Peter M. Waser

Tests for sex-biased dispersal using bi-parentally inherited genetic markers.

Understanding why dispersal is sex‐biased in many taxa is still a major concern in evolutionary ecology. Dispersal tends to be male‐biased in mammals and female‐biased in birds, but counter‐examples exist and little is known about sex bias in other taxa. Obtaining accurate measures of dispersal in the field remains a problem. Here we describe and compare several methods for detecting sex‐biased dispersal using bi‐parentally inherited, codominant genetic markers. If gene flow is restricted among populations, then the genotype of an individual tells something about its origin. Provided that dispersal occurs at the juvenile stage and that sampling is carried out on adults, genotypes sampled from the dispersing sex should on average be less likely (compared to genotypes from the philopatric sex) in the population in which they were sampled. The dispersing sex should be less genetically structured and should present a larger heterozygote deficit. In this study we use computer simulations and a permutation test on four statistics to investigate the conditions under which sex‐biased dispersal can be detected. Two tests emerge as fairly powerful. We present results concerning the optimal sampling strategy (varying number of samples, individuals, loci per individual and level of polymorphism) under different amounts of dispersal for each sex. These tests for biases in dispersal are also appropriate for any attribute (e.g. size, colour, status) suspected to influence the probability of dispersal. A windows program carrying out these tests can be freely downloaded from http://www.unil.ch/izea/softwares/fstat.html

When Should Animals Tolerate Inbreeding

We compare, in an initially outbred population, the number of offspring equivalents expected by an individual that avoids all inbreeding with that expected by an individual that tolerates one inbred mating. The model suggests that for most mating systems, the sole factor determining whether inbreeding tolerance spreads is the cost of inbreeding avoidance. Specifically, most forms of polygyny do not increase the payoff to inbreeding; the critical parameter is not the number of matings an individual engages in but rather how many outbred matings are forfeited when an individual chooses to mate with a relative. The model also suggests that dispersal is unlikely to have arisen primarily as a mechanism to avoid inbreeding, and that father-daughter inbreeding should be more common than mother-son inbreeding.

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 animals ‘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.

Recent demographic bottlenecks are not accompanied by a genetic signature in banner-tailed kangaroo rats ( Dipodomys spectabilis )

Single‐sample methods of bottleneck detection are now routine analyses in studies of wild populations and conservation genetics. Three common approaches to bottleneck detection are the heterozygosity excess, mode‐shift, and M‐ratio tests. Empirical groundtruthing of these methods is difficult, but their performances are critical for the accurate reconstruction of population demography. We use two banner‐tailed kangaroo rat (Dipodomys spectabilis) populations from southeastern Arizona (USA) that are known to have experienced recent demographic reductions to search for genetic bottleneck signals with eight microsatellite loci. Over eight total sample‐years, neither population showed a genetic bottleneck signature. M‐ratios in both populations were large, stable, and never fell below a critical significance value (Mc). The mode shift test did not detect any distortion of allele frequencies, and tests of heterozygosity excess were not significant in postbottleneck samples when we used standard microsatellite mutation models. The genetic effects of bottlenecks like those experienced by our study populations should be strongly influenced by rates of mutation and migration. We used genetic parentage data to estimate a relatively high mutation rate in D. spectabilis (0.0081 mutants/generation/locus), but mutation alone is unlikely to explain the temporal distribution of rare alleles that we observed. Migration (gene flow) is a more likely explanation, despite prior mark–recapture analysis that estimated very low rates of interpopulation dispersal. We interpret our kangaroo rat data in light of the broader literature and conclude that in natural populations connected by dispersal, demographic bottlenecks may prove difficult to detect using molecular genetic data.

Mechanisms and Evolution of Spacing in Animals

Animals of the same species are rarely distributed randomly. Each individual’s movements are influenced by those of its neighbors, with the result that any population exhibits a characteristic pattern of individuals’ locations and activities in space.1 In this chapter, we discuss in turn three approaches to understanding individuals’ spatial relationships: quantitative specification of patterns of spacing; analysis of the behavioral mechanisms that control spacing; and identification of the effects of natural selection on the evolution of spacing. This division separates discussion of the proximate controls of spacing, in our initial sections, from consideration of the ultimate controls, with which we conclude.

Use, misuse and extensions of "ideal gas" models of animal encounter

Biologists have repeatedly rediscovered classical models from physics predicting collision rates in an ideal gas. These models, and their two‐dimensional analogues, have been used to predict rates and durations of encounters among animals or social groups that move randomly and independently, given population density, velocity, and distance at which an encounter occurs. They have helped to separate cases of mixed‐species association based on behavioural attraction from those that simply reflect high population densities, and to detect cases of attraction or avoidance among conspecifics. They have been used to estimate the impact of population density, speeds of movement and size on rates of encounter between members of the opposite sex, between gametes, between predators and prey, and between observers and the individuals that they are counting. One limitation of published models has been that they predict rates of encounter, but give no means of determining whether observations differ significantly from predictions. Another uncertainty is the robustness of the predictions when animal movements deviate from the model’s assumptions in specific, biologically relevant ways. Here, we review applications of the ideal gas model, derive extensions of the model to cover some more realistic movement patterns, correct several errors that have arisen in the literature, and show how to generate confidence limits for expected rates of encounter among independently moving individuals. We illustrate these results using data from mangabey monkeys originally used along with the ideal gas model to argue that groups avoid each other. Although agent‐based simulations provide a more flexible alternative approach, the ideal gas model remains both a valuable null model and a useful, less onerous, approximation to biological reality.

Sociality or territorial defense? The influence of resource renewal

SummaryThe rate with which resources in an area recover from local exploitation should influence the costs to an inhabitant of sharing it with neighbors. I develop a model which predicts the costs of tolerating conspecific foragers (or the benefits of excluding them) as a function of a predators rate of harvesting prey and the preys renewal rate. The predictions are consistent with patterns of social grouping observed in small African carnivores. A generalization of the model considers alternate forms of resource renewal (logistic, constant, or “exponential”) and suggests that not only the average rate of renewal but also the details of its time course should influence animal spacing patterns.

Philopatry, Dispersal, and Habitat Saturation in the Banner-Tailed Kanagaroo Rat, Dipodomys Spectabilis

We report natal dispersal distances for 331 banner—tailed kangaroo rats (Dipodomys spectabilis) at high and low population density. The data were collected in 29 censuses of marked individuals in two populations over 8 yr. One population underwent four—fold variation in density. The other was consistently at high density. Because the number of burrow systems (i.e., breeding sites) remained fairly constant, we interpret changes in density as changes in degree of habitat saturation. Dispersal distances were shorter and the proportion of philopatric young (moving less than a home range diameter) was greater at high density than at low density. This result does not conform to models of dispersal based on studies of cycling small mammals or to a simple competition model of dispersal. All data are consistent with a habitat saturation model, which holds that high—density saturated conditions favor philopatric tendencies in offspring. See full-text article at JSTOR

Old world monkey vocalizations: adaptation to the local habitat?

Abstract Twenty-one representative vocalizations of two species of rain forest monkeys (blue monkeys, Cercopithecus mitis, and grey-cheeked mangabeys, Cercocebus albigena) and two species of savanna monkeys (vervet monkeys, Cercopithecus aethiops, and yellow baboons, Papio cynocephalus) were broadcast in both rain forest and savanna habitats. Broadcast signals were re-recorded at distances of 12·5 and 100 m, digitized, and analysed on a supercomputer to measure the magnitude of distortion of the vocal repertoires of the savanna and forest monkeys in both the ‘appropriate’ and ‘inappropriate’ habitats. Independent distortion analyses were conducted in the time and frequency domains. The results showed that (1) distortion was strong following transmission distances of only 12·5 m in both habitats, (2) distortion scores were further increased, but only by a modest amount, when transmission distance was increased (3) distortion scores were greater in the savanna habitat than in the rain forest habitat, (4) rain forest monkey calls were distorted less in the ‘appropriate’ rain forest habitat than in the ‘inappropriate’ savanna habitat, but (5) savanna monkey calls were similarly distorted in both habitats. The results were consistent with the idea that the rain forest environment is generally more favourable for high-fidelity sound propagation, and that selection for reduced distortion has more strongly influenced the physical form of the vocal repertoire of the two rain forest species than that of the two savanna species.

DISPERSAL AND GENETIC STRUCTURE IN KANGAROO RATS

We used spatial autocorrelation of allele frequencies to examine local structure in a population of bannertailed kangaroo rats for which Wrights isolation‐by‐distance model seems applicable, and for which we can estimate neighborhood size based on 10 years of data on demography and dispersal. The uniform dispersion and strong philopatric tendencies of this species provide a test case for the idea that restricted dispersal can lead to local genetic structure in small mammals. Whether we considered such complications as nonnormal dispersal distances, variation in lifetime reproductive success, fluctuating population density, and adult as well as juvenile dispersal, our estimate of effective population size was fewer than 15 animals. Nevertheless, data from four polymorphic allozyme loci analyzed over a range of separations between 50 m (approximately one home range diameter) and 1,000 m detected no evidence for spatial clustering of alleles. One resolution of this apparent paradox is that “gamete dispersal,” caused by the movements of males away from their residences during the breeding season, may be a significant (and unmeasured) component of gene dispersal. Our analyses also demonstrate that a decline in population density may actually increase neighborhood size. A more general implication is that even extremely philopatric mammals have effective population sizes large enough to prevent the development of local genetic structure.