Sex-biased natal dispersal in Hokkaido brown bears revealed through mitochondrial DNA analysis

Understanding natal dispersal patterns is fundamental in the ecology and conservation biology of large wild carnivores. In this study, we used two approaches to determine genetic variation and dispersal patterns of brown bears in the Shiretoko Peninsula, eastern Hokkaido, Japan. The first approach was a large-scale genetic analysis. We analyzed haplotypes from the mitochondrial DNA (mtDNA) control region of 760 individual samples collected throughout the peninsula during 1998–2016. We detected seven haplotypes, including two that were confirmed for the first time. In females, the distribution of haplotypes was geographically structured, whereas haplotypes in males were distributed widely throughout the peninsula. Only some males in the lower peninsula had haplotypes that were not detected within the peninsula. The second approach was a local-scale genetic analysis, including intensive focal sampling in the Rusha area, a special wildlife protection area on the peninsula. Proportions of mtDNA haplotypes in adult bears were investigated and compared between the sexes. Although more than half of the females had the same haplotype, males had more diverse haplotypes, suggesting that they came to the Rusha area from other regions. Thus, our study revealed that mtDNA haplotype distribution has been maintained by female philopatry, and that bears exhibit male-biased dispersal. Furthermore, the lower peninsula appears to act as a contact zone between the peninsula and mainland Hokkaido, which is important for maintaining genetic diversity.     

Potential for female kin associations in wild western gorillas despite female dispersal

Female philopatry and male dispersal are the norm for most mammals, and females that remain in their natal region often derive foraging or social benefits from proximity to female kin. However, other factors, such as constraints on group size or a shortage of potential mates, may promote female dispersal even when female kin associations would be beneficial. In these cases, female kin associations might develop, not through female philopatry, but through female emigration to the same group. To date, little attention has been focused on the potential for kin-biased behaviour between females in female-dispersing species. Here we investigate the genetic relationships among adults in eight wild groups of unhabituated western gorillas (Gorilla gorilla) at the Mondika Research Center using microsatellite genotyping of DNA collected from hair and faeces. We found that almost half (40%) of adult females had an adult female relative in the same group and average within-group relatedness among females was significantly higher than that expected under a model of random dispersal. This provides the first genetic evidence that females can maintain social associations with female relatives in spite of routine natal and secondary dispersal. In addition, we show that females appear to avoid related silverback males when making dispersal decisions, suggesting that a strategy of non-random female dispersal may also function to avoid inbreeding.     

Sex-biased dispersal in western lowland gorillas (Gorilla gorilla gorilla)

We explored two hypotheses related to potential differences between sexes in dispersal behaviour in western lowland gorillas (Gorilla gorilla gorilla). Direct observations suggest that immature females have more opportunities to move between breeding groups than immature males. The distribution of kin dyadic relationships within and between groups does not, however, support this hypothesis. At larger geographical scales, dispersal is likely to be easier for males than females because of the solitary phase most blackbacks experience before founding their own breeding group. However, previous work indicates that males settle preferentially close to male kin. By specifically tracing female and male lineages with mitochondrial and Y-chromosomal genetic markers, we found that male gorillas in the 6000 km2 area we surveyed form a single population whereas females are restricted to the individual sites we sampled and do not freely move around this area. These differences are more correctly described as differences in dispersal distances, rather than differences in dispersal rates between sexes (both sexes emigrate from their natal group in this species). Differences in resource competition and dispersal costs between female and male gorillas are compatible with the observed pattern, but more work is needed to understand if these ultimate causes are responsible for sex-biased dispersal distances in western lowland gorillas.     

Great bustard population structure in central Spain: concordant results from genetic analysis and dispersal study.

We found significant sex differences in the mtDNA genetic structure and dispersal patterns of great bustards in a population of 11 breeding groups, "leks", in central Spain. The analysis of genetic distances showed that the female population was divided into three groups of leks separated by ca. 50 km, whereas male haplotypes were randomly distributed among leks. Genetic distances among pairs of leks were positively correlated with geographical distances in females but not in males. While female haplotype distributions were homogeneous among leks at close distances, differences in male genetic structure were highly variable even between two close leks. These results from genetic analyses were concordant with those from a radiotracking study on natal dispersal. Natal dispersal distances were higher in males than in females. Also, the frequency of movement of a female between two leks was positively correlated with their genetic affinity and geographical proximity. In males, the frequency of movement was correlated with geographical proximity but not with genetic affinity. Males dispersed among genetically unrelated leks, contributing to keep nuclear genetic diversity in the population, whereas females tended to be philopatric. These results suggest that isolation-by-distance influences the distribution of maternal lineages at a regional level.     

Highly asymmetric fine-scale genetic structure between sexes of African striped mice and indication for condition dependent alternative male dispersal tactics

Sex-biased dispersal is observed in many taxa, but few studies have compared sex-biased dispersal among and within populations. We addressed the magnitude and habitat dependency of sex-biased dispersal in social African striped mice by separating group-related from population-related genetic variance to understand the contribution of each sex to deme structure. As dispersal over unoccupied habitat is likely to be more costly than dispersal within a population, we predicted that individuals leaving the natal population have a lower body condition, being inferior to heavier territorial individuals. Fine-scale genetic structure was detected in both sexes. Female relatedness decreased continuously from R = 0.21 at 25 m to zero at 500 m. Maximum male relatedness R = 0.05 was constant at distances between 25 and 75 m, becoming zero at 100 m. Genetic variance (F(ST) ) among seven locations was significantly higher in females than in males, while inbreeding estimates (F(IS) ) were significantly higher in males than in females. Assignment tests estimated significantly more migrants among males, while Bayesian clustering estimated only a single genetic unit cluster for males among the seven locations. The mean body mass of migrant males (44 g) was significantly lower than for males that remained resident and thus dispersed within their sub-population (48 g). Combined, the results showed habitat-independent male-biased dispersal and high female philopatry, and suggested that body condition was more important than kinship in male dispersal decisions. We suggest that locally inferior males are important for gene flow between sub-populations. Thus, males might follow alternative dispersal tactics.     

Sex-biased dispersal in eastern chipmunks, Tamias striatus

I investigated the effect of male mate competition and inbreeding avoidance on natal dispersal of chipmunks by longitudinally monitoring known individuals from 1986 to 1990. Natal males exhibited greater absolute and effective dispersal distances but dispersed at the same proportion as natal females. Recruitment of juvenile males was negatively affected by density of resident males, but there was no evidence of local mate competition among male kin. Analysis of the spatial distribution of neighbors showed that natal males settled farther from their mothers than did their female siblings and farther than unrelated juvenile males. In addition, mothers apparently tolerated daughters as close neighbors and occasionally shared den sites with grandprogeny. Sexually mature males were never neighbors of their mothers and were never observed at maternal mating bouts. Males may disperse to improve reproductive opportunities by avoiding competition with resident males, and by increasing access to unrelated females. Maternal tolerance of daughters but not sons may result in the close affiliation between mothers and daughters, and indirectly contribute to dispersal of natal males. Hence male-biased dispersal could be a consequence of mate competition and maternal avoidance of incestuous matings. This revised version was published online in July 2006 with corrections to the Cover Date.     

Female-biased natal dispersal in the Siberian flying squirrel

Natal dispersal is usually sex biased in birds and mammals.Female-biased natal dispersal is the prevailing pattern in birdsbut is rare among mammals. Hypotheses explaining sex bias indispersal include the mate-defense mating hypothesis, whichpredicts male-biased dispersal, the resource-defense hypothesispredicting female-biased dispersal, and the competition hypothesis,which predicts that if dispersal is caused by competition forresources between sexes, then the subdominant sex will disperse.We studied natal dispersal of Siberian flying squirrels Pteromysvolans using radio telemetry in Southern Finland in 1996–2004.Of 86 juveniles that survived over the dispersal period, almostall young females dispersed from the natal site, whereas almost40% of males were philopatric. Dispersal was farther for femalesthan males. Females began dispersal on average 2 weeks earlierthan males and were lighter in mass at the onset of dispersalthan later dispersing males. No mate- or resource-defense matingsystem could be found among males, but females seemed to defendnest and apparently food resources, in contrast to the expectationof dispersal bias in resource-defense systems. Competition forresources between sexes does not explain female bias either:in the flying squirrel, the female seems to be the dominantsex. We propose that young females are subordinate to theirmothers and have to disperse to find a vacant, suitable sitefor reproduction.     

Dispersal Patterns Among Olive Colobus in Taï National Park

In Primates, females are more likely to be philopatric than males. However, in some species like Procolobus verus, females or individuals of both sexes disperse. In Taï National Park, Ivory Coast, olive colobus groups are small, with one or two adult males and 6 females. Dispersal is common for juveniles and adults of both sexes. Adult male dispersal is less common than adult female dispersal. Adult females immigrated especially into small, one-male groups indicating that food competition played a role. Furthermore, unknown sexually receptive females visited resident groups and mated with the resident males for a few days before disappearing again. Adult males dispersed when this improved their mating opportunities. All juveniles left their natal groups. The dispersal of juveniles may be a strategy to prevent inbreeding with their parents. Dispersal by juvenile males furthermore seemed to be the result of mate competition. The high dispersal rates, visits by receptive females, and dispersal of all individuals in the population suggest that moving between groups is a strategy that can be used ad hoc in several situations more easily in the olive colobus than in most other primates. The predation risks related to moving between groups were reduced by dispersing in conspecific or allospecific groups and by dispersing to neighboring groups.     

Determinants of female dispersal in Thomas langurs

Female dispersal occurs in a number of primate species. It may be related to: avoidance of inbreeding, reduction in food competition, reduction of predation risk, or avoidance of infanticide in combination with mate choice. Female dispersal was studied for a 5-year period in a wild population of Thomas langurs (Presbytis thomasi) that lived in one-male multi-female groups. Juvenile and adult individuals of both sexes were seen to disperse. Females appeared to transfer unhindered between groups, mostly from a larger group to a recently formed smaller one. They transferred without their infants and when not pregnant, and seemed to transfer preferentially during periods when extra-group males were harassing their group. During these inter-group encounters extra-group males seemed to try to commit infanticide. Thus, the timing of female transfer was probably closely linked to infanticide avoidance. Moreover, females seemed to transfer when the resident male of their group was no longer a good protector. The observations in the present study suggest that females transferred to reduce the risk of infanticide. Female dispersal may have another ultimate advantage as well, namely inbreeding avoidance. Due to the dispersal of both females and males the social organization of Thomas langurs was rather fluid. New groups were formed when females joined a male; male takeovers were not observed. Bisexual groups had only a limited life span, because all adult females of a bisexual group could emigrate. This pattern of unhindered female dispersal affects male reproductive strategies, and in particular it might lead to infanticidal behavior during inter-group encounters. Am. J. Primatol. 42:179–198, 1997. © 1997 Wiley-Liss, Inc.     

Lack of pubertal influences on female dispersal in muriqui monkeys, Brachyteles arachnoides

The hormonal mediation of dispersal in female mammals is poorly understood, in part because of the difficulties of detecting the onset of ovarian cycling and puberty in dispersing individuals. We used noninvasive methods of faecal steroid assays to determine the timing of dispersal relative to puberty and ovarian cycling in wild female muriqui monkeys, a species in which males are philopatric and nearly all females transfer from their natal groups. Natal females had a mean+/-SE age of 73.4+/-7.2 months (N=18) at the time of their transfers. Intergroup transfers occurred when one or more sexually active adult females were present, but did not show any seasonal patterns. Faecal progesterone and oestradiol profiles from nine natal females prior to transfer and four non-natal females that transferred into our study group demonstrate unequivocally that dispersal occurs prior to puberty in this species. All females showed baseline oestradiol levels and low progesterone levels compared with cycling adult females. Immigrants were first observed to copulate at 11.2+/-2.2 months of age (N=4), prior to the onset of normal ovarian cycles, and gave birth to their first offspring at 33.8+/-7.3 months (N=4) after transferring. Mean cortisol levels did not differ between natal emigrants or recent immigrants, and were within the range of those of adult males during the nonbreeding season in 10 of the 11 prepubertal females sampled. These results indicate that female dispersal is not triggered by activational hormones associated with puberty or escape from reproductive suppression in this species. Copyright 2000 The Association for the Study of Animal Behaviour.     

Genetic consequences of sex-biased dispersal in a solitary carnivore: Yellowstone cougars

Male-biased dispersal is a common trait in mammals, including carnivores, but its genetic consequences at the population level have been rarely considered for solitary species. We used long-term genetic data from cougars (Puma concolor) in and around Yellowstone National Park to test predictions based on differences in dispersal behaviour among males and females. Consistent with frequent long-distance dispersal of males, we found support for our prediction of less than expected allele sharing in pair-wise comparisons. In contrast, female residents present at the same time and females separated by few generations failed to share more alleles than expected, contrary to our predictions based on limited female dispersal. However, we find that genetic contributions of females with higher reproductive success were still noticeable in subsequent generations, consistent with female offspring showing fidelity to their natal area. These results highlight the importance of male dispersal for inbreeding avoidance, but do not indicate that short-distance dispersal or philopatry in female cougars results in spatial clustering of related individuals.     

Unraveling the dispersal patterns and the social drivers of natal emigration of a cooperative breeding mammal,the golden lion tamarin

The study of the social drivers of animal dispersal is key to understanding the evolution of social systems. Among the social drivers of natal emigration, the conspecific attraction, aggressive eviction, and reduced social integration hypotheses predict that sexually mature individuals who receive more aggressive behavior and are engaged in less affiliative interactions are more likely to disperse. Few reports have explored these proximate factors affecting emigration in cooperatively breeding species, particularly of Neotropical primates. In this study, we investigated the dispersal patterns and tested the social drivers of natal emigration in the golden lion tamarin (Leontopithecus rosalia) — an endangered species inhabiting Atlantic rainforests fragments in Brazil. We used behavioral and demographic data collected during 7 years from 68 groups of tamarins inhabiting 20 forest fragments. Our analyses from the 160 dispersing individuals showed that dispersal success is higher for males and for those engaged in parallel dispersal, but that males and females use different strategies to enhance their dispersal success, males immigrate into established groups while females form new groups. We did not find high levels of agonistic behavior among group members before natal emigration. Instead we found that conspecific attraction drives natal emigration in both sexes, while additionally the low level of affiliative interactions within the natal group triggers male emigration. We discuss natal emigration in the broader perspective of the cooperative breeding system and the implications of these findings for the conservation of the species.     

Female-biased natal and breeding dispersal in an alpine lizard, Niveoscincus microlepidotus

We measured two aspects of dispersal in the alpine Australian scincid lizard, Niveoscincus micolepidotus : (1) natal dispersal, i.e. shift in home range over the lizard's first year of life, and (2) breeding dispersal, i.e. shifts of home ranges between breeding attempts as adults. On average, displacements were surprisingly small. Female neonates dispersed about twice as far as did males in the same cohort (means of 12 m vs. 6 m). A female's natal dispersal distance was not correlated with her body size or our estimate of physiological performance (sprint speed). However, larger, faster-running male neonates dispersed further than did smaller, slower males. As was the case for neonates, adult females moved significantly further between breeding seasons than did adult males (14.2 m vs. 9.6 m). Because of a female's long gestation period (more than 1 year), two groups of females occur simultaneously in the population, non-ovulated (i.e. with yolking folicles) and pregnant females (i.e. approaching parturition). Females that were not yet ovulated showed a markedly stronger dispersal in response to high reproductive effort (i.e. clutch size in relation to body condition) than did pregnant females. In adult males, body size was negatively correlated with dispersal distance, suggesting that although males have overlapping territories, they exhibit an increasing level of site tenacity with age and/or size. Thus, selection for the relatively more pronounced site tenacity in adult males may have resulted in the more marked philopatric behaviour compared to females also as neonates.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 277–283.     

Dispersal influences genetic and acoustic spatial structure for both males and females in a tropical songbird

Animals exhibit diverse dispersal strategies, including sex‐biased dispersal, a phenomenon common in vertebrates. Dispersal influences the genetic structure of populations as well as geographic variation in phenotypic traits. Patterns of spatial genetic structure and geographic variation may vary between the sexes whenever males and females exhibit different dispersal behaviors. Here, we examine dispersal, spatial genetic structure, and spatial acoustic structure in Rufous‐and‐white Wrens, a year‐round resident tropical bird. Both sexes sing in this species, allowing us to compare acoustic variation between males and females and examine the relationship between dispersal and song sharing for both sexes. Using a long‐term dataset collected over an 11‐year period, we used banding data and molecular genetic analyses to quantify natal and breeding dispersal distance in Rufous‐and‐white Wrens. We quantified song sharing and examined whether sharing varied with dispersal distance, for both males and females. Observational data and molecular genetic analyses indicate that dispersal is female‐biased. Females dispersed farther from natal territories than males, and more often between breeding territories than males. Furthermore, females showed no significant spatial genetic structure, consistent with expectations, whereas males showed significant spatial genetic structure. Overall, natal dispersal appears to have more influence than breeding dispersal on spatial genetic structure and spatial acoustic structure, given that the majority of breeding dispersal events resulted in individuals moving only short distances. Song sharing between pairs of same‐sex animals decreases with the distance between their territories for both males and females, although males exhibited significantly greater song sharing than females. Lastly, we measured the relationship between natal dispersal distance and song sharing. We found that sons shared fewer songs with their fathers the farther they dispersed from their natal territories, but that song sharing between daughters and mothers was not significantly correlated with natal dispersal distance. Our results reveal cultural differences between the sexes, suggesting a relationship between culture and sex‐biased dispersal.     

Socioecological factors of male chimpanzee migration at Bossou, Guinea

For more than 21 years a small semi-isolated group of wild chimpanzees have been studied at Bossou, Guinea, west Africa. All individuals have been identified since the beginning of the study. Remaining rates of infants (0–3 yr) and juveniles (4–7 yr) in the group were 64–80% for both sexes, however, those of adolescents (8–11 yr) dropped drastically, particularly for males (14%). As a result, most natal males as well as females disappeared before fully maturing. Two male visitors and an immigrant were observed in the group. More adult males than females disappeared from Bossou. Group males could be excluded as the genetic father of an offspring born in the group. From these demographic trends it is highly likely that some of these males emigrated rather than succumbed to sickness and death. It seems likely that they left on their own by choice. The reason for male dispersal is hypothesized to be influenced by intra-group male-male competition and the habitat ecology and structure of Bossou. There are no competitive adjacent groups or predators to prevent males from living alone and males can sire offspring out of their natal group.     

Secondary transfer of adult mantled howlers (<Emphasis Type="Italic">Alouatta palliata</Emphasis>) on Hacienda La Pacifica,Costa Rica: 1975–2009

Natal emigration by male and female mantled howlers (Alouatta palliata), and subsequent immigration into breeding groups, is well documented for the free-ranging population on Hacienda La Pacifica, Costa Rica, but secondary transfer was considered rare (Glander in Int J Primatol 3:415–436, 1992). Population surveys in 1998 and 2006 caused us to question our assumptions and to re-evaluate our long-term data set from a post hoc perspective. We first identified all animals observed or captured as adults in more than one non-natal group anywhere in the population. We then systematically analyzed joining or leaving by adults in seven groups tracked for various times from 1975 to 2005 for patterns suggesting secondary transfer. Fourteen adults (nine females, five males) were found in two different non-natal groups as adults. In addition, one male and one female that became dominant and reproduced in their natal group later transferred to a second group, and one female was known to be a tertiary transfer. Data from the seven tracked social groups indicate that 35% of all the males and 29% of all the females were potential secondary transfers. In these groups, males leaving or joining was not associated with group size or absolute number of females. Females leaving or joining was not associated with group size or absolute number of males, but females left groups with more females and joined groups with fewer females. Both sexes left groups with unfavorable sex ratios for their sex and joined groups with sex ratios more favorable for their sex. Since a favorable sex ratio is associated with reproductive success in other howler populations, this suggests secondary transfer as a reproductive strategy. Other factors could also influence secondary transfer.     

Social factors influencing natal dispersal in male white-faced capuchins (Cebus capucinus)

White-faced capuchin males disperse from their natal group at around 4.5 years of age, but there is much variation in dispersal timing: our youngest confirmed disperser was 19 months and the oldest 11 years old. In this study, we investigate possible factors influencing dispersal decisions in this species. Between 1983 and 2010, 64 males were born into three study groups in Santa Rosa National Park, Area de Conservación Guanacaste, and Costa Rica. As of August 2010, 21 died or were presumed dead (<14 months), 13 remained natal residents, and 30 were presumed dispersers. We used backward logistic regression to identify proximate factors that predict the occurrence of male natal dispersal. The occurrence of a takeover (significant positive association) and group size (nonsignificant negative association) were included in the model. Male age, number of maternal brothers, and number of adult males were not significant predictors of natal dispersal. The resultant model correctly classified 97% of dispersed and 89% of resident natal males, for an overall success rate of 95%. The occurrence of a group takeover was the strongest predictor of male dispersal, with natal males being 18.7 times more likely to disperse in the context of a group takeover than during peaceful times. A linear regression model showed that the tenure length of a male's probable father influences the age of natal dispersal, explaining 15% of the observed variation in age. However, when our oldest disperser was removed (an outlier) this effect disappeared. Collectively, these results indicate that group instability, as evidenced by the occurrence of a takeover, shorter tenure length of a natal male's father, and smaller group size, triggers natal dispersal in this species while the converse leads to a delay. These data add to our growing evidence of the enormous impact that takeovers have on the behavioral ecology of this species.     

Restricted dispersal in a long-distance migrant bird with patchy distribution,the great reed warbler

In patchily distributed species dispersal connects local populations into metapopulations. Reliable quantifications of dispersal are therefore crucial to understanding the population dynamics and genetic structure of such metapopulation systems. The great reed warbler (Acrocephalus arundinaceus) inhabits eutrophic lakes and has a patchy breeding distribution. In this study we investigated the dispersal pattern of the great reed warbler based on an extensive capture-recapture effort covering a large census area (22,500 km²). At two adjacent breeding sites (10 km apart) in southern Central Sweden, the "main study area", we ringed the majority of adult and nestling great reed warblers between 1992 and 1999. In 1998 and 1999, we opportunistically searched for territorial males at the majority of the Swedish breeding sites, and were able to examine about 56% of all males in the region. Analyses of recaptured males demonstrated that philopatry predominated. Sixty-nine percent of the recruiting nestlings returned to breed in the main study area (their natal area), and 92% of the resighted adults were found at the same breeding locality in both study years. Breeding dispersal was significantly more restricted than natal dispersal. Additional data on natal and breeding dispersal within the main study area in 1992-1999 suggested that females were as philopatric as males. The overall high level of philopatry, with only occasional longer dispersal distances documented, yielded a root-mean-square dispersal distance of 33 km per generation. High philopatry, short dispersal distances and similar dispersal patterns of male and female great reed warblers contrast the findings among birds in general, but conform to data of species having patchy breeding habitat and isolated populations. Restricted dispersal suggests limited gene flow even among several Swedish populations, which is in line with some previous findings of the population ecology of the great reed warbler: (1) structured mtDNA lineages among European populations; (2) small-scale population differences in song patterns; and (3) low genetic variation and occurrence of inbreeding depression in our main study population.     

Natal dispersal in great bustards: the effect of sex, local population size and spatial isolation

1. We investigated the causes of natal dispersal in four Spanish areas where 35 breeding groups of the polygynous great bustard Otis tarda were monitored intensively. A total of 392 juveniles were radio-tracked between 1991 and 2006 by ground and via aeroplane to avoid potential biases derived from the non-detection of long-distance dispersers. 2. We explored 10 explanatory variables that were related to individual phenotypic features, habitat and conspecific traits in terms of group size and breeding performance, and spatial distribution of available breeding groups. Probability of group change and natal dispersal distances were investigated separately through multifactorial analyses. 3. Natal dispersal occurred in 47.8% of the birds and median natal dispersal distance of dispersers was 18.1 km (range 4.97-178.42 km). Sex largely determined the dispersal probability, with 75.6% of males being dispersers and 80.0% of females being philopatric, in contrast to the general pattern of female-biased dispersal found in most avian species. 4. Both the frequency of natal dispersal and dispersal distances were affected by the spatial distribution of breeding groups. More isolated groups showed a higher proportion of philopatric individuals, the effect being more evident in males than in females. This implies a reduction in gene flow in fragmented populations, as most genetic exchange is achieved through male dispersal. Additionally, dispersers hatched in more isolated groups tended to exhibit longer dispersal distances, which increases the associated energetic costs and mortality risks. 5. The dispersal decision was influenced by the number of conspecifics in the natal group. The individual probability of natal dispersal was related inversely to the size of the natal group, which supports the balanced dispersal model and the conspecific attraction hypothesis. 6. Overall, our results provide a good example of phenotypic plasticity and reinforce the current view that dispersal is an evolutionary complex trait conditioned by the interaction of individual, social and environmental causes that vary between individuals and populations.     

Social and ecological influences on dispersal and philopatry in the plateau pika (Ochotona curzoniae)

Benefits and costs of dispersal and philopatry of the socialplateau pika (Ochotona curzoniae) were studied on the Tibetanplateau for 3 years. Although short-lived, plateau pikas livein cohesive family groups that occupy burrow systems in sedgemeadow habitat Most (57.8%) plateau pikas were philopatric,and dispersal movements were extremely restricted. No juvenilefemales or adult pikas moved more than two family ranges betweenyears; the greatest observed dispersal distances were by twojuvenile males that moved five family ranges from the familyof their birth. Traversing unfamiliar habitat was not a costof pika dispersal because most dispersers settled in familiesthat they could easily visit before dispersal. Dispersal movementsappeared to result in equalization of density among pika families,an expected result if competition for environmental resourcesinfluenced dispersal. Males did not disperse to gain advantagesin competition for mates, as evidenced by their moving to familieswith significantly fewer females. Females, however, moved tofamilies with significantly more males. Males provide abundantpaternal care, and significantly more offspring per female survivedto become adults from families with more adult males per adultfemale. Evidence concerning the influence of inbreeding avoidanceon natal dispersal was indirect. Some males exhibited natalphilopatry; thus some families had opportunity for dose inbreeding.Males and females that dispersed had no opposite-sex relativesin their new families. Philopatric pikas may have benefitedby remaining in families that exhibited low local densities,and philopatric females might have benefited from social cooperationwith relatives.