ing fitness. Because of the high turnover rate of adult males,
young females typically are not exposed to their fathers,
and consequently can breed on or near their natal site with
unrelated males. By contrast, the reproductive life span of
mothers commonly overlaps with that of their sons, and fe-
males show a preference for unrelated males as mates. As a
consequence, young males commonly emigrate from the na-
tal site prior to mating. The timing of dispersal corresponds
with growth and development and the best probability of
successful immigration. Failure to emigrate from the natal
site due to habitat saturation or some socioecological fac-
tor(s) often results in delayed sexual maturation if young
males and females remain in the presence of opposite-sex
parents.
Both philopatry and dispersal have important conse-
quences for individuals, populations, and species. Remain-
ing at the natal site allows for cooperation with close kin,
which can be a foundation for developing complex social
behavior and social systems. An increasingly important con-
sequence of dispersal is its maintenance of genetic diversity
within populations by facilitating gene flow. Loss and frag-
mentation of habitat associated with human activity has
presented barriers to dispersal and gene flow for many pop-
ulations, putting some populations in danger of becoming
extinct (see Lidicker, chap. 38, this volume). Future stud-
ies on rodents and other animals that integrate dispersal
and population genetics will thus be critical in conserving
threatened populations and species.SummaryDispersal is a ubiquitous behavior among rodents and mam-
mals in general, and involves departure from the birth area
or group (natal dispersal) or site of reproduction (breeding
dispersal) and settlement in a new area or group. Natal dis-
persal is more prevalent than breeding dispersal, and tends
to be sex-biased, with males more commonly emigrating
from the birth area and females typically exhibiting philo-
patry, or faithfulness to the natal area throughout life.
Breeding dispersal may allow individuals to avoid mat-
ing with their young, bequeath territories to offspring, or
settle in better habitat. Much of the available data suggests
that avoidance of inbreeding was a primary driving force in
the evolution of natal dispersal, and that polygyny and
greater selectivity in mate choice by females was an impor-
tant ultimate cause of the male bias in mammalian natal
dispersal. Evolutionary causes for natal dispersal among
ordinarily philopatric females are primarily related to com-
petition for environmental resources such as food or nest
sites. Because dispersal mediates the movement of alleles
within and between populations, it has important conse-Dispersal and Philopatry 161Figure 13.6 A model of delayed dispersal and female-biased philopatry in mammals
based on studies of rodents.