the colony (Brody and Armitage 1985). Removal studies
with California ground squirrels also resulted in rapid re-
colonization of vacated habitat (Dobson 1981). However,
male squirrels were less likely to immigrate into removal
areas than were females, suggesting that vacant habitat is a
stronger trigger of dispersal for females.
Presence of potential mates
Female prairie voles, root voles, and common voles (M. ar-
valis) may remain in a reproductively quiescent state while
in the natal area (McGuire and Getz 1991; Gundersen et al.
1999; Heise and Rozenfeld 2002). However, natal female
prairie voles display investigative and affiliative behavior
and become reproductively mature when exposed to unfa-
miliar males (McGuire and Getz 1991). Moreover, female
root voles preferentially settle in habitat with an experi-
mentally high density of males, whereas male root voles are
equally likely to settle in areas with few or many females.
These findings suggest that female voles may be prompted
to disperse by the presence of unfamiliar males in areas out-
side the natal site.
By contrast, Solomon (2003) observed that female prai-
rie voles are as likely to settle in an area with potential
mates as they are to settle in an area from which all males
have been removed. Moreover, Lurz et al. (1997) observed
that food availability was the main factor affecting female
dispersal in Eurasian red squirrels (Sciuris vulgaris), and
that male dispersal was influenced by the distribution of fe-
males. That is, females tended to settle in areas with abun-
dant food, whereas males tended to settle in areas with
potential mates. These findings suggest that although the
presence of mates may influence dispersal in both males and
females, availability of environmental resources may be a
more important consideration for females than availability
of mates.
Presence of opposite-sex parent
As noted previously, dispersal of young from the natal area
can be triggered by the presence of the opposite-sex par-
ent. This has been documented in white-footed mice (Wolff
1992), root voles (Gundersen et al. 1999), and Richardson’s
ground squirrels (Michener and Michener 1977). More-
over, McGuire et al. (1993) observed that emigration from
the natal site was more common in prairie voles when
potential mates in the natal group were parents or sib-
lings than when they were nonrelatives, suggesting that the
presence of opposite-sex siblings as well as parents might
prompt dispersal (see the following).
Interactions with siblings
Jacquot and Vessey (1995) suggested that interactions with
siblings trigger natal dispersal in white-footed mice. They
observed that female mice dispersed farther when they were
from large litters, or when they had many brothers. Jacquot
and Vessey (1995) also observed that male mice with more
than one littermate sister dispersed greater distances than
did males with fewer than two sisters. They proposed that
their results are consistent with the hypotheses that in-
breeding avoidance is an important determinant of disper-
sal for both male and female mice. Bollinger et al. (1993)
also observed in meadow voles that the presence of siblings
can influence dispersal behavior. Voles released onto ex-
perimental plots with same-sex siblings were more likely to
disperse from the plots than were voles released with non-
siblings. McGuire et al. (1993) observed that dispersal was
more common in prairie voles when potential mates were
siblings or other close relatives than when they were non-
relatives. Ribble (1992) similarly observed that dispersal dis-
tances increase with number of siblings in male mice (Pero-
myscus californicus) and with number of sisters in female
mice, and suggested that competitive interaction among
siblings might enhance dispersal tendencies.Attainment of sufficient preparation
Although dispersal is risky, mechanisms exist for minimiz-
ing risks. For example, all male Belding’s ground squirrels
eventually leave their natal areas. However, they delay dis-
persing until they achieve a threshold body mass and suffi-
cient body fat to allow them to meet the potential physical
and energetic demands of dispersing (Nunes and Holekamp
1996; Nunes et al. 1998). Nunes et al. (2004) further ob-
served that young males who engage in social play at high
rates early during the juvenile period have better motor co-
ordination and are more likely to disperse by the end of the
juvenile summer than are males who play at low rates. They
proposed that adequate motor development may be a req-
uisite for dispersal in young male squirrels. Bekoff (1977)
and Thompson (1998) further proposed that interaction
during play bouts with siblings or conspecifics at the same
stage of development might reveal information about a
young animal’s behavioral proficiency and competitive abil-
ity. Individuals might then use this information in deter-
mining when they are physically, behaviorally, and socially
prepared for the potential risks and demands of dispersing.Timing of natal dispersal
In many species, natal dispersal occurs near the time of
reproductive maturity. However, in other species, dispersal
from the natal area occurs long before the attainment of
reproductive competence (Smale et al. 1997). Early disper-
sal is most likely to occur when individuals begin mating at
the onset of reproductive maturity and changes in environ-
mental conditions are highly predictable, allowing individ-154 Chapter Thirteen