2004). Generally, communal denning species that behave
alloparentally are polygynous, or reproduction is confined
to one female and one male in the group. The mara’s social
system is in marked contrast and, so far, appears to be
unique among mammals.
Maras are socially monogamous due to the effect of
sparse, patchily dispersed grazing and the risk of interfer-
ence competition; nevertheless, it is advantageous for pairs
to become gregarious during the breeding season. Com-
munal denning in this way may have a range of advantages
(Taber and Macdonald (1992a). First, there is cooperation
in terms of group defense and vigilance. At warrens with
larger membership, more adults were present for more of the
time, and therefore total vigilance was greater at larger crè-
ches, even though each pair spent less time there (fig. 33.6).
Adults therefore had more time to forage over wider areas
away from the den. Older pups were also active in increas-
ing the corporate vigilance. Second, female maras were seen
to nurse young other than their own. However, it is not
clear whether this represented communal suckling or milk
theft, since, in contrast to capybaras, they seemed anxious
to repel usurpers. Third, coloniality may play an important
role in thermoregulation. Taber and Macdonald (1992a)
found that mortality of pups was inversely related to crèche
size. Patagonia is at the southern end of the species’ range,
and nightly minimum temperatures during the pupping sea-
son fall to less than 5 oC; the shared body heat of larger
crèches may be critical to the survival of newborn pups.
Warrens housing fewer than ten pups over the season might,
at any one time, have insufficient corporate warmth to sur-
vive a colder-than-average night. Consistent with this, in the
warmer northern part of the species’ range, large denning
settlements have not been reported. The unusual breeding
system of the mara may thus be a compromise, conferring
400 Chapter Thirty-Three
Figure 33.5 Occasionally two pairs of maras try to approach the communal
den simultaneously, with resulting hostility. Here, the males of two rival pairs
engage in a standoff to decide precedence in the queue to retrieve their young
from the crèche. Photo by D. W. Macdonald.
Figure 33.6 A crèche of young maras at a communal den. Photo by
D. W. Macdonald.on the pups the benefits of coloniality in an environment
wherein association between pairs is otherwise apparently
disadvantageous (fig. 33.5).
The mara’s blend of monogamy and communal denning
has loose analogues among certain birds. Up to four pairs of
insectivorous groove-billed anis (Crotophaga sulcirostris)
share the tasks of building a communal nest, incubation,
and rearing young, and may thereby reduce adult mortal-
ity during incubation (Vehrencamp 1977, 1978). The clear-
est example of stable bonds between different pairs within
freely mixing groups occurs in the black vulture (Coragyps
atratus). Coalitions of several nuclear families occur to-
gether consistently at roosts and carcasses, and apparently
cooperate to compete over food (Rabenold 1985), while
pairs remain strictly monogamous (Decker et al. 1993).
The glossy black-cockatoo (Calyptorhynchus lathami hal-
maturinus), combines group living with long-term monog-
amy, with large groups nesting together at night, then dis-
persing as mated pairs or trios of the mated pair and a
juvenile to feed (Pepper 1996). Crook (1964) described six
insectivorous species of socially monogamous weaverbirds
(Ploceinae) that nested colonially but were constrained to
forage in pairs due to the disadvantages of interference com-
petition in larger flocks. Colonial nesting may arise (1) with-
out advantages from sociality per se, due to aggregation at
limited nest sites or safe places (Ploceus pelzelninormally
nests solitarily but forms colonies in isolated mangrove
bushes), or (2) because of the advantages of sociality (Buba-
lornis albirostrisbuilds thorny fortress nests). By analogy,
sociality is incidental to the advantages to maras of aggre-
gating near resources at sheep stations where, for example,
predation may be low. Similarly, communal denning may
have arisen without the benefit of sociality if den sites
were limiting (as may occur in rabbits [Cowan and Garson