Rodent Societies: An Ecological & Evolutionary Perspective

tuses were found adjoining one female neighbor at the head
of the uterine horn significantly more than expected. These
results led us to conclude that female capybara overproduce
embryos and subsequently adjust their litter size and sex ra-
tio downward. In the different circumstances of the Vene-
zuelan llanos, Herrera (1998) suspected a different system,
whereby females were more likely to resorb male embryos
likely to be born in the dry season.
Female capybaras are fertile throughout the year, but
there is strong evidence for seasonal peaks in reproductive
effort. This seasonal component is not typical of most trop-
ical mammalian species, which give birth at any time of
year, showing only slight peaks in reproduction (Bronson
1989). The departure of capybaras from this generalization
might be explained by the seasonality of flooding and there-
fore the availability of food and shelter, and the risk of pre-
dation. Moreira and Macdonald (unpublished) described
how the timing of these seasonal peaks in reproduction
varies according to area. For example, in Venezuela (Ojasti
1973) and the Brazilian Pantanal Matogrossense (Schaller
and Crawshaw 1981) the peak is at the end of the rainy sea-
son, whereas on Marajó Island it coincides with the onset
of the rainy season. The explanation probably lies in the
pattern and severity of the floods, which are less extreme
in Marajó than in the llanos (where Azcarate et al. [1980]
found a high rate of newborn mortality due to drowning in
the wet season). Indeed, in the Llanos, dry shelters (offered
by banco habitat) were essential for the survival of new-
born, and the availability of dry land during the peak wet
season was related to capybara group sizes (Herrera and
Macdonald 1989). In this habitat, the risk of drowning di-
minishes only as the floods subside, in turn making the flush
of vegetation accessible in many areas. A further step along
this continuum is illustrated by the natural savannahs of
central Brazil, where there is no flooding at all (Moreira
et al. 2002).
These are adaptive explanations for seasonal breeding,
sensu Rutberg (1987) in tropical ruminants. The onset of
the rainy season is swiftly followed by the appearance of
green foliage, especially grasses (Talbot and Talbot 1963).
Grasses account for approximately 70% of capybara diet,
depending on the period of the year (Escobar and González-
Jiménez 1976). Therefore, coincidence between the birth
of capybaras and a season when forage is available is obvi-
ously advantageous for the survival and growth of the young
and the survival and future reproductive success of the fe-
male (fig. 33.3). Furthermore, reproductive synchrony may
reduce predation in a number of ways (Estes 1976; Bertram
1978). Capybaras do not burrow but benefit from aspects
of sociality for predator defense (see Ebensperger and Cofre
2001). Members of larger groups may also benefit from di-
lution (Hamilton 1971), confusion, and enhanced vigilance

(Yáber and Herrera 1994). Lastly, Macdonald (1981a) re-

ported that allosuckling is common within capybara

groups. This may increase efficiency through cooperation,

since females share the costs of nursing the young. In turn,

this provides another selective force for birth synchrony in

capybaras.

The mara’s social system embraces not only social mo-

nogamy (there are no genetic data) but, often, communal

denning as well (figs. 33.4, 33.5), a combination otherwise

unknown among mammals (Taber and Macdonald 1992a).

Communal denning is taxonomically widespread among

mammals and is often associated with cooperative breed-

ing, defined as situations where individuals other than the

parents provide care in rearing the young. It has been re-

ported in both suborders of the Rodentia; for example, Sci-

urognathi: prairie dogs, Cynomysspp. (Hoogland 1979a)

and Alticola strelzowi(Eshelkin 1976); Hystricognathi:

naked mole-rats, Heterocephalus glaber(Jarvis 1981), and

most of the cavies (e.g.,Cavia aperea, Galea musteloides,

andMicrocavia australis;Rood 1972; Weir 1974: Asher

Social Organization and Resource Use in Capybaras and Maras 399

Figure 33.3 Capybara young. Capybaras are born after a long gestation of

over 5 months, and attain sexual maturity after more than 1 year. Photo by

D. W. Macdonald.

Figure 33.4 Adult mara grazing. Photo by D. W. Macdonald.