Fitness consequences of dispersal decisions for male mountain gorillas (Gorilla beringei beringei)

The fitness consequences of dispersal decisions are difficult to quantify, especially for long-lived species with complex social systems. To calculate those consequences for male mountain gorillas, from the perspective of both subordinate and dominant males, we used behavioral and demographic data obtained over 30 years from the Virunga Volcano population to develop an agent-based model that simulates the life history events, social structure, and population dynamics of the species. The model included variables for birth rates, mortality rates, dispersal patterns, and reproductive skew. The model predicted an average lifetime reproductive success (LRS) of 3.2 for philopatric males (followers) and 1.6 for emigrants. The benefits of philopatry were most sensitive to opportunities for social queuing and to female transfer preferences, but philopatry remained the best strategy over a wide range of group conditions and hypothetical simulations. The average LRS for dominant males was 4.5 when a subordinate stayed and 4.6 when the subordinate emigrated. The dispersal decision of the subordinate male had little impact on the fitness of the dominant male because it came relatively late in the dominant males reproductive life span, and it changed his group composition only incrementally. The fitness consequences for the dominant male were most sensitive to the degree of reproductive skew. Since subordinates suffer a fitness loss when they leave a group, they should accept whatever reproductive restraint is needed to avoid eviction, and the dominant male does not need to offer concessions for them to stay. The dominant male may offer reproductive concessions for other reasons, such as peace incentives or to confuse paternity, or he may not have complete control of reproduction within his group.Communicated by D. Watts     

Reproductive success increases with degree of kinship in cooperative coalitions of female red howler monkeys (Alouatta seniculus)

Evaluation of evolutionary mechanisms proposed to promote cooperative behavior depends on the relative influence of the behavior on the reproductive success of individuals, the reproductive success of the group in which they interact behaviorally, and the degree of gene correlation among cooperators. The genetic relationship within cooperative coalitions of female red howler monkeys was examined for three populations with different densities and growth rates. Patterns of gene correlation change within coalitions is documented using data from the mitochondrial and nuclear genomes, and long-term census monitoring. Differences in fecundity and infant survivorship within and between groups of unrelated (rˉ=0) and related (rˉ≥ 0.25) females are compared. Females that emigrate from their natal groups form coalitions with other migrant females. These coalitions attempt to establish a territory and, once successful in producing offspring, exclude other females from feeding resources. Females in these coalitions had different mtDNA haplotypes and a genetically estimated mean r of 0, supporting demographic data on emigration patterns indicating that these females rarely have the opportunity to form coalitions with kin. Patterns of recruitment and rate of matriline development within social groups supported behavioral data indicating that females actively attempt to promote their own matriline as breeders over that of other females, and that some matrilines are more successful at this than others. Mean r among females was significantly higher in coalitions established as social groups for several generations (rˉ=0.44). In these groups, females all shared the same mtDNA haplotype, and mtDNA haplotype divergence was significantly higher between than within groups. Females in coalitions with kin had significantly higher reproductive success than females in unrelated coalitions in all populations. This difference was not a function of coalition size, number of males, socionomic sex ratio, or primiparity, although anecdotal evidence suggests that allomothering may compensate for inept new mothers in related coalitions more often than in unrelated ones. Differences in territory quality could not be ruled out as a potential causal factor in the saturated populations, but were unlikely in the low-density, growing population. There were substantial differences among long-established coalitions in overall reproductive output in all three populations, and this was significantly correlated with the number of breeding females. Increase in coalition size was a function of both group age and the behavioral tolerance among females. Regardless of the underlying reasons for the patterns observed, reproductive success clearly increases with degree of gene correlation among females within cooperative coalitions, and coalitions that recruit more daughters produce more offspring. The nature of the cooperative relationship among group females directly influences both of these outcomes. This is associated with substantial genetic differentiation among social groups within populations, creating conditions in which genetic tendencies towards cooperative behavior can become tightly associated with group reproductive success. Received: 15 September 1999 / Revised: 27 April 2000 / Accepted: 27 May 2000     

A comparative study of the costs of alternative mayfly oviposition behaviors

Oviposition behavior of insects has associated fitness costs related to the probability that females survive to oviposit. During summer 2003, we observed the oviposition behavior and compared the mortality rates of females of 17 mayfly species in one western Colorado watershed. We dissected adult females collected on terrestrial sticky traps, in drift nets submerged in streams, and in stomachs of brook trout to determine whether the mayflies had oviposited before capture, drowning, or consumption. Females oviposited by either splashing on the water surface releasing all their eggs (splashers), dropping their eggs from the air (bombers), dipping their abdomens multiple times releasing a few eggs at a time (dippers), landing on rocks and ovipositing on the undersides (landers), or floating downstream while releasing their eggs (floaters). Almost 100% of lander and 50% of dipper females had not oviposited when captured on sticky traps, increasing their vulnerability to preoviposition mortality by aerial predators compared to mayflies with other behaviors. In contrast, most females had laid their eggs before drowning or being eaten by a fish (50–90%). However, groups with oviposition behaviors most exposed to the water surface (floaters, then splashers, dippers, and landers) were more vulnerable to drowning before completing oviposition. In addition, splashers and floaters were most vulnerable to predation by brook trout before ovipositing. These data suggest that fitness costs associated with preoviposition mortality may be considerable depending on mayfly oviposition behavior. Furthermore, previously demonstrated benefits of low predation rates on eggs of lander species may be offset in part by costs to female survival. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sex-specific differences in site fidelity and the cost of dispersal in yellow-headed blackbirds

Male migratory birds tend to be more faithful than females to previous breeding sites, suggesting sex differences in costs or benefits of dispersal. In Illinois, greater site fidelity by male yellow-headed blackbirds (Xanthocephalus xanthocephalus) was associated with reduced reproductive success the following year for dispersers relative to non-dispersers. Dispersing females suffered no reduction in reproductive success the following year. Males that attracted few social mates, and thus had low reproductive success, were more likely to disperse, whereas females dispersed in response to low-patch reproductive success, regardless of their individual performance. Males that dispersed appeared to be successful acquiring territories because none was observed as a floater. The rate of dispersal by males in this low-density population was greater than in more dense populations where dispersing males may be less successful at acquiring territories. Despite success at obtaining territories, males that dispersed acquired territories on the periphery of wetlands where fewer females nested, resulting in lower reproductive success. In the second year after dispersing, however, males moved onto more central territories where they acquired larger harems. Thus, dispersal by males may be a long-term strategy requiring at least 2 years for benefits to be realized. Long-term success was enhanced because dispersing males moved to wetlands on which reproductive success was higher than on the wetlands they left. In addition to demonstrating that both individual and patch reproductive success affect dispersal decisions, these data indicate that when evaluating costs and benefits of dispersal, researchers should use a time frame beyond 1 year.     

Asymmetric costs favor female desertion in the facultatively polyandrous northern flicker (Colaptes auratus): a removal experiment

Previous studies of biparental care in birds have focused on the males contribution and experimental removal of males. Woodpeckers, with a high level of paternal care including nocturnal incubation and brooding by males, offer a meaningful system in which to examine the importance of care by females. I studied the reproductive performance of 17 widowed male and 8 widowed female northern flickers (Colaptes auratus) relative to biparental (control) pairs. Of these single parents all widowed shortly after hatching, only one female abandoned its nest. Single parents boosted their provisioning rates to achieve 83% the provisioning rate of control broods, but reared significantly fewer young and young of poorer quality. However, single males, with 85% of the reproductive success of controls, were more successful than females with 43% the success of controls. Among widowed birds, a not significant lower survival was observed, but the chance of re-pairing with the same partner in a subsequent year was only 16% in the natural population, so long-term costs of desertion may be small. Although females seem to have the incentive and ability to desert, a lack of available males may constrain opportunities. This study demonstrates that when parental care roles are reversed in altricial birds, asymmetric benefits can favor female and not male desertion.     

Female dispersal and reproductive success in wild western lowland gorillas (<Emphasis Type="Italic">Gorilla gorilla gorilla</Emphasis>)

This paper presents data on the dispersal patterns and reproductive success of western lowland gorilla females from a long-term study at Mbeli Bai in the Nouabalé-Ndoki National Park, Republic of Congo. We find that female natal and secondary transfer is common. Female immigration rates are negatively related to group size, and emigration rates are positively related to group size, with the net result that larger groups are losing females and smaller groups are gaining females. Furthermore, females transferring between known groups show a preference for significantly smaller groups. However, there is no effect of group size on female reproductive success. Male protection and male quality are considered important in determining female transfer decisions. The case for infanticide is argued and females exhibit strategies that appear to minimise the probability of infanticide following the death of the silverback. Exclusively single-male groups and group formation through female acquisition by solitary males may bias female transfer to lone silverbacks and small groups. The effects of group size on female dispersal and reproductive success are not wholly consistent with an argument for increased foraging costs, and group size effects are more parsimoniously explained by demographic factors. Male protection from intra-group aggression is the most likely factor underlying grouping patterns across gorilla taxa, but differences in population structure and male reproductive strategies may account for inter-specific variation. We stress the need for intra-specific comparisons and more complete data sets on western lowland gorilla feeding behaviour.Communicated by S. Boinski     

The significance of male-male aggression for yearling dispersal in roe deer (Capreolus capreolus)

The significance of adult buck aggression as a proximate factor driving yearling male dispersal in roe deer, Capreolus Capreolus, was studied in Sweden during the dispersal season (April–June) in 1988–1991. In roe there is a well established positive correlation between yearling antler size and dispersal tendency. I tested the prediction that yearlings with large antlers will experience more agonism from adult bucks than those with smaller antlers. Additionally, I studied the relationship between antler size and level of sexual maturity, by recording date of velvet shedding, and sampling testes weights from culled yearlings during the breeding season. The amount of agonism experienced by yearlings was positively correlated with the size of their antlers during April and May, whereas in June no such relationship was found. Aggression towards all yearlings tended to increase as the season progressed. A positive correlation between antler size and testes weight was evident, and the larger the antlers of yearlings the earlier the velvet shedding occured. These results indicate: firstly, that adult buck agonism is an important proximate factor underlying yearling male dispersal; and secondly, that agonism is mainly directed towards the sexually most mature yearlings. This, in turn, suggests mate competition to be the ultimate cause of male-male agonism, and thus male dispersal in this species.     

Bioaccumulation of cadmium,lead and zinc in liver and kidney of red fox (Vulpes vulpes) from NW Spain: influence of gender and age

Cd, Pb, and Zn were quantified in liver and kidney of red foxes (Vulpes vulpes) which were hunted during the 2003–2011 hunting seasons in Galicia (NW Spain). The effects of age and gender were evaluated to determine whether these variables should be included in future biomonitoring studies. The concentrations of hepatic and renal Cd (average 0.6 and 1.3 µg/g) and Pb (0.8 and 0.06 µg/g, respectively) were similar to background levels, with no known toxicological relevance. Similarly, the average levels of Zn in liver and kidney (77 and 17 µg/g) were in the range of physiological levels for canids. Although no significant gender-dependent variations were observed, the effect of aging was evident: the levels of hepatic Pb and both hepatic and renal Cd were higher in adults than in juveniles. Age should be included as a parameter during future biomonitoring programs focusing on trace metal bioaccumulation in red foxes.     

Benefits and costs of earwig (Forficula auricularia) family life

The evolution of parental care and family group formation critically depends on offspring survival benefits and parental fecundity costs of care under given ecological conditions. Investigations of the functional significance of care in insect species that exhibit facultative parental care have been relatively rare but may be of particular interest for better understanding of benefit and cost schedules at an early evolutionary stage. In this study, aspects of benefits and costs of care were addressed in the sub-social European earwig (Forficula auricularia; Dermaptera: Forficulidae) by manipulating the presence of tending mothers and brood size in a fully crossed experimental design. Larvae growing in broods tended by their mother or of reduced size showed a higher survival probability than larvae growing in untended or large broods, as predicted if maternal care is beneficial and shaped by a trade-off between number and quality of offspring. Analysis of patterns of food consumption and developmental time further suggested that the benefit of maternal attendance is mediated by the maternal provisioning of food, while the quality–quantity trade-off seemed to be driven by sibling rivalry. Further, tending mothers delayed the production of a second clutch, indicating a potential cost of care in terms of lifetime fecundity. This study experimentally shows benefits and potential costs of maternal care and family group formation in the European earwig. More detailed behavioural experiments will be required to fully understand how behavioural interactions among family members mediate these reproductive outcomes.     

Intermorph breeding and the potential for reproductive isolation in polymorphic mole salamanders (Ambystoma talpoideum)

The study of reproductive isolation as a prerequisite to sympatric speciation has been limited by the focus on species that have already experienced such isolation. However, a complete understanding of how such processes evolve depends on observing taxa before they complete the speciation process. We studied the potential for sexual isolation in the polyphenic mole salamander, Ambystoma talpoideum, using a series of laboratory and field experiments. This species consists of aquatic paedomorphic adults and terrestrial metamorphic adults which are exhibited by both sexes and which mate in the same aquatic habitat. Previous field studies on this species suggested that intermorph breeding would be less common during the winter months, because paedomorphic adults begin breeding in early autumn and thus may have less energy available for reproduction in the winter. Laboratory experiments conducted during the winter showed that the mating behavior of paedomorphic males occurred at a much lower frequency than that of metamorphic males. In contrast, field experiments that best mimicked natural conditions revealed symmetric intermorph breeding and included multiple paternity shared among males of each morph. This and other studies suggest that there is little evidence of sexual isolation among morphs based on behavioral interactions alone. However, the potential for partial isolation still occurs because of temporal and spatial differences in the frequencies of each morph in nature. Our results suggest that further studies on this system, and other similar polyphenisms, may provide valuable insight into the mechanisms that underlie the evolution of reproductive isolation.     

Indications of fitness differences among vocal clans of sperm whales

Cultural variation can affect the genetic evolution of a species if there are consistent cultural differences that contribute to fitness variation between groups of individuals. In this study, measures of the reproductive success of groups of sperm whales from different cultural clans are used as proxies for fitness. We measure reproductive success using population length distributions from acoustic and photographic measurements and visual observations of the presence of calves. The results obtained are generally consistent between methods; there are large and significant differences between the clans in the measures of reproductive success. The results from this study strengthen the case for cultural hitchhiking in sperm whales by indicating that differences in culture between clans correlate with differences in fitness.     

Offspring genetic diversity increases fitness of female Atlantic salmon (Salmo salar)

Inbreeding has negative effects on individual and population performances. Therefore, enhancement of offspring genetic diversity is believed to play a major role in shaping mating systems. However, no study has clearly separated the direct effect of having multiple partners from the indirect effect of having more outbred offspring on the resulting reproductive success of individuals in the wild. In this study, we report significant associations between both multiple mating and within-individual genetic diversity of offspring, and an increased reproductive success of wild female Atlantic salmon, Salmo salar. Specifically, we found that females with a higher number of mates also have more outbred offspring (within-individuals), and that both of these characteristics increased their reproductive success expressed in terms of offspring surviving when combining all freshwater juvenile stages. Our findings also indicate that determinants of fitness are different among sexes as within-individual offspring genetic diversity was not a strong predictor of male reproductive success, while the number of mates was important. We also show that females mated with more outbred males than on average, which potentially increased their chances of producing outbred offspring. These results therefore suggest that there could be more important indirect genetic benefits of multiple mating for females than for males.Communicated by M. Abrahams     

Differences in reproductive success between laboratory and wild-derived golden hamsters (Mesocricetus auratus) as a consequence of inbreeding

All laboratory golden hamsters (Mesocricetus auratus) originated from a sibling pairing back in 1930. Due to this extreme founder event, domestic golden hamsters are presumed to be one of the most bottlenecked animal populations. Nevertheless, domestic hamsters show no obvious signs of inbreeding depression in commonly used breeding stocks. To explore the existence of potentially masked inbreeding effects, we compared the reproductive success of laboratory (lab) and wild-derived (wild) golden hamsters. We allowed oestrus females to mate consecutively with lab and wild males. The resulting offspring was genotyped using microsatellites to assess paternity. Finally, we compared male reproductive success to genetic variability, sexual behaviour and different sperm characteristics. Both hamster strains exhibited the expected large difference in genetic diversity (H _wild =0.712±0.062 vs H _lab =0.007±0.007. The reproductive success of wild males dramatically exceeded that of lab males (87% of pups were sired by wild males). Sexual behaviour of wild and lab males only varied in the number of long intromissions (intromissions without ejaculation at the end of the mating). No significant differences were observed in relation to mounting, ejaculation and intromission. There were also no apparent differences in sperm motility, velocity and density or testis histology between wild and lab hamsters. We conclude that the reduced reproductive success of lab males represents a hidden inbreeding effect, although its precise physiological cause remains unclear. These results provide first evidence for a major fitness disadvantage in captive golden hamsters.     

A dynamic model of human dispersal in a land-based economy

This paper presents a dynamic state variable model that examines human dispersal in a land-based economy. In humans, like other animals, many individuals disperse during their lifetimes, with extensive variability in the timing and likelihood of dispersal by individuals of different states. The model places human and non-human dispersal in a common framework, specifically by utilizing a cost-benefit approach. As with other animals, individual access to resources likely plays an important role in shaping human dispersal, and the model explores the mechanisms underlying this relationship. Over a series of time steps, individuals must decide whether to stay in their natal area or disperse, given their states (wealth, marital, and inheritance status) and the environmental constraints on dispersal. Costs of dispersal include time and an initial drop in wealth, while the benefit is the potential of the dispersal area to increase wealth. The model determines whether dispersing or staying results in the highest fitness, where fitness is a function of individual wealth and years married. The model shows that dispersal is favorable under a wide range of environmental conditions, but that it also varies predictably according to individual states. Men of middle wealth values disperse under a wider variety of circumstances than poorer or wealthier men. Wealthy men, who are particularly sensitive to the time cost and mortality risk associated with dispersing, appear to adopt a more conservative dispersal strategy than poorer men, who are more sensitive to the wealth cost and wealth growth rate associated with dispersing. Dispersal behavior is also contingent on its effects on an individual's probability of marrying or inheriting wealth. Finally, the model fosters an examination of how the interaction of life events, and their directions of causation, can be studied. Received: 6 May 1998 / Received in revised form: 3 February 1999 / Accepted: 7 February 1999     

Cooperative breeding increases reproductive success in the social spider <Emphasis Type="Italic">Stegodyphus dumicola</Emphasis> (Araneae,Eresidae)

Sociality in some birds, mammals, and social insects was suggested to have evolved through the lengthening and extension of parental care behaviors to nondirect descendents. In these systems, group members care for young cooperatively and, thus, increase the reproductive success of the breeders and fitness of the young. Parental care behaviors, such as regurgitation feeding and matriphagy (consumption of the mother), occur in several subsocial and social spiders. However, it is not known whether females in a colony cooperate in caring for the young of other females and whether such cooperative care improves reproductive success. To answer this question, we created experimental colonies of the social spider Stegodyphus dumicola (Araneae, Eresidae), allowing only one female in a group to produce young, simulating reproductive skew occurring in nests in nature. In this paper, we show for the first time that females of S. dumicola cooperate in providing regurgitated food for young of other females and are even eaten by those young. Young raised by a group of females were larger and had greater survival than young raised only by their mother. Thus, fitness benefits from raising broods cooperatively may have favored the evolution of sociality in spiders.     

Genetic reconstruction of breeding patterns in gundis (Rodentia: Ctenodactylidae)

Current mating system theory predicts that the number of females breeding in a group will depend on the number of females in the group and the accessibility of unrelated males, whereas the number of males breeding in a group will depend on the ability of males to control access to reproductive females. By combining information on group composition with genetic data, I determined whether breeding patterns in a rock-dwelling rodent, Ctenodactylus gundi, were concordant with these expectations. C. gundi breeding patterns varied from facultative monogamy to uni-male polygyny to multi-male polygyny. The number of reproductive individuals of each sex in a group increased with group size. Whereas communal breeding among related females tended to increase female reproductive success, males breeding in the same group were unrelated to other group members and seemed to compete over access to matings. While some males were assigned offspring from neighboring social groups, most group-living males successfully monopolized the reproduction of females within their group. There was no evidence that females had multiply sired litters, although some bred with alternative males in separate breeding attempts. Although numerous individuals were not assigned as parents or offspring, genetic information enabled me to determine that most unassigned individuals were philopatric group members, whereas only a few were unrelated immigrants into their current social group. Together, these results provide evidence that C. gundi social groups represent fairly distinct reproductive units whose breeding patterns are dependent on group size and composition in accordance with theoretical predictions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mating system of a Neotropical roost-making bat: the white-throated, round-eared bat, Lophostoma silvicolum (Chiroptera: Phyllostomidae)

The vast majority of bats strongly depend on, but do not make, shelters or roosts. We investigated Lophostoma silvicolum, which roosts in active termite nests excavated by the bats themselves, to study the relationship between roost choice and mating systems. Due to the hardness of the termite nests, roost-making is probably costly in terms of time and energy for these bats. Video-observations and capture data showed that single males excavate nests. Only males in good physical condition attracted females to the resulting roosts. Almost all groups captured from excavated nests were single male-multifemale associations, suggesting a harem structure. Paternity assignments based on ten polymorphic microsatellites, revealed a high reproductive success of 46% by nest-holding males. We suggest that the mating system of L. silvicolum is based on a resource-defense polygyny. The temperatures in the excavated nests are warm and stable, and might provide a suitable shelter for reproductive females. Reproductive success achieved by harem males appears to justify the time and effort required to excavate the nests. Reproductive success may thus have selected on an external male phenotype, the excavated nests, and have contributed to the evolution of an otherwise rare behavior in bats.Communicated by G. Wilkinson     

Mating patterns and reproductive success in the bushy-tailed woodrat (Neotoma cinerea), as revealed by DNA fingerprinting

The mating patterns and reproductive success of the bushy-tailed woodrat (Neotoma cinerea) were investigated over a 3-year period (1992–1994) using DNA fingerprinting. Paternity was determined by genetic analysis of 58 juveniles of known maternity from 35 litters. Analysis of DNA fingerprints revealed that all offspring within a litter were fathered by a single male; the statistical probability of detecting multiple males mating with a female was high, indicating that multiple paternity would have been detected had it occurred. However, individual males did not father more than one litter from a given female either within or between years. At least 75% of females and 57% of males successfully produced offspring each year. The finding that all littermates are first-order relatives may contribute to the high level of female cooperation in this species. Received: 28 May 1997 / Accepted after revision: 22 March 1998     

Determinants of male mating success in the red bishop (Euplectes orix)

We studied sexual selection in the red bishop, Euplectes orix, a colonial, polygynous weaverbird widely distributed over sub-Saharan Africa. Male reproductive success measured in terms of the number of nests accepted by females and the number of eggs and nestlings in all the nests on a male's territory varied considerably. The standardized variance (variance/mean²) in male reproductive success ranged from 0.505 to 1.737 in different years, indicating a high potential for sexual selection in this species. An analysis of genetic parentage for 432 nestlings by non-radioactive, multilocus DNA fingerprinting confirmed that male reproductive success (number of young sired on the territory) in this species can be reliably estimated by the measures introduced above. In all 4 study years there was a strong positive correlation between male mating success and the total number of nests that males built in their territories. The number of nests built can be partitioned into the number of weeks a male held a territory and his nest-building performance. Both factors exert a significant positive effect on male mating success and in combination explained between 53.3 and 86.3% of the variation in male reproductive success. Male morphological characters were found to be of no importance. Males that established a territory in the following season built more nests and held their territories for longer than males that did not establish a territory in the following season, suggesting that these measures might be indicators of male condition and quality. Male nest-building performance (number of nests built per week) seems to be unrelated to male condition or quality. Received: 8 January 1999 / Received in revised form: 7 June 1999 / Accepted: 13 June 1999