Beaver Natural History
The Eurasian beaver (Castor fiber) and the North American
beaver (Castor canadensis) are the only two extant mem-
bers of the family Castoridae. The Eurasian beaver is re-
stricted to the Palearctic biogeographical region, while the
North American beaver, which is native to and widespread
in the Nearctic region, was introduced into the Palearctic
region (Europe and Russia) in the past 100 years (Nolet and
Rosell 1998; Hartman 1999). North American beavers have
also been introduced into Tierra del Fuego, Argentina (Liz-
arralde 1993).
The two species are morphologically similar (Lavrov
1983; Jenkins and Busher 1979; Sieber et al. 1999) and
have comparable habitat requirements and behavior (Wils-
son 1971; Jenkins and Busher 1979; Hill 1982; Hodgdon
and Lancia 1983; Müller-Schwarze and Sun 2003). For ex-
ample, both species are primarily nocturnal, semiaquatic
(living along all types of freshwater systems), strictly her-
bivorous, and typically live in family groups containing an
adult pair, young of the year (kits) and young from the pre-
vious year (yearlings). Beavers of both species may con-
struct lodges made of mud and sticks and dams that create
ponds, which stabilize the environment. Beavers living on
large lakes and rivers do not build dams; introduced North
American beavers in Russia are more active in construction
behaviors than Eurasian beavers (Danilov and Kan’shiev
1983; Danilov 1995). Both species communicate by tail slap-
ping, scent marking, and through vocalizations and posture.
While the two species are similar in size and appearance
they do have different numbers of chromosomes (C. cana-
densis 2 n40;C. fiber 2 n48) and do not reproduce with
each other (Lavrov and Orlov 1973; Jenkins and Busher
1979).The Beaver Social Group and Mating System:
An OverviewThe two beaver species are in the small percentage of mam-
mals (3% to 5%) and even smaller percentage of rodents
that form monogamous pair bonds (Kleiman 1977; Dunbar
1984; fig. 24.1). Two types of monogamy are generally con-
sidered: (1) genetic (exclusive) monogamy, when the male
and female confine their mating exclusively to the same part-
ner, and (2) social (biparental) monogamy, when the part-
ners maintain a close association after fertilization and care
for the young, but mating may not be exclusive (Barlow
1988; Westneat and Sherman 1997; Reichard 2003).
When male care is critical for reproductive success mo-
nogamy has been considered obligate; when it is less critical
and males will readily mate with additional females it has
been considered facultative (Kleiman 1977; Wittenberger
and Tilson 1980; Clutton-Brock 1989b). However, viewing
monogamy as either obligate or facultative, while useful,
has been questioned (Komers and Brotherton 1997; Smith
1997; Sun 2003). For example, the dik-dik, a dwarf ante-
lope, is considered to exhibit facultative monogamy, yet
while the male provides no parental care he also does not
attempt to mate with additional available females (Brother-
ton and Rhodes 1996; Komers and Brotherton 1997). Ad-
ditionally, monogamy in a broad array of mammal groups