A

t some pointin their lives, nearly all organisms
are faced with the “decision” of being philopat-
ric — remaining where they are, or dispersing —
moving to a new area or joining a new social group or
population. This decision has important consequences for
the evolutionary fitness of individuals. Moreover, because
philopatry and dispersal determine the distribution of indi-
viduals, they have important consequences for the mainte-
nance of ecosystems, the demographic and genetic structures
of populations, the spread of disease, and the conservation
of species (Clobert et al. 2001; Nathan 2001; Bullock et al.
2002; Ostfeld and Mills, chap. 41, this volume).
Dispersal has been extensively studied in mammals, and
in particular within a broad range of rodent species. In
mammalian biology, the term dispersalis used to describe
a process in which individuals leave their home site or so-
cial group and establish residence in a new area or group
(Stenseth and Lidicker 1992). In many cases, the process of
dispersal is preceded by exploratory forays into potential
new home areas or groups. When dispersal occurs, it in-
cludes three general stages: emigrating from the original
home area or group, traversing the landscape to get to the
new home, and establishing residence in the new home area
or group. This movement process is potentially risky for an-
imals. Dispersers may find themselves in unfamiliar areas,
where they are vulnerable to predation (Metzgar 1967), and
may encounter social resistance while attempting to immi-
grate into their new home areas (Wolff 1994a; Solomon
2003). Furthermore, increased vigilance and involvement
in agonistic interactions during the dispersal process may

place energetic demands on individuals (Nunes and Hole-

kamp 1996).

The dispersal process can be categorized as either natal

or post-breeding. In natal dispersal, a young animal leaves

its birth area before reproducing. In post-breeding disper-

sal, an individual leaves an area after it has successfully re-

produced, and settles into a new breeding site. In mammals,

natal dispersal is more common than breeding dispersal,

occurring in nearly all species. Natal dispersal has a strong

sex-bias in mammals, with males typically emigrating to

new home areas at higher rates or over greater distances

than females.

Because of its ubiquity, dispersal is an important phe-

nomenon among taxa. Here, I evaluate dispersal among ro-

dents, examining the ultimate and proximate causes as well

as the consequences of breeding, and natal dispersal and

philopatry. Because rodents are an incredibly large and di-

verse group of animals (Nowak 1991[vol. 1], 1991[vol. 2]),

they exhibit a broad range of dispersal and philopatric be-

haviors, and are an excellent model system for understand-

ing dispersal and philopatry.

Natal Dispersal

Ultimate causes of natal dispersal

In assessing the possible evolutionary forces that brought

about sex-biased natal dispersal in mammals, two impor-

tant questions arise. First, why do young animals leave the

Chapter 13Dispersal and Philopatry

Scott Nunes