ior and large body size make observations easy. However,
many rodent species are small, cryptic, and /or nocturnal,
making mating behavior difficult to observe (Wolff 1989).
Thus the mating patterns of small rodents are usually in-
ferred by (1) lab or enclosure studies, (2) field monitoring
and manipulation using traps or radiotelemetry, or (3) ge-
netic analysis (e.g., Boonstra et al. 1993). Regardless of the
ease or difficulty, the entire range of classical mating sys-
tems has been described in rodents, ranging from monog-
amy to polygyny to promiscuity (table 3.1). However, po-
lygyny has been considered the primary mating system in
rodents, as in mammals in general (Clutton-Brock 1989b),
because gestation and lactation basically emancipate males
from having to provide parental care.
In this chapter I will discuss how males gain access to
females and how the strategies of rival males and females
can constrain a male’s reproductive success. I will address
each of the primary mating systems described for rodents
and discuss the constraints leading to each and the repro-
ductive strategies used by males to maximize their repro-
ductive success within these constraints (fig. 3.1). I will con-
clude that promiscuity, not polygyny, is the most common
mating strategy of males. Since rodents make up 44% of all
mammal species, this conclusion may represent a paradigm
shift for mating systems research on mammals in general.
Monogamy
Monogamy, in which neither sex can access additional
mates (Emlen and Oring 1977), is rare in mammals, occur-
ring in less than 5% of species (Kleiman 1977). Most cases
of monogamy in murid rodents have been inferred from the
spatial distribution of males and females, where the home
ranges of male-female pairs overlap (Wolff 1985a, 1989).
Some of the other traits that have been used to infer mo-
nogamy in laboratory studies include the presence of pair-
bonding, delayed sexual maturation, parental care, and the
absence of sexual dimorphism, or copulatory plugs (Dews-
bury 1981). Rarely, however, do studies examine if the an-
imals are socially monogamous (in which pair bonds are
formed but copulations outside the pair bond may occur)
or genetically monogamous (in which extra-pair copula-
tions do not occur outside the pair bond; Birkhead and
Møller 1998).
Monogamy is especially enigmatic for males, because
male reproductive success is usually a function of the num-
ber of mates he can inseminate (Trivers 1972). Why then
should a male be monogamous? In rodents, two major
patterns emerge. First, monogamy may occur in situations
in which paternal care is critical for the survival of the
offspring (obligate monogamy; Emlen and Oring 1977;
Clutton-Brock 1989b; Ribble 2003). Paternal care could in-
clude directly caring for offspring (e.g., grooming, retriev-
ing, huddling), as well as defending them against predators
or infanticide. Second, monogamy may occur when males
are unable, for various environmental reasons (e.g., widely
dispersed females or resources), to gain access to more than
one female during a mating season (facultative monogamy;
Holmes 1984a; Komers and Brotherton 1997; fig. 3.2).Obligate monogamy
Obligate monogamy has evolved in cases in which males
are not emancipated from providing parental care, because
male care is critical to offspring survival. Therefore, these
males do not have the opportunity to seek additional mates,
and their strategy for maximizing reproductive success is
to maximize offspring survival. For example, in the genus
Peromyscus,California mice (P. californicus) and oldfield
mice (P. polionotus) have been documented as monogamous
(Wolff 1989), both socially and genetically, and males do
not mate multiply even if the opportunity arises (Foltz 1981;
Ribble 2003). Male parental care (grooming, retrieving, and
huddling over the offspring) appears critical to offspring
survival in California mice (fig. 3.2a), especially when the
ambient temperature is cold or resources are low (Guber-
nick and Teferi 2000; Ribble 2003). Removal of males sig-
nificantly decreased pup survival, suggesting that direct pa-
ternal care and not infanticide prevention (no other males
were present) is the primary function of male care in this
species (Gubernick and Teferi 2000). Males also contribute
anogenital stimulation during parturition, potentially aid-
ing the birth process (Lee and Brown 2002). Since male
ranges are often large and could overlap more than a single
female range, female dispersion does not explain monog-
amy in this species (Ribble 2003). Similar advantages of
direct paternal care are found in other species of monog-
amous rodents, such as mound-building mice (Mus spi-
cilegus), in which males spend time covering young and
retrieving stray pups (Patris and Baudoin 2000), and prai-
rie voles (Microtus ochrogaster), in which males contrib-
ute levels of care similar to females (Oliveras and Novak
1986; McGuire and Bemis, chap. 20, this volume). Per-
haps the epitome of monogamy in rodents is the American
beaver (Castor canadensis), which is reviewed extensively
in Busher, chap. 24, this volume).
Maintaining long-term pair bonds is a strategy for mo-
nogamous males that minimizes the risk of not finding a
mate or mating with an infertile female. The Malagasy giant
rat (Hypogeomys antimena) also forms monogamous long-
term bonds that apparently last until one mate dies (Som-28 Chapter Three