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Aggression, dominance, and mating success among capybara males (Hydrochaeris hydrochaeris)

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Abstract

Social groups of capybaras, Hydrochaeris hydrochaeris , averaging 10 adults contained a mean of 3.6 adult males. Of 2911 interactions observed within social groups of capybaras, 34% were among adult males, and these were invariably aggressive. Males were organized in stable, linear hierarchies. The dominant male in each group was significantly heavier than any of the subordinates, but among subordinates, status was not correlated with weight. The dominant male maintained a central position at the core of the group, and time spent in the group by males was correlated with their social status. Each dominant male secured significantly more matings than did each subordinate, but subordinate males, as a class, were responsible for more matings than was each dominant male. Regarding alarm calls, although each subordinate male called less on average than the dominant, subordinates were responsible for the majority of alarm calls in a group. The stability of these hierarchies, in one case over 3 years, and the tendency for males to move up the social ladder in an orderly queue, suggest that many males are unlikely ever to secure dominant status.
Emilio A. Berrera
David W. Macdonald
Wildlife Conservation
Research Unit,
, Department of Zoology,
South Parks Road,
Oxford OXl 3PS, UK
E. A. Herrera is now at the De-
partamento de Estudios Am-
bientales, Universidad Simon
Boliva~, Apartado 89000, Ca-
racas 1080-A, Venezuela.
Received 1 November 1991
Revised 8 March 1992
Second revision 21 April 1992
Accepted 30 April 1992
1045-2249/93/$4.00
@ 1993 International Society
for Behavioral Ecology
Aggression, dominance, and mating
success among capybara males
(Hydrochaerishydrochaeris)
Social groups of capybaras, Hydrochaerishydrochaeris,averaging 10 adults contained a mean of 3.6 adult
males. Of 2911 interactions observed within social groups of capybaras, 34% were among adult males,
and these were invariably aggressive. Males were organized in stable, linear hierarchies. The dominant
male in each group was significantly heavier than any of the subordinates, but among subordinates, status
was not correlated with weight. The dominant male maintained a central position at the core of the group,
and time spent in the group by males was correlated with their social status. Each dominant male secured
significantly more matings than did each subordinate, but subordinate males, as a class, were responsible
f9r more matings than was each dominant male. Regarding alarm calls, although each subordinate male
cillled less on average than the dominant, subordinates were responsible for the majority of alarm calls
in a group. Tbe stability of these hierarchies, in one case over 3 years, and the tendency for males to move
up the socialladder in an orderly queue, suggest that many males are unlikely ever to secure dominant
status. Key words: capybara, dominance, hierarchy, reproductive success. [Behav Ecol4:114-119 (1993)J
Dominance hierarchies are common in group-
living species; they may be confined to males
(e.g., Appleby, 1980; Dewsbury, 1984; Huck and
Banks, 1982), or they may structure the relation-
ship of both sexes as in some primates (e.g., Dun-
bar, 1984; Hrdy, 1977). Dominance hierarchies can
be viewed as a way of limiting aggression within
social groups so that both dominant and subordi-
nate animals benefit by reducing costs associated
with it (Slobodchikoff and Schulz, 1988; Tinber-
gen, 1951). Altematively, "dominant" and "sub-
ordinate" may be separate strategies adopted by
different individuals and, as such, they may be
viewed as the evolutionary resolution of a conflict
through a mixed evolutionarily stable strategy
(Maynard Smith, 1982). An extreme case of this is
exemplified by the two morphs of Coho salmon
described by Gross (1985). Despite the fact that the
study of dominance has pervaded the ethological
literature since its inception, its significance has
been questioned (Appleby, 1983; Rowell, 1974).
Capybaras, Hydrochaerishydrochaeris,weigh about
50 kg and are grazing rodents common in the sea-
sonally flooded savannas (Llanos) of the northem
Neotropics. They live along the shores of ponds
and rivers in stable groups, typically comprising two
to five adults of each sex, together with young of
the year, which disperse as yearlings (Herrera and
Macdonald, 1987). In the Venezuelan Llanos,
groups defend territories of some 10 ha, whose
borders remain rather stable for 3 years and pos-
sibly longer (Herrera and Macdonald, 1989).
Signs of a dominance hierarchy among the males
in capybara groups have been mentioned (Azcara-
te, 1980; Macdonald, 1981a; Schaller and Craw-
shaw, 1981), but not studied in detail. Here we
describe aggressive encounters among male capy-
baras and how they lead to a dominance hierarchy.
\
114 Behavioral Ecology Vol. 4 No. 2
Then we investigate costs and benefits associated
with dominance rank, with special reference to mat-
ing success. We then analyze the hierarchy's sta-
bility and how this relates to fitness among domi-
nant and subordinate males.
STUDY AREA AND METHODS
Capybaras were observed between March and Oc-
tober 1982-1984 around scattered, shallow pools
on a ranch, Hato El Frio (7°46' N, 68°57' W), in
the Venezuelan low Llanos (state of Apure). Here,
flat tropical savannas are flooded between May and
October and endure drought in February and
March, when capybaras suffer a high, density-
dependent mortality (Ojasti, 1978).
We marked individuals with expanding plastic
collars (similar to those used by Clutton-Brock et
al., 1982), painted with unique two-Ietter codeso
We captured individuals each February either by
lassoing them from horseback or using a "dog-
catcher" noose to "fish" them from thorny thickets
of Cassiaaculeata. Hind-foot length was measured
and weight was estimated from a measure of girth
(Herrera, 1986).
We selected groups for intensive study on the
basis of accessibility and numbers of marked ani-
mals. Each group was individually observed in turn
for sessions of 2-12 h. Observations were evenly
spread throughout the daylight hours for each
group. We monitored the size, composition, and
movements of other groups regularly but less fr~-
quently. Ofthe groups intensively watched in 1982,
only one was recognized in subsequent years,
whereas all four groups studied in 1983 (including
the one known from 1982) were studied in 1984.
Groups are referred to by two-Ietter mnemonics.
We identified behavior patterns on the basis of
-
previous studies (Azcarate, 1980; Macdonald,
1981 a). Aggression consists mainly of simple chases
in which one animal walkstoward another that walks
away, and individuals may be classified in terms of
a "winner" and "loser" for each dyadic interaction.
Retaliation was rare and involved the pursued an-
imal facing its opponent, both individuals rushing
toward each other, rearing on their hind legs, and
grappling briefly before the loser broke free and
fled. We also recorded sexual pursuit and mount-
ing; these behavior patterns were treated as instan-
taneous events (sensu Altmann, 1974). When
alarmed, a capybara usually emits a loud bark, at
which its companions stand alert or plunge into the
nearest water (Macdonald, 1981 a). Alarm calling
was also recorded as an evento
State behaviors such as grazing, resting, and wal-
lowing were recorded during scans of the group
every 10 min (Altmann, 1974). The rates at which
individuals performed event-type behaviors de-
pended on the length of time each animal was in
sight, which varied between animals. Therefore, we
used the proportion of time each animal was pres-
ent, calculated from scan data, as a correction fac-
tor for behavioral rates. During scan sampling, the
position of each animal within the group with re-
spect to three imaginary concentric circles (zones)
was recorded, each annulus being of approximately
equal width (Macdonald, 1981a). The size ofzones
did vary, however, depending on how scattered the
group was. Thus, animals were recorded as in the
core (zone 1), the intermediate zone (zone 2), and
the periphery (zone 3). Animals outside the group
were assigned to zone 4.
RESULTS
Aggressive interactions accounted for 71.6% of all
interactions recorded in all groups during the 3
years (2911 interactions in 729 h of systematic ob-
servation).The rest of the interactions wereother
event-type behaviors, which included social inter-
actions such as sniffing another individual, a type
of grooming, and alarm calling. Scent marking is
not included in this sample. At least one male was
involved in 81.3% of all aggressive interactions, and
58% occurred between two males. Thus, aggression
between males made up 34% of all observed social
interactions. Aggression between females was much
less frequent (8.3%).
We then constructed the dominance matrices (one
for each group-season) following Brown (1975)
and Martin and Bateson (1986). In a dominance
matrix, entries indicate the number of times each
animal in a row beats each animal in a column.
Individuals are then ranked so that the lowest num-
bers appear on the lowest half of the matrix. Table
1 shows examples ofhierarchies. In one case, scores
were balanced in a draw, and the animal winning
the greater proportion of his aggressive interac-
tions (including those against unidentified males)
was assigned the higher rank.
In each group, dominance relations among males
were clear-cut: in the majority of dyads one indi-
vidual invariably won. There was only one case of
a reversal, indicating that the hierarchies were
strongly linear. However, according to critics of
earlier studies (e.g., Appleby, 1983; Rowell, 1974),
two conditions are necessary if a hierarchy is to be
Table 1
Dominance matrices for identified males in the
groups of capybaras studied
Loser
Winner Un-
known
WSC WTC OJX
Row animals are winners and column animals are losers.
Entries are total numbers of interactions in each dyad.
The last column and the bottom row are outcomes against
unidentified males. The numbers in parentheses at the
bottom right comer of each matrix correspond to the
total number of interactions seen between unidentified
males and do not indicate which animal won. They are
shown in parentheses because they are not the sum of
their respective columns or rows, nor the total number
of interactions in the group.
considered linear: (1) Dominant-subordinate re-
lationships must be statistically significant and long-
lasting, and (2) all dyadic relationships must be
asymmetrical and all possible triadic relationships
must be transitive, Le., if A dominates B and B
dominates C, then A should also dominate C. In
the present study, the top dominant position in
each group was invariably clear, so the problem of
linearity lies within relationships among subordi-
nates.
Using the binomial test (Siegel and Castellan,
1988) for those cases with fiveinteractions or more
(n = 19), we found that all asymmetriesin dyadic
relationships were significant (4 at p< .05 and 15
atp < .01). The rarity ofaggression led us to define
a permanent relationship as one in which at least
two interactions were recorded. Of 16 such cases
among the subordinates, one was a 1-1 draw, an-
other was 24-1, and the rest were all n-O wins.
Herrera and Macdonald .Capybara dominance 115
Group Cl, 1983
WSC -O 24 14
WTC O -11
OJX 1 O -4
Unknown O 1 1 (3)
Un-
R15 WTE WNA lDG known
Group NC, 1983
R15 -15 9 3 39
WTE O-8 O 8
WNA O O -13
lDG OOO-O
Unknown O 1 16 12 (18)
Un-
165 G14 WW7 W14 known
Group P5, 1984
165 -45416
G14 O -1411
WW7 O O -34
W14 O O O -1
Unknown 1 O 418 (13)
Un-
1BY WJU DII known
Group VQ, 1984
1BY -16 58
WJU O-16
DII O O -O
Unknown O O O(3)
Figure 1
The relationship between
body weight and dominance
rank of male capybaras in
study group ez during the
1982 field season in the
savannas of Venezuela.
Figure 2
Histogram of the percentage
of observation scans that
dominant and subordinate
capybara males spent in four
concentric zones away from
the center of the group. Zone
1, innermost; zone 4,
outermost.
Therefore, with one possible exception, all domi-
nance relationships among capybara males were
permanent.
Considering 17 triads in which each of the three
pairs had interacted aggressively at least twice, all
of 13 such triads that included the dominant male,
and all of four that were among subordinates, were
transitive. The dominance hierarchy among marked
adult males in groups of capybaras of the Vene-
zuelan Llanos is, therefore, strongly linear.
Correlates of rank: benefits and costs
Weight was significantly correlated with rank (pool-
ing all group-seasons, Spearman rank correlation
coefficient r, = -.457, n= 37, P< .01). Although
it would have been more appropriate to calculate
a correlation coefficient for each group, low num-
bers of identified males precluded this. Figure 1
illustrates the hierarchy in the group for which the
most complete data were available.
In four of five cases, the heaviest male was the
Domlnants
60
1/)
~40
20
o234
Zone
Subordinates
60
1/)
a40
s
20
o234
Zone
116 Behavioral Ecology Vol. 4 No. 2
highest ranking. Among subordina tes the weight-
status relationship was less clear, and, if dominant
males are excluded from the pooled data, the cor-
relation between weight and rank is lost (r,=- .292,
n= 26, ns).
Dominant males were found near the core zone
of the group significantly more often than were
subordina tes (using a subsample of independent
scans: X2= 299.03,df = 3,P< .001;Figure2;
independent scans were obtained by using only
those contiguous scans in which at least 50% of the
animals had changed behavior or else used scans
at least 3 h apart). No animal was present 100% of
the time, so the presence of each was analyzed as
a proportion of the total time the group was
watched. Time spent in the group by males was
'significantly correlated with rank (r, =-.633, n =
53, P < .01), although not if dominant males are
excluded (r, = - .145, n= 38, ns). Thus, to the
extent that presence in the group is advantageous,
and despite a linear hierarchy among all males, it
seems that the important distinction lies between
the dominant males and all other males.
Rank did not affect the percentage of time spent
feeding in good-quality feeding habitat (as ranked
by Herrera and Macdonald, 1989; r, = -.122, n
= 53, ns). Nonetheless, field observations indicated
that dominant individuals had priority of access to
food: Characteristically, a subordinate grazing on
a particular patch of grass would move if a higher-
ranking animal arrived to feed there. Similarly,
dominant males commonly displaced subordinates
from wallowing sites. One interesting example of
resource monopolization by a dominant individual
was observed in group CZ on 29 June 1982. A
yellow-headed caracara (Milvago chimachima) was
gleaning for ticks on a subordinate (a symbiosis
described by Macdonald, 1981b). The dominant
male approached, ousted the subordinate, and thus
usurped the attentions of the bird.
We have investigated three costs to which dom-
inants might be subject: (1) energy expended while
chasing other males, (2) risk of injury from retali-
ation, and (3) shortage of time for feeding, etc.,
due to preoccupation with shepherding subordi-
nate males. Capybaras are subject to predation,
principally by feral dogs and caiman. Although it
is conceivable that the dominant male is more at
risk while preoccupied with chasing subordinates,
we have no evidence for this.
With respect to energetic demands, the number
of aggressive interactions initiated per unit time
correlates significantly with rank (r, = -.777, n =
53, P < .01). However, the number of aggressive
interactions received per unit time correlates with
decreasing rank (r, = .595, n= 53, P< .01). Be-
cause winning and losing appear equally costly (both
animals cover the same distance at the same speed),
the costs of aggression for animals of all ranks are
approximately the same. We do not know whether
there is a stress-related cost associated with being
chased. The risk of injury seems to be small because
subordinate males rarely retaliated. On the other
hand, subordinates sometimes suffered serious
gashes to their rumps as they fled, although these
wounds were generally confined to males that lived
on the periphery of the group to the extent ofbeing
solitary.
There was no correlation between rank and feed-
65
.
"1
"O .
=.
E55 .
'"
50 .
45III I I I
2 3 4 5
Domlnance rank
Table 2
Alarm-calling bouts recorded for each age-sex status class
aExpected values are calculated from the average ratio of dominant:subordinate:female of 1:3.8:5.1.
bTotal X2 = 7.07, df = 2, P< .05.
ing rate (Ts= - .121, n= 53, ns), nor any difference
in feeding rates between dominant males as a whole
and subordinates (Mann-Whitney Utest, p= .542,
ni = 15, n2 = 36). Thus, there is no indication that
subordinate males had more time for feeding than
dominants.
Rank and mating success
Capybaras invariably mate in water. During the
mating season dominant males conspicuously pa-
trol their females. When a female is in estrus, the
dominant will follow her avidly, frequently sniffing
her vulva. The consorting pair enter the water and
swim around for several minutes, the male con-
tinuing to follow the female c1osely. The female
may lead the male out of the pond for awide detour
on land and then lead him again into the same or
another pond. Swimming in her wake, the male will
try repeatedly to mate the female, during which she
is forced under water by his weight. In general it
seems that while in the water the female can control
mating by swimming down beneath the male's grasp
when he tries to copulate. Byprolonging the mating
ritual, the female allows a more dominant male to
chase the consorting one, indirectly exerting mate
choice. Sometimes subordinate males willenter the
pond with the mating pair and watch them intently,
occasionally trying to mount the female while the
dominant male is distracted.
In the absence of genetic data with which to
assess paternity, we estimated reproductive success
from observations of sexual behavior. The data used
were the numbers of matings by identified males
that achieved intromission. Ofthese, during 3 years,
15 successful matings were performed by domi-
nants and five by identified subordinates. Taking
into account that there are 15 dominants (15 group-
seasons) and 38 identified subordinates (0.39: 1,
dominants: identified subordinates), the expected
numbers of matings by each c1assdiffered signifi-
cantly from the numbers observed (X2= 21.2, df
= 1, P< .005). It is noteworthy that subordinate
males did secure copulations; in a separate sample
the total number of females that mated (12) with
all subordinate males exceeds that with all domi-
nants (8).
Males commonly interfered with sexual pursuits
initiated by other males. Of sexual pursuits initiated
by subordinates, 53 (29%) were interrupted by the
dominant male and 11% by other, higher-ranking
subordinates. Most of the remainder failed because
the female evaded the subordinate, and none was
interrupted by lower-ranking males. Dominant
males had to stop their mating pursuits seven times
(11% of all pursuits started) to prevent a subordi-
nate from mating with another female.
Alarm calling and rank
Table 2 summarizes the frequency of alarm calling
by age, sex, and status. Using the proportion of
each age-sex c1ass, the expected frequencies of
alarm calling for each were ca1culated. Females
called less frequently than expected, whereas dom-
inant and subordinate males called slightly more
often than expected (X2= 7.07, df = 2, P< .05).
Overall, a group received most warnings from sub-
ordinate males, the category that was most popu-
lous and that predominated, perforce, at the pe-
riphery of the group.
Stability of the hierarchies
The dominant position in hierarchies appeared to
be very stable. For instance, male 165 of group P5
was dominant for all 3 years of the study, and dur-
ing 126 h of observation and 88 interactions, he
was challenged only three times. Groups VQ, Cl,
and C2 each had the same dominant males in 1983
and 1984.
Two takeovers were documented. Between 23
March and 30 April1982, male D02 was dominant
in group CZ. In the morning of 30 April, group
C2 rested alongside another group (as is often the
case in the dry season; Herrera and Macdonald,
1987), and an unidentified male chased D02. This
protracted interaction led the combatants to walk
past male D86, whose group affiliation was un-
known. Male D86 attempted to bite D02, who im-
mediately withdrew. D86 then chased the uniden-
tified male, who had initiated the attack on D02,
but this male retaliated and no winner emerged.
For the next 2 h, D02 sat alone about 30 m from
group CZ. When CZ was next seen, 1 week later,
male D02 had disappeared and D86 was c1early
dominant. Male D02 was never seen again in the
CZ group territory; D86 remained unchallenged
throughout 1982. D02 was seen subsequently some
800 m (about twice the average range width) north-
west of his former territory, in a"group of six ob-
viously younger animals. He won all nine interac-
tions in which he was involved that year, and was
seen throughout the followingyear in the same area
and probably in the same company. In 1983, male
R15 was dominant in group NC, and when chal-
lenged once by WTE (number 2 in the hierarchy),
R15 retaliated and wounded his opponent's face.
By 1984, R15 had disappeared, WTE was domi-
nant, and WNA, previously number 3, was num-
ber 2.
DISCUSSION
The existence of dominance hierarchies has been
known for centuries, and they have been a peren-
Herrera and Macdonald .Capybara dominance 117
Class N%Expecteda Observed X2b
Dominant male 7 11.9 4.4 7 1.54
Subordinate male 23 39.0 16.9 23 2.20
Female 14 23.7 22.7 14 3.33
nial topic for ethologists since Schjelderupp-Ebbe
(1922) formalized the idea of a pecking order among
chickens. Nonetheless, studies demonstrating the
existence of clear-cut linear dominance hierarchies
in wild mammals under natural conditions with an
effect on mating success are few, and none concern
caviomorphs. Dominance hierarchies have fre-
quently been dismissed as artifacts of captivity
(Rowell, 1974). Many studies of dominance in non-
primatemammalshavebeen carried out incaptivity
(e.g., Dewsbury, 1984; Reinhardt, 1985; Shapiro
and Dewsbury, 1986), and work on rats (Rattus
noruegicus) demonstrates the importance of study-
ing dominance under natural conditions (Berdoy
M, Macdonald DW, and Smith P, in preparation).
Our findings indicate that, in the case of the cap-
ybara, dominance relationships are extremely clear-
cut: only in 2 cases out of 50 dyads (4%) did any
rank reversal occur (and this occurred only once
ineachcase).Mostdominancehierarchiesreported
in the literature have been based on less incisive
asymmetries (Appleby, 1983). What, then, has fa-
vored the development of such a clear-cut linear
hierarchy among male capybaras?
For any male, being a member of a group is
crudal to survival because outside of groups the
probability of survival and reproduction may be
negligible. In the Llanos, group living is important
for capybaras' survival and reproduction. Groups
monopolize access to essential resources such as
water and grass (Herrera and Macdonald, 1989),
and members of larger groups probably benefit in
antipredator terms (Herrera, 1986; Macdonald,
1981a) and certainly benefit through increased re-
productive success (Herrera and Macdonald, 1987).
Indeed, groups smaller than four adults failed to
rear any young (Herrera and Macdonald, 1987).
Some adult capybaras spend much of their time
alone, and these are invariably male (Macdonald,
1981a), but even these solitary males try, at least
intermittently, to insinuate themselves into a group
(Herrera and Macdonald, 1987). Outsiders enjoy
none of the antipredator advantages of group
membership. They are harassed by territorial
groups, probably have limited access to critical re-
sources (Herrera and Macdonald, 1989), and cer-
tainly have no access to females. For such solitary
males, group membership under almost any terms
would seem preferable. Whitehead (1990) devised
models to predict the conditions under which a
male should join a group of females or move be-
tween groups of females, with spedal reference to
large mammals. He found that when females are
grouped, being resident was an evolutionarily sta-
ble strategy for males.
The alternatives are, thus, to form bachelor
groups or to be a member of a mixed-sex group
and thus tolerate subservience to and harassment
from a dominant maleoWe have demonstrated that
the latter option has the major advantage of allow-
ing the possibility of mating. Indeed, bachelor
groups of capybaras have not been recorded. There
are no hormonal studies of male capybaras, but
Sachser and Prove (1986, 1988) demonstrate a cor-
relation between sodal status and concentration of
testosterone in the blood in the capybara's closest
relative, Cavia aperea.
Why do dominant males tolerate subordinates?
It seems likely that subordinate males bring several
118 Behavioral Ecology Vol. 4 No. 2
advantages to dominant males: first, they partid-
pate in territorial defense (Herrera and Macdon-
ald, 1987; see also Davies and Houston, 1984), and
second, as a class, they give off more warnings as
alarm calls than other group members. Although
the subordinates' alarms benefit dominant males,
they may also benefit the subordinates (see Sher-
man, 1985). If McNaughton's (1984) suggestion
that group grazing increases primary productivity
is correct, the presence of subordinates may bring
foraging advantages during the growing season.
In terms of cost to the dominant male, the great-
est is probably the matings lost to subordinates. As
females come into estrus synchronously, it is in-
evitable that the dominant male willlose some mat-
ings to subordinates (see Ridley, 1986). Females
cycle every 7 days unless pregnant, and receptivity
lasts possibly only 8 h (Lopez-Barbella, 1982, per-
sonal communication). The loosely knit cohesion
of capybara groups and the lumbering morphology
of the spedes combine to make it very difficult for
a single male to keep at bay evenone, far less sev-
eral, persistent interlopers. This is evidenced by the
plodding persistence with which dominant males
chivvy their rivals to the periphery of resting grou ps
(described in Macdonald, 1981a), by the fact that
larger groups have a retinue of more or less closely
affiliated "hangers-on" (Herrera and Macdonald,
1987), and by the very protracted efforts involved
when attempts are made to drive off an intruder
(detailed descriptions in Herrera, 1986; Macdon-
ald, 1981 a). The cost of excluding interlopers may
be heightened because capybaras, by virtue of their
body size and paudty of sweat glands (Pereira et
al., 1980) may live on the threshold of thermal
stress.
These arguments explain the presente of more
than one male in groups, both from the dominant's
and the subordinate's points of view. But the for-
mation of a hierarchy remains unexplained. Using
game theory, Maynard Smith (1982) has shown that
subordinates and dominants may be the two alter-
natives in a mixed evolutionarily stable strategy (ESS)
for competing animals living in groups. In his mod-
el, other alternatives lead to disadvantages on both
sides and thus reversion to the hierarchy. Slobod-
chikoff and Schulz (1988) also found, in a theo-
retical analysis, that when resources are abundant,
both dominant and subordinates benefit from shar-
ing.
Clearly, avoiding injury while obtaining some
benefits and perhaps securing a chance to succeed
eventually to a higher status seems a favorable com-
promise when compared to a solitary life or fight-
ing. The ESS model does not incorporate the pos-
sibility that male capybaras in a hierarchy are
queuing to inherit the dominant position, and thus
that both subdominance and dominance are a con-
tinuum and therefore part of the same strategy,
being stages through which each individual grad-
uates as it ages (as in male rats; Berdoy et al., in
preparation). Age and size are correlated in male
capybaras only up to about 2 years or 40 kg (Ojasti,
1973). The predictions of both the ESS model and
the queue model are compatible with our obser-
vation that where a dominant disappeared, the sec-
ond and third subordinates became numbers one
and two. The question is whether most individuals
get a chance at both roles.
... After drying at room temperature, the smears were stained with rapid panoptic and observed under a light microscope [22,50]. Other authors utilized a 10% saline solution vaginal wash, whereby 1 mL of the final wash was used to make a smear on the slide that was viewed under the microscope, confirming the presence of different cells according to the estrous cycle phase [54]. In proestrus and estrus, superficial cells predominated, while intermediate cells predominated during metestrus and parabasal cells in diestrus [19,22]. ...
... The pair would then enter the water, however the female would return to land and re-enter the water allowing copulation to take place, thus mating occurred in the water. Subordinate males were usually chased away by the dominant one [54]. ...
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Simple Summary This literature review focused on the reproductive technologies used in female neo-tropical hystricomorphic rodents. Reproductive technologies aid in efficient reproduction, which is important in these species as they are hunted and valued for their meat. Knowledge of the anatomy and physiology would aid in assisted reproductive techniques, thus attention was given to these areas. Within this group of rodent species there were similar characteristics, some of which have been highlighted as well as any unique features. Some reproductive technologies used included colpocytology, ultrasonography, and hormonal analysis. Abstract This is the second of two literature reviews that focuses on the female reproductive anatomy and reproductive technologies used in selected neo-tropical hystricomorphic rodents. The rodents chosen included the agouti (Dasyprocta leporina), the capybara (Hydrochoerus hydrochaeris), and the paca (Cuniculus paca). Over seventy references were used, spanning the years 1919–2021. Knowledge of the reproductive tract is important in understanding any unique features, which may affect the use of reproductive technologies. Some unique characteristics common to these species included the presence of a vaginal closure membrane and a lobulated placenta with a vascular sub-placenta. The capybara had hyperpigmentation of the vagina that was unique to each individual, while the agouti and paca had accessory corpora lutea, in addition to the main one. Some reproductive technologies have been used, with attempts at estrous induction and synchronization taking place within the past five years. Even though most work has been done over the past twenty years, there is still a dearth of information.
... Agoutis usually formed monogamous pairs [69] but could also, in captivity, have several females mated with a male without forming an exclusive bond [70]. On the other hand, capybaras were found to be polygynous in nature [48,71], whereas pacas were solitary animals, except for females and their young, with no male-female pairs seen together [71]. Courtship was similar for both agoutis and pacas, a process that usually involved naso-nasal contact (often with vocalizations), following of the female, enurination of the female by the male (causing female agoutis to go into a "frenzy dance"), thumping of the feet, mounting, intromission and finally ejaculation [69,72]. ...
... Agoutis usually formed monogamous pairs [69] but could also, in captivity, have several females mated with a male without forming an exclusive bond [70]. On the other hand, capybaras were found to be polygynous in nature [48,71], whereas pacas were solitary animals, except for females and their young, with no male-female pairs seen together [71]. Courtship was similar for both agoutis and pacas, a process that usually involved naso-nasal contact (often with vocalizations), following of the female, enurination of the female by the male (causing female agoutis to go into a "frenzy dance"), thumping of the feet, mounting, intromission and finally ejaculation [69,72]. ...
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This review, which is the first of two, focuses on the male reproductive anatomy and reproductive technologies used in Neo-tropical hystricomorphic rodents with the potential for domestication, which are the agouti (Dasyprocta leporina), the capybara (Hydrochoerus hydrochaeris) and the paca (Cuniculus paca). We consider over seventy references spanning from 1965 to 2020, with the majority of work being done in the past twenty years. Knowledge of the reproductive tract and reproductive technologies is critical to the conservation and preservation of these species. Although all three animals had similarities in their anatomy, such as no overt scrotums and testes located intra-abdominally in the inguinal region, some had unique features—for example, the agouti and the paca had penile spines, and two lateral penile cartilages. High spermatogenic efficiency was noted in the agouti and the paca, making them good candidates for increasing their reproductive performance in conservation programs. A review of the literature has shown that there is increasing work taking place on the reproductive technologies used in these animals; however, a lot of work is still lacking, as, to the author’s knowledge, standard protocols and artificial insemination procedures are yet to be established.
... However, because our experiment was conducted in a non-reproductive context, we did not assume that females would be widely more prosocial than males. A strict linear social dominance hierarchy enforced by chasing and, rarely, fights has been described in male capybaras (Herrera and Macdonald, 1989Macdonald, , 1993Maldonado-Chaparro and Blumstein, 2008), and more recently in female capybaras (Nogueira-Filho et al., 2017). Considering the stable and linear hierarchy within our subjects (both males and females), subordinates might be more prosocial towards dominants to avoid punishment. ...
... Wild capybaras are known for their very stable and strictly linear hierarchy, which settles through stereotyped agonistic interactions (Herrera and Macdonald, 1993;Salas, 1999;Herrera et al., 2011;Nogueira-Filho et al., 2017). Our capybaras were no exception, their hierarchy being highly consistent over the seven months we studied them. ...
Article
Prosocial behaviours (providing benefits to a recipient with or without cost for the donor) have been found to be highly influenced by sex and by hierarchy. Rodents, in particular, are good model for studying prosocial responses, as they were found to exhibit intentional prosocial behaviours to reward a conspecific, and are very sensitive to reciprocity. In our study, we conducted a Prosocial Choice Test (PCT) in which four capybaras (Hydrochoerus hydrochaeris) living in a social group could choose between three tokens: choosing the prosocial token rewarded simultaneously the subject and a recipient, while choosing the selfish token only rewarded the subject; and choosing the null token provided no reward to anyone. Dominance within each dyad was also studied, both before and during the PCT experiment. Our results showed an influence of hierarchy: subjects were more prosocial towards the recipient when it was a subordinate than when it was a dominant individual. These results could be interpreted as a desire of strengthening a hierarchical rank regarding the subordinate, of punishing aggressive conspecifics (usually the subject's direct dominant), and of weakening dominant individuals in order to modify the pre-existing hierarchy. Additionally, our results highlighted a direct reciprocity phenomenon, a subject being more likely to be prosocial towards a prosocial recipient. All these findings suggest that prosociality could be well developed in other taxa than Primates and that, in long enough PCT experiments, subtle rules could influence individual prosocial strategies.
... In societies with linear hierarchies, it is often observed that a certain set of individuals dominates over other group members and thus can monopolize resources or ideal positions (Herrera & Macdonald, 1993;Wittig & Boesch, 2003). This effect is referred to as resource holding potential (RHP; (Parker, 1974;Taylor & Elwood, 2003). ...
... e. no agonistic encounter necessary, Instead based on passive visual, auditory or chemical cues) to physical contests of agonistic behaviour. Herrera and Macdonald (1993) suggested that dominance hierarchies can be viewed as a means of actually limiting aggression within social groups, so that both dominant and subordinate animals benefit by reducing costs associated with It. ...
Thesis
p>Free-ranging adult males at two United Kingdom farms were radiotracked to determine home ranges. Home ranges were overlapping, although some contained exclusive sections, and all overlapped with female ranges. Core areas remained similar for all males, and were centred around the main human-mediated resource areas of the farms. Ranges and activity levels were affected by time of day. The size of the home range also correlated positively with body weight and age of the individual. At both farms, an older and heavier male was identified as having a significantly larger range than the other males. One of these males frequently travelled beyond radiotracking range and appeared to engage in aggressive bouts during these trips, suggesting the occurrence of resource defence. Microsatellite analysis of the cats was carried out using DNA obtained from hair samples, in order to measure reproductive success of the individual males. It was found that the older, heavier, wider-ranging and higher-ranking males achieved the greatest rates of reproductive success, but genetic variability was low and some inbreeding was suspected. These males, although now classed as dominant, did not, however, completely monopolise mating opportunities; subordinate males from the farms and unknown males were also found to be fathering kittens in both study areas, suggesting a mating strategy whereby all males maximised their own reproductive success, but this remained relative to their rank within their home colony. Dominant males seeking resources outside their home colony most likely suffered a loss in rank in achieving mating success. Such successful mating system for relatively few males should be considered in the context of the domestic pet cat population, which is subject to heavy neutering rates yet continues to escalate, and the potential problems of a reduced gene pool must be addressed.</p
... Female reindeer with higher status have priority of access to these areas and gain body mass while low-status females lose body mass and have lower reproductive output than dominants (Holand et al., 2004). Social status also impacts access to immaterial resources like social supports (e.g., social bonds, coalitionary support, assistance with offspring care, safe foraging positions; Silk et al., 2010;van Noordwijk and van Schaik, 1987) and mating opportunities (Cowlishaw and Dunbar, 1991;Herrera and Macdonald, 1993). Here, we propose that an animal's social status may also impact their access to sleep opportunities. ...
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Social status among group-living mammals can impact access to resources, such as water, food, social support, and mating opportunities, and this differential access to resources can have fitness consequences. Here, we propose that an animal's social status impacts their access to sleep opportunities, as social status may predict when an animal sleeps, where they sleep, who they sleep with, and how well they sleep. Our review of terrestrial mammals examines how sleep architecture and intensity may be impacted by (1) sleeping conditions and (2) the social experience during wakefulness. Sleeping positions vary in thermoregulatory properties, protection from predators, and exposure to parasites. Thus, if dominant individuals have priority of access to sleeping positions, they may benefit from higher quality sleeping conditions and, in turn, better sleep. With respect to waking experiences, we discuss the impacts of stress on sleep, as it has been established that specific social statuses can be characterized by stress-related physiological profiles. While much research has focused on how dominance hierarchies impact access to resources like food and mating opportunities, differential access to sleep opportunities among mammals has been largely ignored despite its potential fitness consequences.
... The dominant male maintains this status through agonistic behaviors towards subordinates (including persecutions, bites, and threats) in which subordinates are displaced. The dominant leads the group and defends its females and offspring, but that does not guarantee absolute success in mating [16,17]. Nearly 80% of copulations were performed by the dominant male [18,19]. ...
... The neurosensorial information of organisms is processed by the brain, and its regions increase proportionally in relative size according to their functions (Jerison, 1973). Social behavior of capybaras is complex with a hierarchy of territoriality and social domain, with some groups reaching more than 50 individuals (Herrera & MacDonald, 1987;Herrera & MacDonald, 1993;MacDonald, 1981;Ojasti, 1973;Schaller & Crawshaw Jr, 1981). During development, the olfactory bulbs scale isometrically with the cranial endocast volume ( Figure 13b). ...
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Recent studies have analyzed and described the endocranial cavities of caviomorph rodents. However, no study has documented the changes in the morphology and relative size of such cavities during ontogeny. Expecting to contribute to the discussion of the endocranial spaces of extinct caviomorphs, we aim to characterize the cranial endocast morphology and paranasal sinuses of the largest living rodent Hydrochoerus hydrochaeris, by focusing on its ontogenetic growth patterns. We analyzed twelve specimens of different ontogenetic stages and provided a comparison with other cavioids. Our study demonstrates that the adult cranial endocast of H. hydrochaeris is characterized by olfactory bulbs with an irregular shape, showing an elongated olfactory tract without a clear circular fissure, a marked temporal region that makes the endocast with rhombus outline, and gyrencephaly. Some of these traits change as the brain grows. The cranial pneumatization is present in the frontal and lacrimal bones, which we identified as two recesses (frontal and lacrimal) and one sinus (frontal). These pneumatic cavities increase their volume as the cranium grows, covering the cranial region of the cranial endocast. The encephalization quotient was calculated for each specimen, demonstrating that it decreases as the individual grows, being much higher in younger specimens than in adults. Our results show that the ontogenetic stage can be a confounding factor when it comes to the general patterns of encephalization of extinct rodents, reinforcing the need for paleobiologists to take the age of the specimens into account in future studies on this subject to avoid age-related biases. This article is protected by copyright. All rights reserved.
... From each group of capybaras captured, we collared one adult (GPS collar model Lotek Iridium Track M 2D, Lotek Ltd., Newmarket, Ontario, Canada). Because the heaviest capybaras tend to be more dominant (Herrera and Macdonald 1993) and dispersal is male-biased in high-density populations (Herrera et al. 2011), we assumed that the heaviest females should disperse less than males in HMLs. We therefore preferentially selected the heaviest female to collar in HMLs because we needed to collar an adult capybara that was resident to represent the movement pattern of the group. ...
Article
The expansion of human activity forces species to co-exist with people in human-modified landscapes (HMLs). However, living in HMLs demands behavioral adaptations, and the proximity between wildlife and people heightens human–wildlife conflicts. Capybara (Hydrochoerus hydrochaeris) is a thriving rodent species in HMLs in Brazil and as such, is involved in human–wildlife conflicts, such as vehicle collisions and transmission of Brazilian spotted fever (BSF). Despite their public importance, the effects of HMLs on capybara movement behavior have never been investigated. Our study aimed to investigate changes in home range, ranging pattern, and activity, for capybaras in six HMLs and two natural landscapes (NLs) by monitoring capybaras with GPS collars. We found home ranges 2.43 times greater in NLs than in HMLs and differences in ranging pattern in HMLs. Capybaras tended to be more nocturnal and move shorter distances across HMLs than NLs. Our results confirm the impacts of the HMLs altering capybara movement. The aggregation of capybaras in very small home ranges might imply on greater risks of tick infestations. In addition, capybara–vehicle collision may be increased during capybaras’ nocturnal activity. Therefore, we recommend that transportation agencies avoid the construction of transportation infrastructures (roads, railways, airstrips) in capybaras’ home ranges, which should be of restricted access to people in BSF endemic areas.
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Individuals within both moving and stationary groups arrange themselves in a predictable manner; for example, some individuals are consistently found at the front of the group or in the periphery and others in the center. Each position may be associated with various costs, such as greater exposure to predators, and benefits, such as preferential access to food. In social bats, we would expect a similar consistent arrangement for groups at roost-sites, which is where these mammals spend the largest portion of their lives. Here we study the relative position of individuals within a roost-site and establish if sex, age, and vocal behavior are associated with a given position. We focus on the highly cohesive and mobile social groups found in Spix's disc-winged bats (Thyroptera tricolor) given this species' use of a tubular roosting structure that forces individuals to be arranged linearly within its internal space. We obtained high scores for linearity measures, particularly for the top and bottom positions, indicating that bats position themselves in a predictable way despite constant roost-switching. We also found that sex and age were associated with the use of certain positions within the roost; for example, males and subadults tend to occupy the top part (near the roost's entrance) more often than expected by chance. Our results demonstrate, for the first time, that bats are capable of maintaining a consistent and predictable position within their roosts despite having to relocate daily, and that there is a link between individual traits and position preferences.
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Disputes typically flared up when the animals were competing over supplemental food but dominance-subordination relationships were the same at feeding and idling. The bison avoided escalations of conflicts generally solving them by means of spontaneous withdrawing and threat demonstrations. Rank disputes were observed between the partners of all pairs of animals. Dominance depended on age and sex, with older animals being dominant to young ones and males dominant to female age-mates. Animals were particularly aggressive against opponents close in rank. Agressiveness was not a prerogative of either sex.-from Authors
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(1) Groups of capybaras living in the seasonally flooded savannas of Venezuela showed seasonal shifts in habitat use in relation to changes in availability of favourable habitat (mainly grassland) caused by the alternation of drought and flooding. (2) Group home-ranges were stable; certain activities (e.g. resting, grazing, wallowing) were largely restricted to specific localities within each range. Although groups fed on the most favourable (`grazing') habitat in relation to its availability, individuals fed for similar amounts of time in this habitat irrespective of its availability per head. (3) Groups of capybaras were territorial. The small amount of overlap between neighbouring home-ranges corroborated observations of intruder rejection. Territory size varied between 5 and 16 ha (mean = 10.4 S.D. +- 3.7 ha) and was positively correlated with group size. (4) All home-ranges (with one exception) had a section of each of four major habitat types. Comparing home-ranges, the areas of two of these habitat types (bush scrub and low-lying grassy patches) correlated significantly with group size. Furthermore, the area of bushy scrub in each home-range correlated with the reproductive success of females occupying it. These two habitats are essential for survival in the dry season (grassy patches) and wet season (bushy scrub). (5) 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 different seasonal conditions. These results are discussed in relation to the pattern of availability and dispersion of the critical resources.