difficult. Additionally, a polygynous mating system would
mean more beavers living in a defined area with greater re-
source use, which would require the beaver group to move
more often. In a polygynous system the costs of moving and
new construction would be greatly increased (Buech 1987;
Sun 2003).
However, Smith (1997), also studying populations at
northern latitudes, suggested that polygyny can occur in
high density, poor-quality habitat populations, and related
it to the “choosy generalist” foraging behavior exhibited by
beavers. In this population, once the preferred food (aspen)
was depleted beavers fed on a variety of other species that
were readily available. Smith believed that in most habi-
tats there is sufficient alternative food to support additional
breeding females in a territory, and noted that beavers
within a territory are often related, and that overall fit-
ness would be increased if related females reproduced in the
same territory. While this remains largely untested, it is
interesting that while few instances of polygynous mating
have been documented, all of the cases have occurred in
northern-latitude populations.
Hypothesis 3
The third hypothesis states that female aggression prevents
males from acquiring additional mates and has been sug-
gested to play at least a partial role in the evolution and
maintenance of monogamy in beavers (Wittenberger and
Tilson 1980; Buech 1987). Female aggression has been
cited as a potential proximal mechanism for maintaining
the pair bond (Wittenberger and Tilson 1980; Buech 1987).
However, female dominance within a family has only been
demonstrated in one study (Hodgdon and Larson 1973),
and all other studies indicate that adult males and females
are codominant (see the earlier section on social behavior).
Only anecdotal evidence exists that female aggression helps
maintain the pair bond. It is difficult to find much support
for this hypothesis, and studies designed to investigate the
role female aggression might play in maintaining the pair
bond should be conducted.Hypothesis 4
The evolution of monogamy as a male mate-guarding strat-
egy (Brotherton and Rhodes 1996; Brotherton and Komers
2003) has not been specifically tested in beavers. However,
data on beaver territorial behavior, time budgets, and tim-
ing and duration of estrus may support this hypothesis.
Since beavers evolved in northern latitudes and breeding
occurs when mobility is restricted by environmental condi-
tions, male roving costs would be high and the potential for
extra-pair copulations low. Additionally, estrus lasts only288 Chapter Twenty-Four
Table 24.2 Alternative hypotheses for the evolution of monogamy in beavers
Hypothesis Supporting life history characteristics Selected references
- Indispensable, non-shareable male parental care Precocial young, long gestation and weaning periods, Wilsson 1971; Kleiman 1977; Sharpe and
is critical for female reproductive success. Implies long development to sexual maturity, territorial be- Rosell 2003; Hodgdon 1978; Jenkins and
exclusive mating. havior, food-caching, alarm behaviors, complex, Busher 1979; Hodgdon and Lancia 1983;
construction behaviors, pair is ecologically isolated Busher and Hartman 2001 provide re-
from other potential mates during reproductive period views of beaver behavior.
over much of range. - Shareable (not indispensable) male parental care Same as above. Behavior of yearling and two-year-old Wittenberger and Tilson 1980; Buech
is important for female reproductive success and beavers in the family group may provide some of the 1987, 1995. Also refer to reviews of
the female gains no advantage in a polygynous same benefits as a polygynous system. behavior above.
mating system. - Female aggression constrains the male from addi- Female dominance during pair bond formation or within Wilsson 1971; Hodgdon and Larson 1973;
tional mating. existing family group; female territorial behavior; ago- Wittenberger and Tilson 1980; Buech
nistic encounters with nonfamily members. 1987. - Monogamy evolved as a male mate-guarding Male territorial behavior, scent marking, alarm behavior, None specifically for beavers. However,
strategy. female territorial behavior. see Brotherton and Rhodes 1996; Broth-
erton and Manser 1997; Komers and
Brotherton 1997; Brotherton and Komers
2003. - Female dispersion does not allow a male to Female dispersal patterns, female territorial behavior, None specifically for beavers. However,
monopolize more than one female. Females scent marking, female aggression and dominance. see Brotherton and Manser 1997;
occupy relatively small, isolated territories. Komers and Brotherton 1997. Sun et al.
2000 and Hartman 1995 may provide
support.
SOURCE: Hypotheses 1–3 are summarized from Wittenberger and Tilson (1980). Hypotheses 4 –5 are summarized from Brotherton and Rhodes (1996), Brotherton and Manser
(1997), and Komers and Brotherton (1997). Specific cases and references are discussed in the text.