also individually. Large groups also had more subordinates
that have lower marking rates. Since larger groups defended
larger territories (Herrera and Macdonald 1989) it is likely
that the home ranges of large groups were less thoroughly
covered with scent than were those of smaller groups. Male
members of large groups tended to be more loosely asso-
ciated with their social units than were those in smaller
groups (Herrera and Macdonald 1987), and this difference
in cohesiveness of membership may also have had bearing
on the reduced marking activity of members of large groups.
The testes mass of the capybara is closely associated
with the size of androgen-dependent scent glands (Herrera
1992) and with the proportion of nonspermatogenic tissue
in the testes. This suggests that testes size in capybaras is
more associated with androgen-dependent chemical signal-
ling than with sperm production (Moreira, Clarke, and
Macdonald 1997, Moreira, Macdonald, and Clarke 1997).
Although capybaras do not burrow, their partly noctur-
nal habits may increase the importance of olfactory signals.
Overall, the pattern appears to be of female-defense polyg-
yny, with dominant males marking ranges that they strive
to make at least partly exclusive, and with males within the
group competing for sexual access to females.
In comparison to the capybara, scent marking in the
mara is less frequent (Taber and Macdonald 1984). Olfac-
tory communication among maras includes anal dragging,
enurination (the spraying of urine over a conspecific), and
the formation of dung piles. Both anal dragging and enuri-
nation are more frequent during the breeding season and
are most commonly done by males. The function of both
behavior patterns is unknown. Scent marking in maras is
associated with intense social interaction in diverse con-
texts, and thus generally in circumstances where individ-
ual recognition might be important. It seems probable that
at least some functions of enurination are different for the
two sexes, with males typically marking estrous females,
whereas female urine spraying seems to be associated with
repelling the advances of males and of unrelated pups try-
ing to steal suckles. Although anal marking and enurination
are widespread amongst hystricomorph rodents (reviewed
in Macdonald 1985), proof of their function is elusive.
While it seems plausible that male enurination on females
could be olfactory mate guarding, the seemingly diverse
contexts of this behavior leave plenty of room for hypothe-
sizing alternative functions.
Reproductive Biology
The hystricognath rodents share several reproductive char-
acteristics that are unusual among rodents. Compared to
myomorph rodents, the hystricognath group are character-
ized by long lifespans, long and variable estrous cycles, pro-
tracted gestations, precocious young, and moderate to
small litter sizes (Weir 1974; Kleiman et al. 1979), resulting
in the majority of hystricognaths having low reproductive
potential. However, the capybara is an outlier to the nega-
tive correlation between litter size and body weight found
among the hystricognaths (Kleiman et al.1979), having one
of the largest litter sizes (mean close to four), and being
the heaviest species within the suborder. It is these traits
that make the capybara particularly suitable for sustainable
harvest, with the result that they have been hunted through-
out their range for many years (Ojasti 1991). While the im-
pact of hunting is difficult to estimate, it is believed to be
selective, thereby affecting the social and age structure of
populations (Moreira and Macdonald 1993). A reduction
in capybara numbers in Venezuela indicates the need for
further evaluation of hunting pressures (Ojasti 1991); mod-
els that predict birth probabilities from hunting data (e.g.,
Moreira and Macdonald, unpublished) may offer an oppor-
tunity for exploring ways of maintaining populations while
allowing cropping by impoverished local communities.
Capybaras den communally when breeding, and females
appear to suckle infants indiscriminately within their group
(Macdonald 1981a). An average of 4.2 capybara young are
born per litter, each weighing approximately 1.5 kg (Ojasti
1973). The gestation period averages 150 days. On Marajó
Island, Moreira et al. (2001) found that around 18% of im-
planted capybara embryos are lost during gestation. Fur-
ther analyses of the 275 embryos of 50 females in the last
4 months of gestation (Macdonald and Moreira unpub-
lished) revealed that male embryos were heavier than female
ones, and appeared to demand more nutrients due to their
higher rate of development. Following sex allocation theory
(Trivers and Willard 1973), we suggested that, if one sex
gains more than the other from extra parental investment,
then parents with relatively more resources should bias
their investment toward the sex with the greater rate of re-
productive return. Since a dominant male capybara secures
most of the copulations within his multimale group, his
reproductive success is potentially much higher than that
of any female or subordinate male. Females in better con-
dition, therefore, might be expected to produce a relatively
higher proportion of sons compared with weaker females.
Moreira et al. (2001) and Macdonald and Moreira (in press)
did indeed find that females in better condition had more
dead fetuses in their uteri, and that their litters were more
likely to be male biased. Indeed, they found that, irrespec-
tive of its sex, a fetus positioned between two male fetuses
weighed significantly less than one positioned between two
female fetuses. Further, female fetuses were found more of-
ten, and male fetuses were found less often, between male
fetuses than expected by a binomial distribution. Male fe-398 Chapter Thirty-Three