rapidly exhausted. Rain brings regrowth of grasses and
forbs in the scrub, where the dispersion of sparse grazing
forces maras towards isolation. While their ranges drift, ma-
ras are constrained for other reasons (the benefits of com-
munality and reduced predation risk, discussed earlier), to
remain in the vicinity of the clearing and the outstation. As
the next summer advances, regeneration time of fallowed
grazing patches increases as vegetation becomes parched, at
which point the pairs of maras follow the sheep (and their
dung) to the lagoons /corrals, where the flush of grass is
more attractive. According to this model, where resource
dispersion alternates between favoring separation and con-
gregation, the size of settlement memberships will be de-
termined by whichever is the narrower seasonal bottleneck
in resource availability. Due to the apparent uniformity of
scrub and the variability in features of the outstations, and
the fact that mara settlements develop only near lagoons /
outstations, it seems likely that qualities of the dry season
resources, in terms of lagoon-outstation patch richness, are
a good determinant of settlement size.
Summary and Conclusions
Two large, unusual, South American rodents, the capybara
and mara, have complicated societies that reveal both inter-
and intraspecific variation that appears to be related to
the extremes of resource dispersion that they face. We pro-
pose that these species provide fertile models for testing
relationships between patterns of resource availability and
sociality.
Capybaras live in groups, often in seasonally flooded
habitats. Changes in availability of favorable grassland,
caused by the alternation of drought and flooding, result
in shifting habitat use, but their home ranges remain stable
and territorially organized. Territory size is positively corre-
lated with group size and, within their home range, areas
of key habitat types are also correlated with group size.
Capybaras depend upon access to permanent surface water;
where there is such water, it appears that their territories
are configured to encompass sufficient resources to ensure
survival under widely differing seasonal conditions.
Maras face extremes of resource dispersion between the
wet and dry seasons, with associated pressures that lead to
a different social system. In the wet season, sparsely dis-
persed grazing and interference competition favor spacing
out, leading to the formation of monogamous pairs. In the
dry season, clumping of resources facilitates pairs congre-
gating in settlements around outstations and dry lagoons.
Superimposed upon the ecological factors favoring spac-
ing out during the wet season are the sociological factors
that cause maras to den communally — for example, pro-
tection from predators through increased vigilance, and bet-
ter thermoregulation. The resulting compromise is a social
system so far unique among mammals.
The Resource Dispersion Hypothesis proposes that the
spatiotemporal dispersion and richness of resources can
lead to the passive formation and maintenance of groups,
even in the absence of any direct benefits of group living per
se. Put simply, where resources are patchily distributed, ter-
ritory owners must defend an area that guarantees mini-
mum food security. However, much of the time this mini-
mum food security is likely to be exceeded, in which case
additional animals can join the territory with low com-
petition costs, forming a group. Additional factors, notably
predator defense, also clearly affect the functional signifi-
cance of group living. In both species, competition arising
from group living would seem to be lower among females
than males, and resource dispersion may particularly favor
female sociality. Other issues, such as an inability to chase
other males away, kin selection, and sharing in the defense
of the territory may be playing a role in male tolerance of
each other (concepts such as peace incentivesand staying
incentives,as modeled by Kokko and Johnstone (1999),
may prove relevant). Capybaras offer an interesting subject
on which to test such ideas. Indeed, within groups of capy-
baras there is a dominance hierarchy, which seems to func-
tion much as modeled by Kokko and Johnstone (1999),
while females are grouped because they can share resources
and protect themselves against predators. Although doubt-
less this is only one factor among many that affects the
function of their societies, despite their contrasting life his-
tories, we suggest that resource dispersion provides a frame-
work for explaining why capybaras live in groups and why
monogamous maras form settlements. Indeed, as Johnson
et al. (2002) argue, the Resource Dispersion Hypothesis
lays a foundation for considering the sociological factors
that shape mammalian societies and the underlying influ-
ence of ecological conditions on social organization.402 Chapter Thirty-Three