Gorilla Society: What We Know and Don't Know
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Evolutionary Anthropology 16:147–158 (2007)
ARTICLES
Gorilla Society: What We Know and Don’t Know
A. H. HARCOURT AND K. J. STEWART
Science is fairly certain that the gorilla lineage separated from the remainder As to the succession of the king-
of the hominoid clade about eight million years ago,2,4 and that the chimpanzee ship there is no certainty, but the
lineage and hominin clade did so about a million years after that.1,2 However, facts point to the belief that on the
just this year, 2007, it was discovered that although the human head louse sepa- death of the king, if there be an adult
rated from the congeneric chimpanzee body louse (Pediculus) around the same male he assumes the royal preroga-
time as the chimpanzee and hominin lineages split,3 the human pubic louse tive, otherwise the family disbands,
apparently split from its sister species, the congeneric gorilla louse, Pthirus, 4.5 and they are absorbed by or attached
million years after their host lineages split.3 No tested explanations exist for the to other families. Whether this new
discrepancy. Much is known about hominin evolution, but much remains to be leader is elected in the manner that
discovered. The same is true of primate socioecology in general and gorilla other animals appoint a leader, or
socioecology in particular. assumes it by reason of his age, can-
not be said; but there is no doubt
that in many instances families
Over a century ago, Richard Gar- The largest family of gorillas that I remain intact for a time after the
ner5 described gorilla society thus: have ever heard of was estimated to death of their leader.’’
‘‘In the beginning of his career, in in- contain twenty members. But the If one ignored the fanciful inter-
dependent life, the gorilla selects a usual number is not more than ten pretations and updated the wording,
wife with whom he appears to sus- to twelve: : : . When the young gorilla Garner’s portrayal would be remark-
tain the conjugal relations thereafter, approaches the adult state, he leaves ably close to any current description
and preserves a certain degree of the family group, finds himself a of gorilla society. Even the whimsical
marital fidelity. From time to time he mate, and sets out in the world for account of a jungle-book type coun-
adopts a new wife, but does not dis- himself. I observed that, as a rule, cil with the ‘‘king’’ presiding as judge
card the old one; in this manner he when one gorilla was seen alone in could be a description of the end of
gathers around him a numerous the forest it was usually a young a day-rest period, with animals gath-
family, consisting of his wives and male, but nearly grown; it is proba- ered near the silverback, exchanging
their children: : : . The father exer- ble that he was then in search of a grunts that coordinate departure.6
cises the function of patriarch in the wife. : : : The last 50 years of gorilla
sense of a ruler: : : . To him the others In the matter of government, the research have, of course, done more
all show a certain amount of defer- gorilla : : : leads the others on the than just confirm the general accu-
ence: : : . march, and selects their feeding racy of Garner’s accounts. Gorilla so-
grounds and places to sleep; he ciety has now been studied in many
Alexander Harcourt is a professor in the breaks camp, and the others all obey sites across the gorilla’s geographic
Department of Anthropology and the him in these respects. Other animals range in Africa. The findings, per-
Graduate Group in Ecology at the Uni- that travel in groups do the same haps especially those concerning
versity of California, Davis. Kelly Stewart
is a research associate in the Depart- thing; but in addition to this, the similarities and differences between
ment of Anthropology at the same insti- natives aver that the gorillas from populations, have allowed the main
tution. They are most recently the
authors of Gorilla Society. Conflict, Com- time to time hold palavers or rude advance over Garner’s description,
promise and Cooperation Between the form of court or council in the jungle. namely deep socioecological under-
Sexes, University of Chicago Press, On these occasions, it is said the king standing of why the gorilla has the
2007. E-mail: Eahharcourt@ucdavis.edu
presides; that he sits alone in the sort of society that it does.7–10 More
centre, while the others stand or sit recently, genetic analyses have wid-
Key words: competition; cooperation; infanticide; in a rough semicircle about him, and ened the vista of answerable ques-
predation; socioecology
talking in an excited manner. Some- tions on a host of topics from mating
times the whole of them are talking behavior to population structure.11,12
V
C 2007 Wiley-Liss, Inc. at once, but what it means or alludes Fifty years ago, at the dawn of quan-
DOI 10.1002/evan.20142 to no native undertakes to say, except titative vertebrate socioecology, mam-
Published online in Wiley InterScience
(www.interscience.wiley.com). that it has the nature of a quarrel. : : : malian societies could be described,
148 Harcourt and Stewart ARTICLES
but they were not understood. Thanks ern gorillas live in west central foods are unavailable) such as bark
in great part to primatology, much is Africa; eastern gorillas live in the or other woody vegetation.21,24,25
now understood. Congo-Nile divide region of eastern In short, the gorilla’s large body
We emphasize the importance of central Africa.8,10 Ch. 3 Opinion dif- size and terrestrial habits enable it to
primatology for three reasons. One, fers on whether gorillas should be exploit relatively low-quality but
science is nothing if not detail, and classified as two species (western abundant food that is available
primatologists have been able to col- and eastern gorillas) or one. We are throughout the year.26–28 This diet
lect extraordinarily detailed data on going to avoid the argument and talk has had a major impact on their
their subjects. Two, evolution by nat- as if there were only one species Go- social evolution. By switching to foli-
ural selection is fundamentally based rilla gorilla. Rather than using spe- age when fruit is scarce, gorillas can
on variation between individuals, cific or subspecific names, we distin- afford to be more permanently gre-
and primatology, perhaps more than guish the subtaxa by the region in garious than can the ripe-fruit-seek-
any other nonhuman discipline, has which they are found (for example, ing chimpanzees and orangu-
concentrated on the behavior of indi- Cross River gorillas or Virunga goril- tans.29,30
viduals. And three, from the start, las) or by widely recognized designa-
primatology’s abiding interest was in tions (western gorillas, eastern low-
social processes and the nature of land or Grauer’s gorillas, and moun-
the resulting society rather than the tain gorillas). Be warned, however,
demography and structure of popula- that for some primatologists, G. go-
Social System
tions. It is no accident that primatol- rilla refers to only the western popu- Gorilla populations across Africa
ogy helped launch the field of socio- lation, while the eastern population appear remarkably similar in many
ecology and continues to be a main is G. beringei.13 aspects of social structure, such as
player. group size and composition, despite
We will concentrate on four issues considerable differences in ecologi-
here. First, we argue that despite the cal factors such as diet, daily travel
high degree of sexual dimorphism of
Gorilla Ecology distance, and seasonality.19,31 For
gorillas, with females half the size of Until the 1980s, most of our instance, the median number of
males, females nevertheless strongly knowledge about gorilla ecology adult females per group varies from
influence males’ behavior and the na- came from studies of mountain goril- 3.5 in the west to just five among
ture of male-male competition. Sec- las in the Virunga Volcanoes, earn- mountain gorillas.10 Ch. 4 Gorillas
ond, we propose that the traditional ing the genus the label of consum- everywhere live in polygynous, usu-
view that protection from predation mate folivore.14,15 Since then, studies ally one-male groups in which adult
favored females’ association with of other populations have indicated membership can be stable for years
powerful males should not yet be that gorillas’ diets differ markedly (Fig. 1).32–36 Groups are based on
rejected in favor of the current view from region to region, the most strik- female-male association and on
that protection from infanticide is ing contrast being in degrees of fru- intense competition between males
the main cause of the association. givory.7,16–18 This variation is closely for sole, long-term access to fertile
Third, we ask why most gorilla linked to altitude and the availability females.10 Ch. 7,11,37
groups contain only one adult male, of fruit in the habitat.10 Ch. 4,18–20 In all populations, males and
when evidence suggests that residing Although gorillas eat fruit when females commonly emigrate from
in multi-male groups provides con- they can get it, they are not obligate their natal groups.10 Ch. 8,32,38,39 Emi-
siderable reproductive advantages to frugivores, as are the chimpanzees grating females transfer between
both sexes. Finally, we use gorilla so- with whom they are sympatric at all groups, often more than once in
ciety to illustrate the point that while sites except the Virunga Volcanoes. their lives.33,35,40 Emigrating males,
the societies of species can differ All gorillas, even those in fruit-rich in contrast, do not join other groups,
because their individuals are follow- lowland forests, consume large but travel alone until they can attract
ing different evolutionary rules, dif- amounts of herbaceous vegetation. females.41,42 Males that do not emi-
ferences between societies can also This diet is lower in energy than is grate become dominant breeders
arise as a result of the same rules fruit, but more abundant and perma- either by taking over the group when
operating in different social and nently available. Most importantly, the dominant male dies or by usurp-
physical environments. the gorillas’ staple foods, the foods ing top rank from the dominant
that are eaten all year round, are all male.10 Ch. 11,33,36,43 Processes of
nonfruit species; specifically, they are group formation, transition, and ex-
THUMBNAIL SKETCH OF GORILLA high-quality herbs.16,21 When fruit is tinction are similar in all popula-
ECOLOGY AND SOCIAL SYSTEM scarce, gorillas do not persist in tions where data exist.8 Researchers
searching for it, as do chimpan- have documented infanticide by
Taxonomy zees.18,22,23 Instead, they switch to a males among mountain and eastern
The gorilla occurs in two main predominantly herbaceous diet, lowland gorillas and strongly suspect
populations separated by the 1,000 incorporating lower quality ‘‘fallback’’ it in at least one western popula-
km of the Congo Basin forest. West- foods (those used when preferred tion.10 Ch. 11,40,44,45ARTICLES Gorilla Society 149
Figure 1. A group of Virunga gorillas. Ó A. Harcourt/Anthrophoto.
Along with these general similarities mates. The opposite is the case for pletely dominant over females. For
are some intriguing contrasts between males. instance, the female Virunga gorilla is
populations. The most striking of these supplanted from food nearly ten times
is the higher proportion of multi-male as often by the group male as by
groups in mountain gorillas compared Males Influence Females another female.10 Ch. 6
to eastern lowland (Grauer’s) and
western gorillas.10 Ch. 11,17,41 In apparent contrast to this frame-
work, however, male gorillas do not Females Influence Males
move to females. Gorilla society is
better understood as one in which The power of male gorillas and the
females go to males. Females rarely movement of females between males
FEMALES INFLUENCE MALES both imply an overriding influence of
WHICH INFLUENCE FEMALES travel unaccompanied by adult males.
When females emigrate, they usually males on females. Nevertheless,
WHICH INFLUENCE MALES : : : . females strongly influence males.
transfer immediately to a group or
According to a powerful, cohesive lone silverback.8,10 Ch. 8 In addition
framework in socioecology, one that Schaller’s14 still-accurate description
explains a lot of variation across of the power of the male over the A low density of females forces
mammalian societies, the distribu- females leaves little room for female males to associate with females
tion of food influences the distribu- influence.14 Ch. 6 Gorillas seem to con-
tion of females, which in turn influ- tradict the socioecological framework. Because female gorillas live at such
ences the distribution of males.46,47 The power of the male over females low density and have such short
The argument is that female mam- is not surprising. Male gorillas, at estrous periods, and because both
mals differ in their lifetime reproduc- twice the size of females, are one of sexes move such short distances each
tive success more because of differ- the more sexually dimorphic primate day, males need to stay with any
ential access to resources than species.48 Presumably as a result of female they find if they are to have a
because of differential access to this contrast in size, males are com- chance of being with a female when150 Harcourt and Stewart ARTICLES
she is in estrus.10 Ch. 10,49 How do we interact affiliatively with their females preferentially join groups
know this? infants.37 with more than one adult male,
The claim comes from the results Evidence that long-term friendly regardless of group size.43 One
of computer models of the move- relationships can influence female obvious explanation for this prefer-
ments of individuals around their mate choice comes from data on the ence is that infanticide is far less fre-
range, which are based on old mod- fission of a mountain gorilla group quent in multi-male than in one-male
els in chemistry of the chances of after the death of the leading groups, in large part because with
molecules hitting one another. The male.37,42 Rates of affiliation between more than one male present females
model simulates the chance of a the sexes during the two years before do not have to disperse to infantici-
male (one molecule) ‘‘hitting’’ a the fission, when the two new group dal, nonfather males at the death of
female (another molecule) while she leaders had been subordinate, pre- the leading silverback.43,52
is in estrus, given the density of dicted reasonably well which of the
females and the distance at which subsequently dominant silverbacks
individuals can detect one another. the females would join: Nine of the Interaction of Male and Female
The model’s results indicate that a 12 females in the original group went Influences
roaming gorilla male might bump with the male near which they previ- Females influence males but, in
into just one tenth the number of ously spent the greater proportion of turn, the consequence of the females’
estrous females that a male who time.37 influence on males is males’ influ-
stayed with encountered females The primary social arena in which ence on females. For instance, if the
would have access to. female choice exerts an influence on threat of emigration by females
males is transfer of females between allows long tenure of the resident
males. Almost all females leave the male, the resident male’s long tenure
Female choice affects male group in which they were born. Sub- forces emigration on his daughters
breeding success, competitive sequently, many transfer again, indi- (Fig. 2).10 Ch. 8
tactics, and tenure cating that females are choosing We can see this feedback from the
among the available males.8,10 Ch. 8
fact that when only one male is pres-
Females’ choice of males occurs in This choice could have a profound
ent in a gorilla group, all females
two arenas, within the group, and effect on the nature of gorilla society
born into the group emigrate before
within the local population.35 Within because the effect of emigration by
breeding, most likely to avoid mating
multi-male groups, females can females could be to extend the ten-
with their potential father.10 Ch. 8
choose which male to mate with. In ure of the breeding male.10 Ch. 11 Go-
However, when more than one male
the population, emigrating females rilla society is unusual in that take-
is present, some females stay to have
choose which of the available males overs of a group of females by an
their first offspring in their natal
to transfer to. incoming male are extremely rare.8
Both behavioral and genetic data This is in marked contrast to the so- group.51 Indeed this relationship
indicate that dominant males in ciety of some female-resident spe- seems to be general among primates.
multi-male groups of Virunga moun- cies, such as baboons, macaques, Females leave their natal group in
tain gorillas do about 85% of the and hanuman langurs. The lack of species in which males do not immi-
mating and produce about 85% of takeovers in gorilla society is not grate, whereas in taxa with male im-
the offspring.11,50 This skew is due because male gorillas do not fight. migration and, therefore, a supply of
primarily to dominant males’ success They do. However, even though the unrelated mates, females remain in
at aggressively preventing subordi- fights can occasionally be lethal,8 it their natal group.53 Across mam-
nate males from mating with fertile might be that they are not more fre- mals, females emigrate in taxa in
females.10 Ch. 11,50 quently serious because the reward which the breeding tenure of the
However, females’ mating prefer- cannot be guaranteed. In a society in male is longer than the time it takes
ences also play a role in dominant which females can and do leave the females to mature, whereas females
males’ breeding success. Adult dominant, breeding male, the bene- remain in their natal group in taxa
female Virunga gorillas initiate fits of winning a fight are too uncer- in which the father has gone by the
about 65% of copulations with the tain to warrant the costs. In brief, if time the females mature.54
dominant male.51 Although females males cannot be guaranteed the
also copulate with subordinate package, they are not going to fight
males, they initiate fewer of these hard for it. Food Affects Females, which
matings.35 Therefore, subordinate It is not yet clear which character- Affect Males: The
males have to work to attract istics of males the females might be
females. For example, some younger choosing when they transfer, apart
Socioecological Framework
males will actively foster friendly from the fact that they choose unre- If emigration by female gorillas
associations with females, initiating lated, fully mature males. However, influences the tenure of male goril-
proximity and grooming, in a rever- there is some evidence regarding the las, and that tenure influences emi-
sal of the usual direction of male- choice of group characteristics. In gration by females, where does the
female friendly interactions, and the Virunga gorilla population, cycle start? It begins with food andARTICLES Gorilla Society 151
Figure 2. Scheme to show interacting influences of the environment, males and females. Solid arrows ¼ cause; dashed arrows ¼
permit.7–10,32,34,35,37–43,46,47,52–61,66,80,86,88,89
the payoffs to females of residing with females of species for which poor- between the sexes fundamentally
cooperative relatives (Fig. 2).55–57 season diet is less predictable. starts with the female. The low den-
In general, the default for a female Moreover, competition tends to be sity of female gorillas forces males to
mammal must be to stay where it scramble (first come, first gets it) associate with females. Moreover, the
is.58 A familiar environment with a rather than contest (the better fighter females’ threat to emigrate, which is
familiar spatio-temporal distribution wins). Thus there is little to no direct allowed by the wide and abundant
of resources and dangers must be effect of rank on reproductive abil- distribution of their folivorous diet,
better than an unfamiliar one. Famil- ity.59 If ability in contest competition allows long breeding tenure of males.
iar competitors and partners are is unimportant, help in contests from
more easily coped with and used familiar partners is not as advanta-
than are unfamiliar ones, especially geous as it is in species with more ARE LEOPARDS AND HUMANS THE
as most of the evidence indicates frequent or intense contest competi-
MAIN PREDATORS OF GORILLAS
that mammals are as xenophobic as tion.8,10 Ch. 5,60 Finally, because male
humans are. In short, emigration is gorillas defend incoming females OR ARE MALE GORILLAS?
almost necessarily costly. against aggression from residents, they Female gorillas, especially moun-
In the case of the folivorous female minimize the costs of immigration.61 tain gorillas, are effectively always
gorilla, however, neither the familiar- In gorilla society, then, we see an with a male, one that is twice their
ity nor the potential aggression from obvious direct influence of males on size and completely dominates them
residents is so important.10 Ch. 2,5,6,10 females: Males outcompete females in competition for food. When
While foliage of course varies in for food; they minimize the effects of females emigrate, they usually do so
quality, a gorilla’s environment is competition and cooperation among only when another male is close by,
green; that is, its food is widespread females; they protect females and within a few hundred meters.10 Ch. 8
and common. Knowledge of the their offspring; and their long tenure We once saw a Virunga female on
food’s spatio-temporal distribution is forces daughters to emigrate. Never- her own for two days. She had trans-
therefore less important than it is for theless, the direction of influence ferred to a lone male and was trans-152 Harcourt and Stewart ARTICLES
Figure 3. Two Virunga males in an aggressive, protective stance between the photographer and the rest of the group. Ó A. Harcourt/
Anthrophoto. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
ferring back to her previous male. gorillas obtain from association with verting the local population of males
We think she made a mistake in a male.39,62 into potential fathers, and hence
judging where and how far away her Certainly, among gorillas and many noninfanticidal males. Females of
home group was. Previously com- other species, males protect their off- several species seem to use this strat-
pletely habituated to our presence, spring from being killed by other egy.64 Female chimpanzees and
she was clearly extremely frightened males.63 Indeed, some of the best evi- baboons, with their huge sexual
of us as we followed her over the dence for protection of offspring by swellings, which obviously are attrac-
two days during which she desper- primate males comes from the gorilla. tive to males, are classic examples.
ately searched for her group. Only However, the question is not whether Why does the female gorilla not do
after she had found her previous males protect the infants of the the same as the female baboon or
group and the male did she relax. females that have joined them, mated chimpanzee? We cannot record in-
with them, and produced their off- fanticide rates suffered by female
spring; of course they do. The ques- gorillas ranging on their own and
Protection from Infanticide tion here is whether the main benefit mating polyandrously because this
The obvious reason for a female that female gorillas obtain by joining never happens. However, using a
gorilla to search out and associate a male is protection against infantici- computer, we can simulate a roam-
with a male is for protection (Fig. 3). dal male gorillas or protection against ing, polyandrous female, just as we
Traditionally, predation was consid- predators.10 Ch. 7 simulated the roaming male search-
ered the key threat. However, current The alternative to joining one male ing for estrous females. The simula-
views stress protection against infan- for protection against all others is to tion needs to estimate the chances of
ticide as the main benefit that female mate with all the others, thus con- an estrous female meeting males andARTICLES Gorilla Society 153
converting them to noninfanticidal females associating with males is that Distinguishing the Hypotheses
potential fathers by mating with the gorilla is so large that it is rela-
Both the anti-infanticide and anti-
them. The model needs also to esti- tively safe from predation.62 How-
predation hypotheses have logic and
mate the subsequent chances of the ever, it is almost an axiom of prima-
evidence to support them; neither
nursing female meeting males with tology that terrestriality exposes ani-
negates the other. The difference
which she has not mated, which will, mals to greater risk of predation than
between them lies in the frequency
in consequence, be infanticidal. The does arboreality.66 While the axiom
and cost of the death that the associ-
modeling has been done.65 might not be true for smaller prima-
ation is meant to prevent. Being
The model’s answer is that nearly tes,67 it probably is for the great apes.
killed by a predator is a lot more
60% of a roaming female’s infants are Their main nonhuman predators are
costly to a female than losing a baby
killed by males the female did not the large cats.67 The gorilla, of
to infanticide: An infant can be
mate with and meets while she is course, is mainly terrestrial because
replaced within a year.69
nursing. This proportion is over four of its very large size. So we remain
times the 14% rate observed in the unconvinced that the gorilla is at less
mostly single-male groups in the risk of predation than, for example,
WHY DON’T GORILLA MALES
Virungas that were the study groups the chimpanzee.10 Ch. 7 LIVE TOGETHER?
when this rate was calculated.44 It In any case, gorillas are certainly The question of why males so rarely
seems that a gorilla female could preyed upon by both leopards and live together is still a question for mam-
never mate with enough males to con- humans,10 Ch. 7 and males have malian socioecology as a whole.46,70,71
vert enough of the local population evolved an anti-predator, screaming, Gorillas exemplify the puzzle.
into noninfanticidal potential fathers. roaring charge14 Ch. 7 that they do Western and eastern lowland goril-
That being the case, her only recourse not use in fights with each other. las comprise perhaps 99% of the total
is to associate with a protective male. From personal experience, we can gorilla population in Africa, probably
Models are, of course, only as good attest that the display is extremely numbering more than 70,000 animals
as the data that go into them and their effective. to the mountain gorilla’s 700.72 Over
manipulation of the data. What evi- 95% of western and eastern lowland
dence is there that the model reflects groups are one-male groups.10 Ch. 4 In
reality? First, for the Virunga popula- contrast, ever since studies on
tion of gorillas the model produces a We are far from Virunga gorillas started in earnest, a
value for rate of infanticide, 17%, that understanding either large minority of multi-male groups
is very close to the real 14% observed has been reported,14 Ch. 4 even over
for single-male groups. Second, when why the society of 40% in some years.10 Ch. 4,73,74 We are
the values for the chimpanzee and gorillas is mostly single- far from understanding either why
orangutan are entered, the model the society of gorillas is mostly single-
reproduces the low rates of infanti- male or why multi-male male or why multi-male groups are
cide recorded in these populations. groups are far more far more common among mountain
The model seems to work. gorillas than among western or east-
Be that as it may, the mistake common among ern lowland gorillas.
must not be made of assuming that mountain gorillas than
because the model supports the anti- What Determines Whether
infanticide hypothesis for female
among western or
Maturing Males Stay or Leave?
gorillas joining a male, this hypothe- eastern lowland gorillas.
sis can replace the anti-predation hy- The answer to the question of why
pothesis. Evidence in favor of one most gorilla groups have only one
hypothesis is far from evidence silverback needs to be answered by
against another one. Showing that considering the factors that influence
data fit is, of course, a vital first step Finally, with regard to evidence that maturing males’ decisions to stay or
in arguing for any hypothesis. But it male gorillas protect females against leave, given that multi-male groups
is fundamental scientific error to predators, so that females associate form when males remain in the
suggest that a hypothesis is the cor- with them for the benefit of that pro- group in which they mature.8 Which
rect one just because the data fit. tection, female gorillas behave as if is the better reproductive strategy for
The second step of replacement must they feel safer from predators when a the maturing male?
always be to show that the new hy- familiar silverback is nearby. Thus, Calculations indicate that Virunga
pothesis is better than the old. Yamagiwa68 nicely showed that while males that attain a breeding position
in the presence of a male, females in in the group they were born in do
one group nested off the ground about twice as well as do males that emi-
30% of the time (measured as percent grate, producing about three off-
Protection from Predators of nests of females off the ground), in spring on average compared to the
The main argument against the his absence, females nested off the leavers’ one-and-a-half.43,75 The pri-
predation-protection hypothesis for ground 80% of the time. mary advantage to stayers is that they154 Harcourt and Stewart ARTICLES
start with more females than do emi- mountain gorillas and all other pop- have groups with more than 20
grating males, which begin as solita- ulations in the frequency of multi- members been observed.8
ries and acquire mates incrementally. male groups. Because males do not
As a result, stayers enjoy more immigrate into breeding groups, a Lower risk of infanticide
females on average during their ten- multi-male group takes many years
ure than do leavers. In addition, emi- to develop, long enough for at least Some researchers have suggested
grating males are more likely to suffer one male to have reached full silver- that multi-male groups of western
complete reproductive failure than back age (15 years) while the father and eastern lowland gorillas do not
are stayers. Males that stay also gain is still alive or for two sons to have persist because infanticide is not as
the protective and attractive (to matured. The chances of that hap- great a risk as it is in mountain
females) advantage of multi-male pening will be affected by the num- gorillas.33 In this case, neither
groups mentioned previously. These ber of reproducing females, as well males nor females gain the repro-
benefits outweigh the costs of losing as birth and death rates. ductive benefits conferred by better
fertilizations (about 15%) to subordi- There currently are not enough survival of offspring in multi-male
nate co-resident males.11 published data to assess differences groups, which would reduce the
So why do most gorilla groups have in life-history variables between pop- advantages of co-residence to both
only one silverback? Although the ulations.32 However, many popula- subordinate and dominant silver-
finding that multi-male groups do bet- tions of western and eastern lowland backs.75 Although infanticide has
ter indicates that for males emigrat- gorillas are declining because of dis- been either observed or strongly
ing is an inferior strategy to staying eases such as Ebola, hunting, and suspected in Grauer’s and western
and queuing, in fact many males do environmental destruction,72 whereas gorillas, infants have also been
not have any choice.75 For example, if the mountain gorilla populations in spared in situations when infanti-
a group disintegrates after the death Bwindi and the Virungas are either cide was strongly predicted, such as
of the breeding male, the maturing increasing or remaining stable.17,73 proximity to a nonfather male.33,40
males, which cannot immigrate into The most commonly suggested ex- What conditions might lower the
breeding groups as can females, planation for differences in number risk of infanticide?
become leavers by default. In addi- of males in gorilla groups is that One suggestion for a lack of infan-
tion, if a male faces too much mating more males will be found where the ticide is that neighboring males are
competition because the leading male environment is rich enough to sup- related to the females or familiar
is still in his prime, because of a long port large groups of many females with them.12,33 Long-term observa-
queue, or because of an unfavorable producing many fast-growing, sur- tions of mountain and eastern low-
female-male ratio, for example, it may viving young. Within mountain go- land gorillas show that males emi-
be worthwhile for him to try his luck rilla populations, although the num- grate gradually from their groups,
elsewhere. Indeed, all of these factors ber of adult females does not differ sometimes paying visits after absen-
seem to distinguish leavers from between one-male and multi-male ces of several months. The same pro-
stayers.43 groups, weaned group size does, per- cess occurred during fission and the
In general, more resources pro- haps implying that the multi-male gradual separation of a subgroup in
duce more competitors.76 A factor groups are experiencing better con- eastern lowland gorillas.33 During its
strongly implicated in primate males’ ditions.10 Ch. 4 intermittent return visits, an emigrat-
decisions to compete or not for resi- For gorillas, prime ecological con- ing male continues to be accepted by
dence in a group is the number of ditions are high densities of herba- the resident silverbacks as if it were
females in a group. Thus, larger ceous vegetation6 such as those still part of the group. Thus, it seems
groups of females tend to have more found in mountain gorilla habi- to take a long time for a group male
males in them.77 However, multi- tats.17,19 Although differences in diet to be seen as an outside male.
male mountain gorilla groups do not and related variation in foraging Also, if emigrating males establish
have significantly more adult females effort suggest that mountain gorillas ranges in the neighborhood of their
than do single-male groups.73,78 It face fewer ecological constraints parent group, as they have been
seems to be the ratio of females to than do other populations,7,79–81 the reported to do in both western and
males,43 not the absolute number of average group size is about the same eastern lowland sites,12,33 then they
females, that is the determining fac- across Africa, varying across western may not be unfamiliar with the lac-
tor in males’ choice to stay or leave. sites from 9–22, and across eastern tating mothers that might transfer to
sites from 10–17.10 Ch. 4 Mountain them from those groups. In fact, they
gorillas, however, have larger maxi- might even have mated with those
Contrasts Between Western and
mum group sizes than do other pop- females in previous months, making
Eastern Populations ulations (one now numbers over 50), the infants potential offspring, or at
Demographic and ecological and a higher proportion of groups least relatives.
influences numbering over 20 individuals.17 If infanticide really is less frequent
Among western gorillas, only in one in western and eastern lowland goril-
Demographic variation might help area, Odzala National Park and its las, for whatever reason, then there
explain the striking contrast between surrounds in the Republic of Congo, will be less benefit to females inARTICLES Gorilla Society 155
Figure 4. Conflict and coperation among gorilla females. The adult female at bottom left has cuffed the young male at bottom center
away from a food source (the dead tree). The young male’s mother (top center), grandmother (mid-right), and aunt (bottom right) are
all aggressively vocalizing at the aggressor, who then departed with her daughter (very bottom left). Ó K. Stewart/Anthrophoto. [Color
figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
residing in a group with more than cercopithecines may be more likely female gorillas, who have been
one male. Females may then base than members of other taxa to use described as merely tolerating one
their choice of group on minimizing cooperation, especially with kin, as a another, are the competitive, steep
feeding competition rather than means to compete.83 Similarly, dif- dominance hierarchies of female sav-
maximizing protection, a process ferent taxa can show different behav- annah baboons and various species
that will inhibit the development of ioral and physiological responses to of macaque, in which strong friendly
multi-male groups.33,40,41,82 the same social situation, such as the relationships among kin correlate
presence of strangers of each sex.84 with females remaining for life in
DIFFERENT RULES OR DIFFERENT At the same time, differences in the their natal group and whole matri-
ENVIRONMENTS PRODUCE nature of societies could be a conse- lines ranking with respect to one
quence of the fact that the same
DIFFERENT SOCIETIES? another.85 Seemingly completely dif-
rules operating and interacting in
The societies of different lineages ferent behavioral propensities, or
different environments can produce
of primate taxa, clades, can differ different outcomes. ‘‘rules,’’ are operating in gorillas com-
fundamentally,83 implying that dif- pared to savannah baboons and
ferent taxa might be operating Same Rules, Different some of the macaques.
according to different evolutionary Nevertheless, we suggest that a lot
rules. For example, in otherwise sim-
Environment, Different Society of gorilla society, and a lot of the dif-
ilar social and environmental cir- A contrast to the relatively egalitar- ferences between gorillas and other
cumstances, it seems that Old World ian, uncompetitive relationships of species, can be explained by the fact156 Harcourt and Stewart ARTICLES
that while gorillas follow the same ingredient of the gorilla’s poor-sea- yses of Martha Robbins and David
fundamental behavioral rules, show son diet, their fallback diet of Watts must be singled out for special
the same fundamental behavioral pro- ground vegetation, is so widespread mention. Because of page con-
pensities, as do the other species, the and of such low quality that compe- straints, we have not cited their or
different environmental stages on tition and cooperation are not fre- others’ original studies nearly suffi-
which these players strut and fret quently useful enough to make the ciently. We thank Diane Doran-
their lives result in different societies. benefits of staying and helping kin Sheehy and Jessica Lodwick for
For instance, female gorillas com- outweigh the potential inbreeding detailed, helpful commentary. As will
pete over food, as do female baboons costs of doing so.8,10 Ch. 5,39 be obvious to readers, this article is
and macaques.86,87 Over 90% of sup- The other answer is that in the quite heavily based on our recently
plants and aggression among females small groups in which gorillas usu- published book on gorilla society.
in two Virunga gorilla groups were ally live, the dominant male can eas- That book benefited from many gen-
over food; in the same groups, 12 of ily influence interaction among the erous reviewers, especially Martha
20 pairs of females showed a clear females, and in fact largely negates Robbins, who read and commented
hierarchy between the members of the consequences of competition and on the whole of it.
each pair.86 Also, female gorillas are cooperation among the females
aggressive to immigrants,88 as are themselves.10 Ch. 6 The issue is not
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