was found to evolve in the absence of male parental care and
in species where females occupied small, exclusive ranges
(Komers and Brotherton 1997).
Beavers appear to exhibit both social and genetic mo-
nogamy since they mate exclusively with one partner (al-
though in most cases this has yet to be confirmed by DNA
analysis), have a high degree of biparental care of the young,
and the pair bond is maintained for multiple years. Beavers
are also considered to exhibit obligate monogamy based on
the extent of male parental care and lack of behavioral di-
morphism (Kleiman 1977; Sharpe and Rosell 2003). Many
aspects of beaver ecology and behavior support the for-
mation and maintenance of the monogamous pair bond.
Among these are territorial defense, relatively slow matura-
tion of young (approximately 2 years to sexual maturity),
and presence of older family members living with the fam-
ily group (Kleiman 1977).
A beaver group defends a territory, which can be a
length of shoreline on a large river or lake or an entire sec-
tion of a smaller stream (including ponds created by dam
construction), and this territory includes their lodge(s),
dams, and winter food cache. In many northern latitudes
there is reduced plant productivity during the winter when
the ponds and streams freeze, restricting beaver mobility
and access to food. In this type of climate a monogamous
mating system may help ensure access to a mate (breeding
generally takes place in January –February and the estrus
period is short). Beavers illustrate all the criteria for long-
term pair bonds (Wilson 1975), and the beaver mating sys-
tem represents an ideal model for investigating the evolu-
tion of monogamy.
Past reviews of monogamy in beavers have relied pri-
marily on a few studies of the Eurasian species (Eisenberg
1966; Kleiman 1977), but more recently a number of re-
searchers in North America have investigated beaver social
behavior. These studies have generally examined the eco-
logical constraints, primarily habitat quality (Buech 1987;
Smith 1997), the amount of parental care that influences
beaver social organization (Svendsen 1980b; Busher 1980;
Busher and Jenkins 1985; Buech 1987; Woodard 1994), or
both (Sun 2003). Rosell and Pedersen (1999) summarized
the social behavior of the Eurasian species. However, at
present no comprehensive analysis of the social organiza-
tion of both beaver species has been undertaken.
This chapter will review the past and current knowledge
of the social organization of the two extant beaver species
and critically evaluate the beaver social group in light of the
evolutionary and ecological constraints that shape the mat-
ing system.The Beaver Social GroupTypical group composition
The beaver social group is commonly called a colony, re-
gardless of the fact that it does not fit the precise sociobio-
logical definition of a colony (Wilson 1975). The idea of
a beaver colony is believed to have originated with the Eu-
ropean fur trappers and explorers (Morgan 1868; Taylor
1970). However, modern quantitative studies document the
beaver social group as being a family group.
In one of the first quantitative studies of the beaver social
group Bradt (1938) analyzed forty-two groups in Michigan
that were completely trapped out and the beavers assigned
to sex and age classes. He reported that 59.5% (25/42) con-
sisted of an adult pair (male and female) with or without
young and 45% (19/42) consisted of an adult pair with at
least one litter. This led Bradt to conclude that “A typical
beaver colony is shown... to consist of an individual fam-
ily, including the parents, the kits, and often the yearlings
born the previous year.” Throughout the beaver’s range in
North America authors have consistently documented the
family group as the unit of social organization, and have
found the mating system to be strictly monogamous (Hall
1960; Taylor 1970; Hodgdon 1978; Busher 1975, 1980,
1987; Svendsen 1980b, 1989; Buech 1987; Payne 1982;
Patenaude 1983; Peterson and Payne 1986; Wheatley 1994,
1997; Woodard 1994). Data on the composition of family
groups with adult pairs and groups with adult pairs and
young from selected studies illustrate the variation among
populations as well as the general pattern of groups with
adult pairs (fig. 24.2).
Studies of Eurasian beavers in Russia, The Netherlands,
and Sweden also support the definition of the beaver social
group as a family, with the adults exhibiting mating exclu-Social Organization and Monogamy in the Beaver 281Figure 24.1 A mated pair of Eurasian beavers (Castor fiber). The male is on
the left with the right eartag and the female is on the right with the left eartag.
The pair had been together at least eight years when the picture was taken, and
were 15-16 years old. These beavers were reintroduced into the Netherlands in
- Photograph and age data provided by Frank Rosell.