However, observations of adult males have also been made
inside lodges and add insight into male investment. The
adult male was observed being as active as the adult female
in mutual grooming within the lodge, although the adult fe-
male was more apt to initiate mutual grooming with the
adult male (Patenaude and Bovet 1984). During parturition
the adult male and yearlings were actively involved in
preparation of the lodge, and physically formed a protective
enclosure around the newborn beavers. However, the adult
male was less active in ingestion of the placenta than the
adult female, and less active in licking the newborn beavers
(Patenaude and Bovet 1983).
Ecological and Evolutionary Considerations
and the Beaver Family Group
Reviews of monogamy in mammals by Eisenberg (1966),
Kleiman (1977), Wittenberger and Tilson (1980), and Clut-
ton-Brock (1989b) have all summarized conditions under
which specific mammal groups exhibit a monogamous mat-
ing system. Additionally, the recent volume edited by Re-
ichard and Boesch (2003) examines the evolution of social
monogamy in numerous animal groups. The two beaver
species are often cited as examples of socially (if not socio-
genetic) monogamous mammals because they defend terri-
tories, invest in their territories by the construction of dams
and lodges, have young with an extended period of matu-
ration, and the adult male exhibits a relatively high degree
of both direct and indirect parental care.
The specific type of mating system is an outcome of the
individual reproductive interests of both the adult male and
female, and these interests in reproduction are usually dif-
ferent (See also Waterman, chap. 3 and Solomon and Keane,
chap. 4 this volume). Thus what is reproductively advanta-
geous for one sex will pose constraints on how the other sex
will mate. Three preconditions that may be necessary for
the evolution of monogamy are: (1) the female must gain
unique benefits from the pair bond, (2) the female must en-
sure and be certain of male fidelity, and (3) males must re-
main with the female, and that males will gain no reproduc-
tive advantage by deserting the female (Wittenberger 1979;
Wittenberger and Tilson 1980).
Five hypotheses for the evolution of monogamy that are
appropriate for beavers are: (1) indispensable, nonshareable
male parental care is critical for female reproductive suc-
cess, (2) shareable (not indispensable) male parental care is
important for female reproductive success, and the female
gains no advantage through a polygynous mating system,
(3) female aggression constrains the male from additional
mating, (4) monogamy evolved as a male mate-guarding
strategy, and (5) female dispersion does not allow a male to
monopolize more than one female (Wittenberger 1979; Wit-
tenberger and Tilson 1980; Clutton-Brock 1989b; Brother-
ton and Rhodes 1996; Brotherton and Manser 1997; Kom-
ers and Brotherton 1997). Hypotheses 1 and 2 involve the
importance and degree of male parental care, while hypoth-
eses 4 and 5 relate to male mate-guarding and female dis-
persion. These hypotheses, selected beaver life-history traits,
and supporting references are summarized in table 24.2.Hypothesis 1
This hypothesis, which stresses the critical role of male
parental care, is the traditional hypothesis suggested to ac-
count for beaver monogamy (Eisenberg 1966; Kleiman
1977). The primary argument is that male care is indispen-
sable and that females will have less reproductive success
without male parental investment. Data on subadult mor-
tality rates for transplanted beavers and nontransplanted
populations tend to support this hypothesis, since subadult
mortality in transplanted beavers was found to be 100%,
but only 50% or lower in nontransplanted animals (Henry
and Bookhout 1969; Gunson 1970; Taylor 1970; Payne
1984; McKinstry and Anderson 2002). Lower subadult
mortality may result from protection provided in the parent
colony, and relocation appears to disrupt the family struc-
ture. A time-budget analysis of a Norwegian Eurasian bea-
ver population found no behavioral dimorphism between
mated adult males and females, and male care was consid-
ered critical for successful reproduction (Sharpe and Rosell
2003).
However, a time budget model of parental investment
in North American beaver families living on a large lake in
Minnesota did not support hypothesis 1. Male investment
was not considered “required,” and the model suggested
that a female could potentially raise at least one kit by her-
self (Buech 1987). Additionally, the model indicated that fe-
males living in high-quality habitats with preexisting struc-
tures (lodges and dams) and possibly help from younger
animals could successfully raise more than one offspring per
litter. Most studies of beaver behavior indicate that adult
males invest (both directly and indirectly) in the family area,
and that male care is important in maintaining the mating
system.Hypothesis 2
The second hypothesis, which states that it is more advan-
tageous for a female to be monogamous because the cost
of polygyny is too high (Verner 1964; Orians 1969) was
favored by Buech (1987) for beavers in northern latitudes.
The extended period of parental care and the investment in
lodges and dams make determining the polygyny thresholdSocial Organization and Monogamy in the Beaver 287