with two or more non-natal males, the dominant sired 91% of 33 infants. Together, males pursuing one of several alternative reproductive tactics, such as roaming among several groups or immigrating peacefully, sired only 11% of infants. Thus, female sifakas do not control group composition by offering reproductive opportunities to subordinate males as staying incentives, intrasexual selection is not relaxed, and dominant males prevail in a tug-of-war over subordinate males. Because male reproductive skew in sifakas is even more pronounced than in harem-living anthropoids studied to date, intrasexual selection is clearly not relaxed, and the lemur syndrome is more puzzling than ever.     

Why breed communally? Factors affecting fecundity in a communal breeding mammal: the banded mongoose (<Emphasis Type="Italic">Mungos mungo</Emphasis>)

Fecundity is an important component of fitness. In cooperatively breeding species, studies aimed at understanding the factors that affect fecundity have largely been restricted to species that exhibit high reproductive skew, where reproduction is monopolised by a few individuals. In such species, dominant suppression and inbreeding avoidance are the principal explanations for low fecundity in subordinate females. In this paper, we evaluate the relative effects of individual, social, and environmental factors on female fecundity in a low skew cooperative breeding mammal: the banded mongoose (Mungos mungo). Most females (80%) conceived in each breeding event, and most pregnant females (93%) carried their litter to term. The principal determinants of a females fecundity were intrinsic qualities, particularly age and body size. However, there was no evidence of dominant suppression of subordinate reproduction or inbreeding avoidance. Similarly, there was little indication that social or environmental factors influence fecundity. We suggest that in the banded mongoose, the apparent lack of costs to inbreeding, and the absence of dominant female suppression of reproduction in other females result in low reproductive skew. Indeed, in banded mongooses, like lions (Felis leo), multiple breeding may be a consequence of benefits to rearing young communally.Communicated by F. Trillmich     

Do stress hormones suppress helper reproduction in the cooperatively breeding red-cockaded woodpecker (<Emphasis Type="Italic">Picoides borealis</Emphasis>)?

In the cooperatively breeding red-cockaded woodpecker (Picoides borealis), male helpers are subordinate to male breeders and do not mate with females, even when unrelated to the breeding female within their group or through extra-group matings, yet exhibit reproductive hormone profiles similar to those of breeders. We investigated whether reproduction might be suppressed in helper males via high levels of the stress hormone corticosterone. We also examined effects of group size and season on corticosterone levels by comparing baseline and maximal plasma levels of corticosterone between helper males and breeding males, and among helper males and breeders of both sexes living in groups of different sizes throughout the reproductive cycle. We also measured plasma levels of estrogen, progesterone, and testosterone to examine other potential hormonal differences between helpers and breeders. Male status did not explain variation in any hormones; therefore, our data do not support the hypothesis that helper males are reproductively suppressed via corticosterone or the other hormones investigated. However, the presence of two or more helper males in a group tended to reduce baseline corticosterone in breeding and helper males, but not breeding females, suggesting that helper males reduce parental effort of other male group members. Seasonally, maximal corticosterone peaked during the nestling provisioning phase for breeding and helper males, but not breeder females, suggesting that males show an increased response to stressors posing a potential threat to survival of offspring.     

Male care of infants in a siamang (Symphalangus syndactylus) population including socially monogamous and polyandrous groups

While male parental care is uncommon in mammals, siamang (Symphalangus syndactylus) males provide care for infants in the form of infant carrying. I collected behavioral data from a cohort of five wild siamang infants from early infancy until age 15–24 months to identify factors affecting male care and to assess the consequences of male care for males, females, and infants in a population including socially monogamous groups and polyandrous groups. There was substantial variation in male caring behavior. All males in polyandrous groups provided care for infants, but males in socially monogamous groups provided substantially more care than males in polyandrous groups, even when the combined effort of all males in a group was considered. These results suggest that polyandry in siamangs is unlikely to be promoted by the need for “helpers.” Infants receiving more care from males did not receive more care overall because females compensated for increases in male care by reducing their own caring effort. There was no significant relationship between indicators of male–female social bond strength and male time spent carrying infants, and the onset of male care was not associated with a change in copulation rates. Females providing more care for infants had significantly longer interbirth intervals. Male care may reduce the energetic costs of reproduction for females, permitting higher female reproductive rates.     

Relatedness, polyandry and extra-group paternity in the cooperatively-breeding white-browed scrubwren (Sericornis frontalis )

We used DNA fingerprinting to examine the genetic parentage and mating system of the cooperatively breeding white-browed scrubwren, Sericornis frontalis, in Canberra, Australia. Our analyses revealed a remarkable variety of mating tactics and social organization. Scrubwrens bred in pairs or multi-male groups that consisted of a female and two or more males. Females were always unrelated to the pair male or alpha (dominant) male. Among multi-male groups we found three different mating tactics. Firstly, when alpha and beta (subordinate) males were unrelated, they usually shared paternity in the brood. This resulted in both males gaining reproductive benefits directly. Secondly, when beta males were not related to the female but were related to the alpha males, beta males sired offspring in some broods. In this situation, beta males gained reproductive benefits both directly and potentially indirectly (through the related alpha male). Thirdly, when beta males were related to the female or both the female and alpha male, they remained on their natal territory and did not sire any offspring. Thus beta males gained only indirect reproductive benefits. Overall, when group members were related closely, the dominant male monopolized reproductive success, whereas when the members were not related closely the two males shared paternity equally. This positive association between monopolization of reproduction and relatedness is predicted by models of reproductive skew, but has not been reported previously within a single population of birds. Other cooperatively breeding birds with both closely related and unrelated helpers may show a similar variety of mating tactics. Finally, we found that extra-group paternity was more common in pairs (24% of young) than in multi-male groups (6%), and we discuss three possible reasons for this difference. Received: 21 May 1996 / Accepted after revision: 14 December 1996     

Kin-preferential cooperation,dominance-dependent reproductive skew,and competition for mates in communally nesting female house mice

Little is known about the behavioural mechanisms facilitating kin-preferential communal breeding in wild house mice (Mus domesticus). We evaluated the effect of kinship and male availability on aggression, social structure and reproductive skew in groups of female mice freely interacting and reproducing in semi-natural indoor enclosures. Triplets of either sisters or non-sisters were established in enclosures provided with either one or three littermate males, which were unrelated and unfamiliar to the females. Sisters were more spatially associated and less aggressive than non-sisters, leading to higher incidences of communal breeding and reproduction. This is in agreement with theoretical considerations on kin selection in house mice. Reproductive success was highly skewed in favour of dominant females due to subordinate infertility or complete loss of first litters, which might have been caused by dominant females. In spite of this, subordinates only rarely dispersed from the enclosures, suggesting that perceived dispersal risk generally outweighed relatively reduced reproductive potentials. Aggression levels among females were significantly higher when one male was available, compared to when three males were available. We suggest that this might result from higher female-female competition for mates, due to the risk of missing fertilisation when synchronously oestrous females encounter limited numbers of males in a deme. Our results indicate that, first, communal nursing in house mice might have evolved to make the best out of a bad job rather than to enhance offspring fitness; and, second, that female-female mate-competition might play an important role in shaping female social structure in this polygynous mammal.Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by F. Trillmich     

Mutual tolerance or reproductive competition? Patterns of reproductive skew among male redfronted lemurs (<Emphasis Type="Italic">Eulemur fulvus rufus</Emphasis>)

The social organization of gregarious lemurs significantly deviates from predictions of the socioecological model, as they form small groups in which the number of males approximately equals the number of females. This study uses models of reproductive skew theory as a new approach to explain this unusual group composition, in particular the high number of males, in a representative of these lemurs, the redfronted lemur (Eulemur fulvus rufus). We tested two central predictions of “concession” models of reproductive skew theory, which assume that subordinates may be allowed limited reproduction by dominant group members as an incentive to remain in the group, thereby increasing the group’s overall productivity. Accordingly, relatives are predicted to receive less reproduction than non-relatives, and the overall amount of reproductive concessions given to subordinates is predicted to increase as the number of subordinates increases. In addition, we tested whether the number of females in a group, a variable not previously incorporated in reproductive skew theory, affected reproductive skew among males. Using microsatellite analyses of tissue DNA, we determined paternities of 49 offspring born into our study population in Kirindy forest (western Madagascar) since 1996 to determine patterns of male reproductive skew to test these predictions. Our analyses revealed remarkable reproductive skew, with 71% of all infants being sired by dominant males, but both predictions of reproductive skew models could not be supported. Instead, the number of females best predicted the apportionment of reproduction among the males in this species, suggesting that current reproductive skew models need to incorporate this factor to predict reproductive partitioning among male primates and perhaps other group-living mammals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Both Peter M. Kappeler and Markus Port contributed equally to this paper.     

Reproductive dominance and differential ovicide in the communally breeding burying beetle Nicrophorus tomentosus

Communal breeding can be characterized by the division of reproduction among cooperating individuals and, if the distribution of reproduction is inequitable, by the mechanisms for achieving skewed reproductive success. The burying beetle (Nicrophorus tomentosus) is a facultative communal breeder. Unrelated adults, especially females, provide extensive parental care to broods of mixed parentage. The frequency and degree of reproductive skew between two females were examined experimentally. On medium-size carcasses, the proportion of eggs attributed to each female was not significantly different from random in 42% of the broods, skewed in 42% and not shared in 16%. Although reproduction was usually skewed in favor of the larger female, the relative sizes of the two females did not predict the degree of skew. On large carcasses, the proportion of eggs attributed to each female was not different from random in 87% of the broods and weakly skewed in 13%. Several mechanisms for biasing reproductive success were investigated. Females increase the proportion of their offspring in the brood by committing differential ovicide. Secondly, burying and preparing a carcass cooperatively stimulates ovarian development of the larger female and slows it for the smaller female, reducing or delaying oviposition by the subordinate. Thirdly, larger females are more likely to be dominant and are more fecund than smaller females. Received: 20 July 1996 / Accepted after revision: 30 November 1996     

Complete reproductive skew within white-browed sparrow weaver groups despite outbreeding opportunities for subordinates of both sexes

The distribution of reproductive success within societies is a key determinant of the outcomes of social evolution. Attempts to explain social diversity, therefore, require that we quantify reproductive skews and identify the mechanisms that generate them. Here, we address this priority using life history and genotypic data from >600 individuals in 40 wild groups of the cooperatively breeding white-browed sparrow weaver, Plocepasser mahali. We show that groups comprise up to six males and seven females, but within-group reproduction is completely monopolised by a single dominant male and female, while extra-group males sire 12–18 % of offspring. Strong within-group kin structure could frequently explain these monopolies, as subordinates had typically delayed dispersal from their natal groups and so frequently (1) lacked within-group outbreeding partners, and/or (2) stood to gain little from contesting dominant reproduction, being almost as related to the dominant’s young as they would have been to their own. Kin structure alone cannot account entirely for these monopolies, however, as they remained complete following the immigration of unrelated males and females. That subordinate females remain reproductively quiescent despite also showing comparable body condition to dominants, overlapping them substantially in age, and showing no evidence of elevated stress hormone levels raises the possibility that they exercise reproductive restraint due instead to a threat of action by dominants and/or deficits in offspring fitness that might arise if subordinates bred. Our findings highlight the complexity of the mechanisms that generate reproductive disparities in animal societies and the challenge of identifying them when skews are complete.     

Components of lifetime reproductive success in communally and solitarily nursing house mice — a laboratory study

Under laboratory conditions, communal nursing among familiar and closely related female house mice (Mus domesticus) improved lifetime reproductive success compared to females rearing litters alone or females living with a previously unfamiliar, unrelated partner (reproductive success was measured within an experimental lifespan of 6 months, standardized as 120 days after mating at the age of about 2 months). An analysis of the contribution of three multiplicatively combined components to variation in reproductive success among breeding females revealed that, in all three social groups, survival of young until weaning contributed most to differences in lifetime reproduction (46–64% of the total variance). Females living with a sister had a significantly higher probability of reproducing successfully than females in the other groups, and also reared significantly more litters communally than females sharing nests with an unrelated partner. Weaning probabilities of young were highest in litters cared for by sisters and lowest in nests of unrelated females. Young were found dead either directly after birth (within the first 2 days of lactation) or after they had been cared for and nursed for at least 1 day. The loss of an entire litter typically occurred directly after birth. In monogamous females rearing litters alone the death of almost all young coincided with such early entire-litter mortality. In polygynous groups, however, offspring died at an older age and more litters suffered the loss of some young. Still, rearing young with a sister improved survival directly after birth and fewer litters were lost entirely in comparison with females in the other groups. In polygynous groups, pregnant females were observed to kill some of their partner's dependent young shortly before they gave birth themselves. As a consequence, individual young had reduced survival when they were firstborn in a communal nest (another litter was born within 16 days). Analyzed over a lifetime, communal care among familiar and closely related female house mice seems to be an adaptation to maximize the survival of offspring until weaning.     

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.     

Sex differences in fitness following a group take-over among Toque macaques: testing models of social evolution

Summary A group of toque macaques took-over the home range of one of its subordinate neighboring groups and fused with it to form a larger cohesive group. In the 7 years before the take-over, the dominant group had consistently won all contests at common feeding sites, yet the fitnesses of the females of these two groups did not differ significantly (Fig. 2A). After the take-over the females of the subjugated group occupied the lowest ranks in the combined dominance hierarchy of the merged groups (Fig. 1) and thereby lost the advantages of an own home range, such as priority of access to food. Consequently, in the merged group, survivorship and reproductive success among the subjugated females were significantly less than among the females of the dominant subgroup (Table 2, 4). The dominant matrilines grew numerically and replaced all of the subjugated females, and all but one of their offspring, within 8 years after the take-over (Fig. 2B). These data support the hypothesis that cooperation among female kin in defending resources against strange females is important in the evolution of female-bonded groups. Before the merger all 5 natal males of the subordinate group had transferred to the dominant group, where they occupied high and mid-level dominance ranks (Fig. 1). These males survived at a significantly greater rate than their subordinate female kin. Thus, the cost of group transfer seems to be greater for females than for males, and this may be one reason that females generally do not emigrate or that groups do not fuse. The data suggested three hypotheses. First, since large body size and other adaptations for fighting, giving males an advantage in male-male competition for mates, are also of advantage in resource competition with males and females, such male characters may also be favored by non-sexual selection, especially where male reproductive strategy involves group transfer. Second, female bonded groups evolved as female defensive coalitions against not only female but also male resource competitors, there having been a mutual influence in the coevolution of large-sized males and female gregariousness. Third, female defensive coalitions against large-sized aggressive males are also advantageous out-side the context of food competition, or, independent of foraging strategy.     

Female control of reproductive skew in cooperatively breeding brown jays (<Emphasis Type="Italic">Cyanocorax morio</Emphasis>)

Brown jays (Cyanocorax morio) are long-lived, social corvids that live in large, stable, territorial groups (mean = 10 individuals). In this study, I determined the distribution of reproductive success within groups using multi-locus DNA fingerprinting. Breeding females produced virtually all (99%) of the young within their own nests. Reproduction within groups was highly skewed towards a single primary female, although long term data indicate that secondary females (female breeders that were usually younger and subordinate to the primary female) were sometimes successful. The high reproductive skew observed for females was associated with primary female aggression. Successful reproduction by secondary females may have been due to parental facilitation or the inability of primary females to completely suppress secondary females. Multiple paternity occurred in 31–43% of broods and extra-group paternity occurred in a minimum of 22% of broods. Patterns of paternity also varied between years, since females often switched or included new genetic mates. Although male consorts of nesting females fathered relatively few offspring (20%), they still had a higher chance of fathering offspring than any other single group male. Reproduction was less skewed for males than females as a result of female mating patterns. Female reproductive patterns are consistent with some of the predictions and assumptions from optimal skew models, while male reproductive patterns are not. The factors affecting skew in species with complex social systems such as incomplete control by breeders over subordinate reproduction, female control of paternity, and resource inheritance have not been well incorporated into reproductive skew models.Communicated by: J. Dickinson     

Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable

In a genetic analysis of the mating system of cooperatively breeding Arabian babblers (Timalidae: Turdoides squamiceps), we identified which individuals in the population are breeding, and how reproductive success was distributed among group members with respect to their dominance rank, for both males and females. The population was characterized by an asymmetrical distribution of reproductive success; behaviorally dominant males produced 176 of 186 (95%) of the offspring in 44 social groups analyzed, and alpha females produced 185 of 186 (99.5%). We evaluated models of reproductive skew by examining genetic and demographic correlates of reproduction by␣subordinates. Subordinate (beta) males that sired young were more likely to be recent dispersers from their natal groups or members of newly formed groups than betas that did not reproduce. Breeding beta males had spent smaller proportions of their lives with the current alpha male and female as alphas than had beta males that did not sire young. One consequence of the linkage of dispersal with breeding in newly formed, nonnatal groups is that beta males that sired young had significantly lower genetic similarity to the alpha males in their groups (based on band-sharing coefficients using multilocus minisatellite DNA fingerprinting) than those that did not sire young. This pattern may occur generally in species in which group membership accrues both through nondispersal of young (forming groups of relatives) as well as through dispersal involving coalitions that sometimes include nonrelatives. Received: 22 July 1997 / Accepted after revision: 5 February 1998     

Male reproductive success in free-ranging feral horses

In the social organization of feral horses, adult males compete to monopolize groups or bands of females, sometimes called harems. Alternative male strategies are to remain alone or with other bachelors or, less commonly, to accept subordinate status within a harem. The hypothesis that dominant harem stallion status confers a reproductive advantage was tested in free-ranging feral horses. The presence of foals in harems headed by vasectomized (VSX) versus intact stallions was used to assess the ability of these stallions to control reproduction in their harems. Of harems headed by VSX stallions, 17 and 33% contained foals during years 2 and 3 post-treatment, respectively. In contrast, 86 and 80% of harems headed by non-VSX stallions contained foals in those years. Acquisition of pregnant mares appeared more likely than sneak copulations by bachelor stallions to account for foals in harems with a single stallion. However, most foals were born into harems that included a subordinate stallion, an occurrence that was undoubtedly exacerbated by the extended breeding season resulting from the sterility of the harem stallion. Thus, in comparing alternative reproductive tactics, bachelors appeared less successful than subordinate stallions within a harem. However, the highest reproductive success was achieved by the harem stallion, further demonstrating that alternative tactics are not equally profitable. Received: 13 July 1999 / Received in revised form: 24 July 1999 / Accepted: 24 July 1999