(Cephalophus monticola), and Kirk’s dikdik (Madoqua kir-
kii) all pair bond and live in much smaller ranges (10 ha)
compared to maras (as reviewed in Taber and Macdonald
1992b). None of these species has to cope with abundant
sheep or the need for scarce shared pupping dens. The maras’
pupping dens are large, generally with multiple entrances;
their scarcity suggests they may be a limiting resource, but
we have no evidence on how difficult they are to dig.
Social Structure: Monogamy versus Group Living
Taber and Macdonald (1992b) observed and radio-collared
wild maras over 3 seasons on the Valdés Peninsula. They
found that maras’ social units could be easily distinguished
by the close proximity of their members (within 15 m of
each other) and the wide spacing between them (usually
100 m). Of 342 independent sightings of such units, 65%
consisted of a male-female pair, and, apart from aggrega-
tions, groups of more than three pairs were never seen. In
short, most maras in the studied population lived in mo-
nogamous pairs.
Monogamy, having one social mate at a time, is in itself
a relatively rare mating system among mammals (Crook
1977; Kleiman 1977; Clutton-Brock 1989b; Waterman,
chap. 3 this volume). A number of hypotheses have been
put forward concerning the adaptive significance of mo-
nogamy in mammals (e.g., sequestering females, indivisible
paternal care; Kleiman 1977, 1981; Dunbar 1984; Zeveloff
and Boyce 1980; Wittenberger and Tilson 1980). Monog-
amy may be favored by patterns of resource dispersion that
determine that the carrying capacity of a territory is insuffi-
cient to support additional breeding females (e.g., gibbons,
Hyoblates klossi;Tenaza 1975; see also Kleiman 1977).
This finding is supported by Komers and Brotherton’s (1997)
model, which showed that female dispersion was the main
factor favoring monogamy in mammals. This assertion is
exemplified by Hendrichs and Hendrichs (1971), who con-
cluded that defense of rich, widely dispersed food patches
precluded larger groups of dik diks. Taber and Macdon-
ald (1992b) suggest that, because the grasses and forbs on
which maras feed are swiftly depleted, slow to replenish,
and patchily dispersed, interference competition forces ma-
ras to avoid each other; the paucity of the resources and
their dispersion militates against social units larger than a
pair. Further, synchrony in births suggests that most fe-
males come into estrus (which lasts for only a few hours) in
relative synchrony during the wetter season, when more
forage is available throughout the thorn scrub and maras
are at their most dispersed. Rather than solitary territorial-
ity, though, the spatial organization and movement patterns
imposed by the mara’s diet, and the brevity of the females’
sexual receptivity, means that males probably reproduce
most successfully by monopolizing one female. Males do
not care for the young directly, but they do save time for the
foraging female by being vigilant. Furthermore, with few
opportunities for promiscuous mating, males may maxi-
mize their reproductive success by guarding their young
and their mate from predation.
The mara’s social system is strikingly different from that
of other large caviomorphs such as the capybara. The only
other caviomorph rodent (out of 188 species; Macdonald
2001) that is reported to live in pairs is the agouti (Genus
Dasyprocta;Smythe 1978). Agoutis live in territorial pairs
in dense tropical rainforests, and differ markedly from ma-
ras in that the paired animals are rarely together and live an
essentially solitary existence (Kleiman’s [1977] facultative
monogamy). Interestingly, the mara demonstrates a mor-
phological convergence with some ungulates, with its long
legs and body and its reliance on speed and crypsis to es-
cape from predators (some of these, such as dik diks and
duikers, are also pair-living). Most strikingly, stotting, char-
acteristic ungulate escape behavior otherwise unknown in
other orders (Caro 1986) is part of their predator aversion
repertoire. Although polygyny is common in ungulates, five
species are known to be monogamous, and probably repre-
sent convergence with the circumstances of maras. (These
fall into a category between small solitary ruminants of
dense forests and generally larger, group-living ungulates
of open savannahs). However, the mara’s drifting range dis-
tinguishes it from the monogamous ruminants, which main-
tain stable, exclusive territories. This difference may arise
as a compromise necessitated by the mara’s communal den-
ning behavior, which forces pairs into closer contact than
foraging considerations alone would dictate.
By contrast, capybaras live in mixed-sex groups that
typically comprise a dominant male, one or more females
(which are probably kin), several infants and young, and
one or more subordinate males. At El Frio ranch in Vene-
zuela, group sizes ranged from four to sixteen individuals
(Herrera and Macdonald 1987, 1989), while on a ranch
just 50 km away in the same general ecosystem (El Cedral
ranch) groups ranged from eight to twenty-five individuals
(Salas 1999). The differences between the Salas (1999) and
Herrera and Macdonald (1987, 1989) studies seem to be
related to resources: El Cedral had a more homogeneously
distributed, more abundant, and less seasonally variable
resource base than El Frio. In both study areas, there is a
well-defined dominance hierarchy among the males, main-
tained by aggressive interactions that usually take the form
of simple chases. Dominant males are heavier and obtain a
greater proportion of matings than do subordinates (He-
rrera and Macdonald 1993; Salas 1999). Females are much
more tolerant of each other, although the precise details of
their social relationships, hierarchical or otherwise, are un-
known. The membership of these groups appears to be quite396 Chapter Thirty-Three