sivity and defending a territory against other beavers (Wils-
son 1971; Djoshkin and Safonov 1972; Nolet and Rosell
1994; Hartman 1994, 1995; Rosell and Pedersen 1999).
The extended family group
While the majority of studies of both exploited and unex-
ploited populations document the family as the most en-
countered social group, variations in group composition
have been reported. Additional older individuals (2-year-
olds or older) have been observed living in family groups
(Hay 1958; Rutherford 1964; Novakowski 1967; Leege
and Williams 1967; Taylor 1970; Bergerud and Miller
1977; Hodgdon 1978; Payne 1982; Busher et al. 1983;
Smith 1997; Busher and Lyons 1999; Müller-Schwarze and
Schulte 1999; fig. 24.3). Most authors believed that these
additional adult beavers are older offspring that have de-
layed dispersal, which usually occurs the spring or sum-
mer when they are 2 years old (Jenkins and Busher 1979;
Müller-Schwarze and Sun 2003).
In high-density populations from California to Canada
between 22.2% to 87.5% (mean 38.2%) of families had
additional adults, while in low-density populations the
range was 0.0% to 12.5% (mean 5.2%). In high-density
populations young adult beavers are less likely to disperse
from the natal colony and more likely to remain living with
their family (Müller-Schwarze and Schulte 1999; see also
Nunes, chap. 13 this volume). Delayed dispersal appears to282 Chapter Twenty-Four
Figure 24.2 Percent of beaver groups that consisted of a mated pair of adults and the percent of those groups that consisted of a mated pair of adults with
young. Svendsen (1989) did not report the percentage of groups consisting of a pair plus young. The Busher and Lyons data are from an unpublished research
report for The Center for Field Research–Earthwatch (October 1988).
Figure 24.3 A group of subadult Eurasian beavers (Castor fiber) in Norway,
feeding together near the lodge. These individual are 2 –3 years old and have
not dispersed from the family territory, most likely due to high population den-
sity in the area. Photograph and data provided by Orsi Bozér.