productive success at the beginning of the reproductive life-
span of early dispersers.
Finally, early dispersal by young males may allow them
to establish social relationships with potential mates. In
many species, females are selective in choosing mates, pre-
ferring some males over others, and might be more likely to
mate with males if they have had the opportunity to fully
assess them. Early dispersal might also provide males with
the opportunity to become familiar with potential mates as
well as other males in their new home area, which may be
beneficial to them as they compete for mating opportunities
during the breeding period.Consequences of natal dispersal
Genetic structure of populations
The development of new techniques in molecular genetics
has contributed considerably to our understanding of the
relationship between dispersal and the genetic structure of
populations (e.g., Dobson, chap. 14, this volume). Molecu-
lar techniques have been used to provide support for the hy-
pothesis that dispersal by male rodents and male mammals
facilitates gene flow within and between populations. Ehric
and Stenseth (2001) used mitochondrial and microsatellite
DNA analysis to evaluate the genetic structure of Siberian
lemming (Lemmus sibiricus) populations and observed ex-
tensive genetic variation. Moreover, local groups of females
were more genetically similar than local groups of males,
suggesting that gene flow is more prominently facilitated by
male than female dispersal.
Genetic studies of other rodent species have also indi-
cated that dispersal is important in maintaining the ge-
netic diversity of populations. In black-tailed prairie dogs,
long-established colonies tend to be more genetically simi-
lar than newly formed colonies, presumably because gene
flow facilitated by dispersal has counteracted founder effects
in older colonies (Roach et al. 2001). Santos et al. (1995)
observed that genetic diversity decreased in populations of
root voles when opportunities for dispersal were limited.
Limited opportunities for dispersal have also been linked to
reduced gene flow and reduced genetic diversity in Mala-
gasay jumping rats (Hypogeomys antimena) in Madagascar
(Sommer et al. 2002).
In a study of North American collared lemmings (Di-
crostonyx groenlandicus), Ehrich et al. (2001a, 2001b) sug-
gested that the degree of genetic similarity among distantly
separated populations could only be explained by long-
distance dispersal. They noted that lemmings are occasion-
ally observed on sea ice, and suggested that transport by ice
may facilitate long-distance dispersal. Dispersal distances
over a kilometer have been observed in white-footed mice,
yellow-bellied marmots, and Belding’s ground squirrelsDispersal and Philopatry 155Figure 13.1 Body mass of male Belding’s ground squirrels evaluated according
to their age at dispersal. The same eight males are represented at each time
point. Early dispersal appears to be associated with a body mass advantage early
in males’ second summer, when they begin mating.
uals to reliably anticipate and prepare for their first breed-
ing season via early settlement in the new home area. In
the following I assess three possible adaptive advantages of
early dispersal in male Belding’s ground squirrels, a species
in which most males disperse between 7 to 10 weeks of age
but do not begin mating until they are two years old.
First, early dispersal by young male squirrels may reduce
the social resistance they encounter as they establish resi-
dence in their new home areas. Males who disperse as ju-
veniles leave home at the end of the breeding season, when
adults are fattening for hibernation and agonistic interac-
tions are rare. By contrast, males who disperse as yearlings
leave home during the following breeding season, when ag-
gression is high among adult males as they compete with
each other for mates and among reproductive females as
they defend maternal territories.
Second, early dispersal may provide an energetic or size
advantage at the time of breeding. According to this hy-
pothesis, early dispersal allows young males to increase
growth or acquisition of energy reserves, possibly by avoid-
ing social resistance as they settle in their new home areas
(see the preceding). If young males are able to time disper-
sal so that agonistic interactions with residents in their new
home are minimal, they not only can reduce energy expen-
diture, but also increase time available for foraging. Pre-
liminary work with Belding’s ground squirrels suggests that
young males may in fact gain an energetic advantage by
dispersing early. Males who disperse as juveniles tend to be
larger at two years of age than are males who disperse as
yearlings (fig. 13.1). Because size is correlated with fight-
ing success, and fighting success is correlated with mating
success, this size difference may parlay into increased re-