12 –24 hours in beavers (Wilsson 1971; Doboszynska and
Zurowski 1983) and this, coupled with reduced mobility,
may have selected for mate-guarding as a way to ensure
reproduction (Sun 2003). However, an adult male does
not always closely attend the adult female (Hodgdon 1978;
Busher 1980), and the interaction rate during the activ-
ity period between adults is low (Hodgdon 1978; Busher
1983a; Woodard 1994). Male mate-guarding as the ulti-
mate factor influencing beaver monogamy is intriguing, and
more studies specifically designed to test this hypothesis are
required.
Hypothesis 5
Data exist that support the role of female dispersion in the
evolution of monogamy in beavers. The general pattern is
for 2-year-old beavers to disperse from their parent colony
during the spring or summer. While most dispersing bea-
vers of both sexes move to sites near the parent colony, a
significant difference in average dispersal distances between
male and female beavers has been documented. In a popu-
lation in New York, female beavers dispersed an average
of 10.15 km while males dispersed an average of 3.49 km
(Sun et al. 2000). Adequate data to quantify the dispersion
of females are not available for beavers, but relatively wide
spacing would provide support for the female-dispersion
hypothesis if females were isolated in exclusive territories
(Brotherton and Manser 1997; Komers and Brotherton
1997). Additional support for this hypothesis comes from
the general pattern of range expansion by a reintroduced
Eurasian beaver population in Sweden. Beavers spread along
a very flat dispersal front from the introduction sites, with
long dispersal distances (Hartman 1995, 2003). These data
generally support the hypothesis that monogamy in beavers
may have been influenced by female dispersal into relatively
isolated territories and the subsequent mating with a single
male. Mate-guarding and investment in the territory by
construction activities and defense of food resources would
solidify and help to maintain the pair bond.
Summary and Additional Considerations
For beavers, the ultimate ecological factors in the evolution
of the pair bond may have been their foraging and con-
struction behaviors, and the fact that they choose large prey
items (trees). This behavior selected for large body size,
which in turn selected for a longer developmental period.
The extended length of care provided within the family
group increased the need for and amount of parental invest-
ment by both sexes, but especially by the male. Since the
male invests heavily in the family area through construction
and defense activities, it is less advantageous for the male to
desert the female after reproduction.
Female dispersion and subsequent male mate-guarding
behavior may be the ultimate social factors in the evolution
of monogamy in beavers. Sexually mature females disperse
alone from the natal territory and may disperse farther
than males. Given the short estrus period, once a pair bond
forms it would be advantageous for the male to engage in
mate-guarding to ensure reproduction.
However, the complexity of the environment, the elabo-
rate construction behaviors, the central place foraging, and
food storage make it difficult to separate out the causal
agents. No other socially monogamous rodent shows the
same degree of construction activities or foraging and food
storage behavior. Territorial behavior by both sexes of bea-
vers may have evolved from a mate-guarding strategy, but it
also may be a resource defense strategy (both food and the
physical family area). However, the available evidence sug-
gests that male mate-guarding behavior was important in
the evolution of monogamy in beavers.Problems and Future ResearchThroughout this chapter I have attempted to indicate areas
of concern where data were equivocal or inconsistent. Here
I summarize a few major problems and suggest potential ar-
eas of research. Since both species of beaver have large geo-
graphical ranges and live in a variety of freshwater habitats
it is difficult to generalize about social behavior, in that so-
cial systems might vary with habitat conditions (Busher and
Jenkins 1985; Buech 1987, 1995; Sharpe and Rosell 2003).
For example, a comparative behavioral study of beavers liv-
ing along a large river in the southeastern United States or
the Central Valley of California with those living at more
northern latitudes may provide insight into the evolution
of the beaver social group. Certainly river-dwelling beavers
should engage in less construction behavior and partition
the shoreline resource differently than beavers on small
streams or in ponds. If male investment is critical for the
reproductive success of the female, how does a male on
a large river system invest in the family? Additionally, if
movement is not restricted by ice or food availability, what
mechanisms keep the male from deserting his mate for ad-
ditional mating opportunities? Perhaps in these situations
female aggression could restrict the ability of the adult male
to seek out other mates; alternatively, the pair bond could
be maintained by mate-guarding. Additional data on female
dispersion and male access to multiple versus single mat-
ing partners would provide a better understanding of the
role of male mate-guarding in beaver monogamy. Specific
socioecological conditions could stimulate more individualSocial Organization and Monogamy in the Beaver 289