on bajios throughout the year, but particularly in the dry
and early-wet seasons; animals spend less time in this habi-
tat as it becomes more deeply flooded late in the wet season.
Bancos are virtually unused until the peak of the rains,
when deep flooding and displacement of the best grasses by
reed and water hyacinth force the animals up to these
higher, bushier areas, which become the only dry land
available. These seasonal shifts in habitat use do not involve
migratory movements. On the contrary, in their search for
dry land in the wet season, capybaras rarely move their cen-
ter of activity more than 300 m from their dry season wal-
low. Because capybaras are confined to the vicinity of water
their distribution is restricted, and the nutritious grazing,
together with an apparent paucity of competitors, facilitates
high local population densities. Home ranges are therefore
relatively small (10 ha for a ten-adult strong social group)
compared to those of other similarly sized tropical herbi-
vores (c.f. Eisenberg 1981).For instance, the density and
biomass of capybaras within an average home range is
roughly ten times greater than those of the Bohor reedbuck
(Redunca redunca,Hendrichs 1975). This species is perhaps
the closest equivalent of capybaras in terms of size (30 –
50 kg) and habit (grazing herbivore of tropical marshes in
Africa), but the reedbuck shares its habitat with as many as
ten sympatric species of grazing antelope.
Capybaras did not substantially alter the size or juxta-
position of their home ranges between years (Herrera and
Macdonald 1989), in contrast to the large-scale movements
of some other tropical, mammalian grazers living in sea-
sonal regions (e.g., Leuthold 1977; Maddock 1979). The
dispersion of group ranges showed minimal overlap be-
tween neighboring home ranges and an effectively contigu-
ous blanket of home ranges throughout the study area. Her-
rera and Macdonald (1989) found that home range size
was significantly correlated with both the number of adults
in the group, and the total group size. They argued that this
trend was related to the habitat composition of the home
ranges, with larger home ranges also having greater areas of
bushy banco and bajio. Furthermore, the total area of
bushy banco in each home range was also highly correlated
with the number of surviving young per female.
The conclusion that groups are territorially organized is
supported by Herrera and Macdonald’s (1987) observa-
tions of the expulsion of intruders — principally males, ex-
cluded by males. Analyses of territory economics have
shown that for territoriality to evolve, there must be some
critical resource that makes a territory economically de-
fendable; that is, the costs of defense are lower than the
benefits thereby obtained. The territoriality of capybaras
may be explained by their dependence on water. Water sus-
tains their food plants, and capybaras also thermoregulate
in water, mate in water, and seek refuge from predators in
water (Ojasti 1973; Azcarate et al.1980; Macdonald
1981a; Herrera 1986). As the dry season progresses, lakes
that form the flooded landscape contract, becoming little
more than puddles. These remaining pools and the associ-
ated food become limiting and highly patchy resources. It
seems likely that it is the need to guarantee access to a per-
manent dry-season lagoon that favors territoriality; Her-
rera and Macdonald (1989) found that every territory did
indeed contain a water hole next to a grazing patch, mak-
ing the piece of land viable and defensible.
In their study of maras in Argentina, Taber and Mac-
donald (1992b) found that radio-collared animals also dif-
fered seasonally in their use of habitat, predominantly stay-
ing in thorn scrub in autumn and winter (the wet season),
while grazing on dry lagoon beds in summer. The daily ac-
tivities of maras on the Valdés Peninsula involved almost
continuous movement; excluding travel and search time,
maras devoted at least 36% of daylight hours to grazing on
grasses and forbs, probably due to the paucity of available
food. Mara pairs had unstable (drifting) home ranges, in
that they were continually utilizing new areas and vacating
old ones. Over a year, pairs ranged through areas of around
200 ha. While ranges overlapped substantially, utilization
differed within areas that overlapped, and pairs generally
avoided each other. The segregation of pairs seemed to arise
through avoidance, and active exclusion by males defend-
ing a 20 m space around their mate. Daily ranges were scent
marked by males using anal dragging.
It seems likely that the functional explanation of this
spatial avoidance is that the grazing of an area by one pair
renders it useless to other pairs for at least several weeks.
The grass and forbs on which maras graze are swiftly de-
pleted, slow to replenish, and, on the fine scale of foraging
movements, patchily dispersed. The need for a period of fal-
lowing before an area is reusable necessitates a shifting pat-
tern of range utilization; it is also advantageous for forag-
ing animals to avoid areas recently depleted by others.
Although these arguments would favor solitary territorial-
ity, there are countermanding pressures (see the following)
that result in a unique social compromise: maras forage in
pairs that occupy drifting territories whose members con-
vene in the vicinity of a shared breeding den.
The argument that a system of fallowing and depletion
explains drifting territoriality is based on the deduction
that a fixed territory accommodating sufficient areas of re-
cuperating vegetation would be too large to defend against
other maras, and against sheep, from whom, unlike capy-
bara, maras face considerable competition. Therefore pairs
defend a prevailing range, vacating exhausted patches and
drifting into the next fruitful one they encounter. In contrast,
the similarly sized antelopes of steppe, savannah, and open
forest, klipspringer (Oreotragus oreotragus), blue duikerSocial Organization and Resource Use in Capybaras and Maras 395