Fitness consequences of dispersal: is leaving home the best of a bad lot?

Using 20 years of demographic and genetic data from four populations of banner-tailed kangaroo rats (Dipodomys spectabilis), we asked whether dispersing individuals gain benefits during adulthood that might compensate for the substantial survival costs they experience as juveniles. Compared to philopatric animals, within- and between-population dispersers gained no measureable advantages in adult survival, fecundity, or probability of recruiting offspring to adulthood. Restricting analyses to members of two central populations living more than 15 times the median dispersal distance from the study site edge, and using peripheral populations only to detect dispersal or offspring recruitment "offsite," did not change this result. Population density during year of birth had small negative effects on adult survival and fecundity, but there were no interactions with dispersal phenotype. We found no evidence that dispersers gained access to superior habitat or that their offspring suffered less inbreeding depression. Our results are consistent with fitness advantages being indirect; by leaving, dispersers release their kin from competition. Our results are also consistent with the possibility that dispersal is the "best of a bad lot." If dispersal is a conditional strategy, then the benefits may be obscured in observational data that compare dispersing individuals to philopatric individuals rather than to individuals whose dispersal phenotype is experimentally manipulated.

How much can parentage analyses tell us about precapture dispersal?

Estimating rates of movement among populations is never simple, and where young animals cannot all be captured at their birth sites, traditional field methods potentially underestimate dispersal rates. Genetic assignment tests appear to hold promise for detecting 'precapture' dispersal, and recent evidence suggests that even on the scale of dispersal between populations, genetic parentage analyses can also be informative. Herein, we examine the performance of both types of analysis with data from a 17-year study of dispersal in banner-tailed kangaroo rats Dipodomys spectabilis. We compare estimates of precapture dispersal from (i) the commonly used parentage analysis program cervus (ii) a pedigree-reconstruction program, MasterBayes, that combines genetic with spatial and other nongenetic information and (iii) genetic assignment procedures implemented by the program geneclass2, with (iv) rates of dispersal observed through recapture of a subset of animals initially marked shortly after weaning. geneclass2 estimates a larger proportion of precapture dispersers than MasterBayes, but both approaches as well as those based on field data alone, suggest that approximately 10% of adults in local populations are immigrants and that interpopulation dispersal is slightly female-biassed. All genetic procedures detect precapture dispersal between populations, but dispersers identified by MasterBayes are particularly compatible with what is independently known about body mass at dispersal, dispersal distance and distance between parents. Parentage analyses have considerable potential to infer the value of this otherwise elusive demographic parameter when most candidate parents can be genotyped and when nongenetic information, especially the distance separating candidate mothers and fathers, can be incorporated into the procedure.

Quantifying male-biased dispersal among social groups in the collared peccary (Pecari tajacu) using analyses based on mtDNA variation.

Recent advances in the statistical analysis of microsatellite data permit calculation of sex-specific dispersal rates through sex- and age-specific comparisons of genetic variation. This approach, developed for the analysis of data derived from co-dominant autosomal markers, should be applicable to a sex-specific marker such as mitochondrial DNA. To test this premise, we amplified a 449 bp control region DNA sequence from the mitochondrial genome of the collared peccary (Pecari tajacu), and estimated intra-class correlations among herds sampled from three Texas populations. Analyses on data partitioned by breeding group showed a clear signal of male-biased dispersal; sex-specific fixation indices associated with genetic variation among social groups within populations yielded values for females (F(GP)=0.91), which were significantly larger than values for males (F(GP)=0.24; P=0.0015). The same general pattern emerged when the analyses were conducted on age classes (albeit nonsignificantly), as well as categories of individuals that were predicted a posteriori to be dispersers (adult males) and philopatric (adult females and all immatures). By extending a previously published methodology based on biparentally inherited markers to matrilineally inherited haploid data, we calculated sex-specific rates of contemporary dispersal among social groups within populations (m(male symbol)=0.37). These results support the idea that mitochondrial DNA haplotype frequency data can be used to estimate sex-specific instantaneous dispersal rates in a social species.

Genetic detection of sex-biased dispersal.

We investigated the application of a recently developed genetic test for sex bias in dispersal. This test determines an animal's 'assignment index' or the expected frequency of its genotype in the population in which it is captured. Low assignment indices indicate a low probability of being born locally. We investigated the use of this test with the white-footed mouse, Peromyscus leucopus, in which dispersal is predominantly male-biased, but not extreme. We found that male P. leucopus had significantly lower assignment indices than females. These data suggest that the genetic test for sex bias in dispersal has potential to be used with species that do not have extreme sex-biased dispersal tendencies.

Mangabey (Cercocebus albigena) ranging patterns in relation to fruit availability and the risk of parasite infection in Kibale National Park, Uganda.

Two opposing hypotheses concerning determinants of mangabey (Cercocebus albigena) ranging patterns have been advocated. One hypothesis suggests that ranging patterns of mangabeys are largely a response to fruit availability, while the other hypothesis advocates that concerns of fruit availability are supplemented or overridden by concerns of fecal contamination and that the risk of parasite infection, especially during dry weather, determines their pattern of range use. In this 9 month study of mangabeys in the Kanyawara study area of Kibale National Park, mangabeys moved longer distances during the wet season than during the dry season. There were no seasonal differences in group spread, number of 50 by 50 m quadrats used, or in quadrat overlap between sequential sample periods. Intensity of quadrat use was closely related to the number of fruiting trees/lianas in the quadrats, irrespective of season. These findings are consistent with the hypothesis that fruit availability is a main factor influencing mangabey ranging patterns. The results are not consistent with the hypothesis that mangabey ranging patterns largely reflect differential seasonal risk of parasite infection.

Spontaneous lactation is an adaptive result of pseudopregnancy.

Lactation is almost exclusively associated with pregnancy and giving birth. Although lactation can be induced without a preceding pregnancy in some species, this requires exogenous hormones, artificially intense or extended suckling or both. Spontaneous lactation, lactation by females that have neither been pregnant nor experimentally manipulated, is extremely unusual among eutherians. Among nondomesticated animals, spontaneous lactation has been observed repeatedly only in the dwarf mongoose Helogale parvula. We report here spontaneous lactation by free-living dwarf mongooses using data on urinary oestrogen conjugate concentrations (n = 560, 65 females) and body weight (n = 3,096, 25 females) from a population in Serengeti National Park, Tanzania. We use demographic data from this population to demonstrate that spontaneous lactation, and thus the endocrine phenomena that induce it, increase the evolutionary fitness of lactating females.

Polyspecific associations of Cercocebus albigena: geographic variation and ecological correlates.

The characteristics of mixed-species groups and interspecific interactions involving the mangabey, Cercocebus albigena, are described. Polyspecific associations are common and behaviorally complex. In Uganda, ecological data suggest that associations with some species are related to foraging, although evidence that these associations benefit the individuals involved remains equivocal. West African data indicate considerable geographic variability in polyspecific association tendencies.

Postreproductive survival and behavior in a free-ranging female mangabey.

The survival and behavior of a postreproductive free-ranging female Cercocebus albigena is documented; her patterns of grooming, aggression, and other social and spatial relationships with members of her group are described and compared with those of other, cycling, females. The phenomenon of female postreproductive survival in free-ranging nonhuman primates is briefly reviewed.

Spatial associations and social interactions in a "solitary" ungulate: the bushbuck Tragelaphus scriptus (Pallas).

The bushbuck population in a study area of 0.7 km2 in Queen Elizabeth National Park, Uganda, was observed between June 22 and July 30, 1971. The animals (4 male male, 10 female female, 4 immat.) were individually recognizable. Observations consisted of census sightings and longitudinal observations; during the latter location, neighbours, social encounters and other behaviour of focal animals were recorded continuously. 71% of the census sightings were of single animals. Hardly any aggression was observed, nor did the bushbuck actively avoid oneanother. The definition of the term "solitary" and possible advantages of a dispersed social system for the bushbuck are discussed.