Rodent Societies: An Ecological & Evolutionary Perspective

stable, with few changes occurring, even across years (Her-
rera and Macdonald 1987). This stability is comparable to
that of several territorial species of primates and carnivores
(e.g., Hrdy 1977b; Rasa 1985).
Why do capybaras live in groups? The niche occupied
by the capybara has much in common with those of many
species of tropical ungulates. Like them, the capybara is a
relatively large grazer and is subject to predation on open
plains. Capybaras are, or have been in the recent past, prey
to puma, jaguar, feral dogs, caiman, and man. Macdonald
(1981a) described how a group of capybaras, under attack
by feral dogs, bunched together with youngsters within a
protective cordon. There are three reasons why capybaras
might form groups in response to predation: (1) increased
vigilance to forestall attack, (2) increased possibility of in-
timidating the predator through greater strength of num-
bers, and (3) the selfish herdeffect (Hamilton 1971), in
which individuals lower their risk of capture by hiding in
the group. Yáber and Herrera (1994) found that total vigi-
lance (number of heads up per hour) increased with group
size whereas individual vigilance (number of times an ani-
mal would lift its head per hour) decreased. Also, subordi-
nate males made the greatest contribution to vigilance and
paid the higher cost in terms of frequency of head-up be-
havior. Indeed, Ebensperger and Cofré (2001) suggest that
capybara sociality may be related to predation, whereas in
other caviomorphs it is more related to cooperation in bur-
rowing and limited burrowing sites. Apart from defense
against predators, it is also possible that aggregation by her-
bivores may help maintain a high quality and quantity of
forage (McNaughton 1984).
Despite the differences in social structure, both capy-
baras and maras are, at times, gregarious feeders. In the dry
season, capybara groups coalesce around dwindling pools,
forming temporary aggregations of 100 or more animals.
When the wet season resumes, these large aggregations
split up into the original groups that formed them (Herrera
and Macdonald 1989). Similarly, despite their monogamy,
as many as twenty-five pairs of maras often form aggrega-
tions, known as settlements.Taber and Macdonald (1992b)
found pairs in a settlement maintained proximity to clear-
ings and lagoon beds. Settlement members also invariably
chose sites adjoining a shepherd’s outstation with a water
trough, corral, and a shepherd (the latter perhaps having
an impact on predator distribution and behavior). The au-
thors suggest that, although the lagoons are dry during the
dry season, the vegetation around them would have had the
most recent access to water and be closer to the water table.
Furthermore, in the study area, sheep congregated around
the dry lagoons; their nitrogenous waste in the form of dung
may have resulted in faster regeneration and /or higher nu-
tritive value of the maras’ forage. Taber and Macdonald
(1992b) also suggested that maras might den near outsta-

tions so that their young can benefit from the reduced num-

ber or increased timorousness of predators there. This ben-

efit could be enjoyed where food resources (possibly also af-

fected by the shepherd’s activities) allowed maras to cluster

around dwellings. In this context, Sunquist and Sunquist

(1989) noted that, perhaps in a similar way, the aggrega-

tions of chital deer (Axis axis) around park dwellings at

dusk afforded the deer protection against predators.

Importance of Olfactory Communication

A conspicuous feature of the sociality of both capybaras

and maras is scent marking, but it is far more frequent in

capybaras (Herrera and Macdonald 1994). Capybaras

have two kinds of scent gland (Macdonald et al.1984). The

first, highly developed in males but almost nonexistent in

females, is located on top of the snout and is a dark, oval

shaped, naked protrusion known as the morrillo(fig. 33.2).

The second is found in both sexes and takes the form of two

glandular pockets located on either side of the anus. Male

anal glands are filled with easily detachable hairs abun-

dantly coated with layers of hard, crystalline calcium salts

(Macdonald et al.1984). Female anal pockets also have

hairs, but theirs are not detachable and are coated in a

greasy secretion rather than with crystalline layers. The

proportions of each chemical present in the secretions of in-

dividual capybaras are different, providing a potential for

individual recognition via personal olfactory fingerprints.

Capybara males scent mark more and have more ag-

gressive interactions than females (Herrera and Macdonald

1987). It appears that the snout scent gland is mainly in-

volved in signaling dominance status, while the anal gland

may be important in group membership recognition and

perhaps territoriality. Larger groups had lower scent mark-

ing rates than did smaller groups, not only as a unit, but

Social Organization and Resource Use in Capybaras and Maras 397

Figure 33.2 Scent marking. A male deposits a white sticky secretion from its

morrillo. Photo by D. W. Macdonald.