these species can be quite variable, depending on local re-
source dispersion, with the annual number of mates per
male varying greatly.
Many species of rodents have variable mating systems,
ranging from monogamy to promiscuity (Wolff 1985a,
1989). Prairie voles (fig. 3.2b) have been studied extensively
in the field and laboratory, and although they show many
characteristics of monogamy, a large portion of males ap-
pear to be polygynous and multi-male mating has been re-
ported (Getz et al. 1993; Carter and Getz 1993; Wolff and
Dunlap 2002). Wandering males do not appear to differ
phenotypically from monogamous males (Solomon and
Jacquot 2002); thus, prairie voles might be a good example
of facultative monogamy. Prairie vole males can form long-
term bonds, and males guard females from other males (De-
Vries and Carter 1999). In deer mice (Peromyscus manicu-
latus), male movements and spatial overlap with females
suggest that mating may range from monogamy in some
cases to polygyny or promiscuity in other populations (Bird-
sall and Nash 1973; Merritt and Wu 1975; Kleiman 1977;
Mihok 1979). Similarly, in white-footed mice (P. leuco-
pus), mating systems range from monogamy to polygyny
to promiscuity, depending on density and spatial distribu-
tion of females (Wolff and Cicirello 1990). In low densities,
white-footed mice appeared to form monogamous bonds
(Mineau and Madison 1977), whereas in a moderately
dense population, females mated multiply (Wolff 1989).
Wolff (1989) suggests that multiple mating in higher densi-
ties is a female strategy to reduce infanticide by males (see
also Ebensperger and Blumstein, chap. 23, and Solomon
and Keane, chap. 4, this volume). A similar pattern of fac-
ultative monogamy is seen in California voles (Microtus cal-
ifornicus), montane voles (M. montanus), and Townsend’s
voles (M. townsendii), in which mating patterns vary with
density (Kleiman 1977; Wolff 1985a; Lambin and Krebs
1991b). Likewise, density of female giant kangaroo rats (Di-
podomys ingens) influences the number of mates per male
(Randall et al. 2002), with mating by both sexes ranging
from a single partner at low densities to multiple partners
at higher densities. Because males must defend a food terri-
tory, their mating opportunities are restricted to neighbor-
ing females (Randall et al. 2002) and they are unable to pur-
sue a strategy of competitive searching (see the following).
Facultative monogamy in marmots appears to be influ-
enced by the distribution and quality of resources. In yel-
low-bellied marmots (Marmota flaviventris), males that oc-
cupy marginal habitat are usually associated with a single
female, whereas males in better habitats are polygynous
(Armitage 1986b). Polygynous males have twice the repro-
ductive success of monogamous males because annual re-
productive success is directly linked to the number of fe-
males residing in a male’s territory (Armitage 1986b, 1998).
Males will expand territories to include more females if the
opportunity arises, but they are constrained from moving
into new territories by the need for a decent hibernacu-
lum and sufficient resources to prepare for hibernation (Ar-
mitage 1986b). Hoary marmots (M. caligata) in Alaska are
monogamous, most likely because the burrows females need
for hibernation are too widely spaced and resources are too
low for clumping of females (Holmes 1984a). Males likely
do not have the opportunity to travel from one valley to an-
other to seek additional fertile females, especially consider-
ing the period of female breeding is fairly short (Holmes
1984a). In more southern populations, when resources per-
mit, females clump and males are polygynous (Barash 1981).
Similar patterns have been observed in other species of mar-
mots; 19% of Olympic marmots (M. olympus) and 50% of
woodchucks (M. monax) were reportedly monogamous
(Barash 1973; Armitage 1986b).
The distribution of resources or females, not just abun-
dance, may also affect mating patterns. If mating systems
are associated with resource dispersion, when resources are
patchy, females should form groups to defend the patches,
whereas when resources are uniform, females should not
clump (Slobodchikoff 1984). Thus males should associate
with a group of females when resources are patchy, but only
with a single female when resources are uniform. When this
hypothesis was tested in Gunnison’s prairie dog (Cynomys
gunnisoni), altering habitat to a more uniform distribution
of resources did result in a greater number of monogamous
associations (Slobodchikoff 1984; Travis and Slobodchikoff
1993; Travis et al. 1995). However density, which reflects
resources, is also a strong predictor of mating patterns in
this species, as monogamy occurred only at the lowest den-
sities with the lowest patchiness. Similarly, in grey-sided
voles (Clethrionomys rufocanus), males had greater overlap
with other males when females were clumped and less so
when females were dispersed (Ims 1988). However, in field
voles (Microtus agrestis), the spatial distribution of females
did not affect male spacing, whereas female density did
(Nelson 1997).PolygynyPolygyny, in which a male monopolizes access to more than
a single female, is considered the most common mating sys-
tem in mammals (Clutton-Brock 1989b). Males should
attempt to mate with as many females as possible to maxi-
mize their reproductive success (Trivers 1972). Many dif-
ferent types of polygyny have been described, but these
fall into two major categories: defense and non-defenseMale Mating Strategies in Rodents 33