SOCIOECOLOGY
Socioecological
models try to explain
social
organization in terms of ecological factors that influence social
interaction patterns. Most models of primate socioecology
concentrate on aspects
of feeding ecology, particularly the degree and nature of competition
that individuals face both within and outside their groups.
More
recently, the protection that males may offer females and their young
against infanticidal male rivals has also featured in these
models. However, none of the published models, which are
meant to
apply to primates generally, accurately describes the African guenons
(including blue monkeys). These species are largely
frugivorous,
like other forest living monkeys (e.g. macaques), but their social
relations appear to be substantially different, especially in terms of
agonistic behavior. The idea that females bond with
protective
males is also not well supported by the observations we have made.
My
field
work includes collecting
systematic
records on social behavior of adult female and juvenile blue monkeys to
document
patterns of affiliative and agonistic behavior, as well as patterns of
feeding
and space use. These data have revealed very low rates of
agonistic
behavior and the absence of a detectable dominance hierarchy over
periods
in which such a hierarchy would be easily detected in other Old World
monkeys.
By combining data across several years, an hierarchical organization to
dyadic dominance relationships becomes apparent, but it is definitely
not a salient feature of blue monkey society. Dominance rank
correlates with few
behavioral measures, and so far seems unrelated to reproductive
success. In addition, and in contrast to other
more ‘dominance-oriented’ primate species, coalition formation is
exceedingly rare. We are currently working on a comprehensive
description of the dominance system of blue monkeys, based on 12 years
of data from multiple groups (see Keren
Klass's
page). It appears that these hierarchies
are generally moderately linear, and asymmetries within dyads are
marked, however the hierarchy is not a very steep one. As in
other cercopithecines, there is evidence for inheritance of rank among
females.
Whether
there is an ecological
explanation for these
weak agonistic asymmetries is still an open question, and one that I
continue to work on by expanding data collection to cover all the
seasons of the
year. The results so far suggest that the nature of food
resources
(whether fruit or leaf, clumped or dispersed) is not as important as
the
way individuals use those resources: in particular, when animals can
spread
out as blue monkeys do, overt competition is avoided, and societies can
be non-hierarchical. Karen
Pazol’s
thesis supports this conclusion with meticulous data on food
availability. By contrast, Steffen
Foerster's thesis did show some rank effects related to food
competition and diet, and ultimately to fecal glucocorticoids in one of
two groups. These are the first substantial rank-related
fitness correlates we've found, but it remains to be seen if they do
ultimately influence reproductive success. Groups may differ because of
the way in which their members "handle" competition, i.e. whether they
spread out or not, and also according to the main dietary items they
consoume.
My new project aims to understand how group size may affect the social lives andreproduction of females and males. Recent group-fissions give us an unprecedented opportunity: however, so far, quite a few of the patterns we've seen don't seem (again) to fit what the models tell us should happen. Here is the public abstract from the NSF website about this project:
Sociality is a hallmark of the primate order, and understanding the selective advantage of group living has proved an enduring question. The researchers are studying six groups of wild blue monkeys (Cercopithecus mitas stuhlmanni), differing five-fold in size, in the Kakamega, Kenya rain forest to understand the evolutionary basis of group living. This project focuses on how variation in group size influences biological fitness of wild female and male blue monkeys. Group size is expected to reflect costs and benefits of group living to individual group members. Females compete aggressively with group-mates over access to food, and engineer group splits when groups become large. Such observations suggest costs to living in larger groups, while collective territorial and predator defense, mainly by females, also suggest benefits. The research team investigates how group size influences female fitness, measured directly in terms of inter-birth interval, infant survival, and age at first birth, and indirectly by aspects of feeding and social behavior. Males of this species try to monopolize groups of females, and should have more reproductive opportunities in larger groups. Models of male reproductive partitioning suggest additional important factors, however, like female reproductive synchrony, intruder pressure and a male's own reproductive history. This project clarifies which factors best explain the considerable variation in reproduction among males. The researchers monitor demographic changes, female social and sexual behavior, and collect fecal samples for genetic paternity assignment using microsatellite loci. The study population is one of very few in which long-term demographic data exist. This project directly measures reproductive outcomes as related to social and ecological drivers.
My new project aims to understand how group size may affect the social lives andreproduction of females and males. Recent group-fissions give us an unprecedented opportunity: however, so far, quite a few of the patterns we've seen don't seem (again) to fit what the models tell us should happen. Here is the public abstract from the NSF website about this project:
Sociality is a hallmark of the primate order, and understanding the selective advantage of group living has proved an enduring question. The researchers are studying six groups of wild blue monkeys (Cercopithecus mitas stuhlmanni), differing five-fold in size, in the Kakamega, Kenya rain forest to understand the evolutionary basis of group living. This project focuses on how variation in group size influences biological fitness of wild female and male blue monkeys. Group size is expected to reflect costs and benefits of group living to individual group members. Females compete aggressively with group-mates over access to food, and engineer group splits when groups become large. Such observations suggest costs to living in larger groups, while collective territorial and predator defense, mainly by females, also suggest benefits. The research team investigates how group size influences female fitness, measured directly in terms of inter-birth interval, infant survival, and age at first birth, and indirectly by aspects of feeding and social behavior. Males of this species try to monopolize groups of females, and should have more reproductive opportunities in larger groups. Models of male reproductive partitioning suggest additional important factors, however, like female reproductive synchrony, intruder pressure and a male's own reproductive history. This project clarifies which factors best explain the considerable variation in reproduction among males. The researchers monitor demographic changes, female social and sexual behavior, and collect fecal samples for genetic paternity assignment using microsatellite loci. The study population is one of very few in which long-term demographic data exist. This project directly measures reproductive outcomes as related to social and ecological drivers.
Our
descriptions of blue monkey
societies are
also related to a very different and active area of research, namely
the evolution of intelligence in the primate order. An
influential hypothesis explaining the evolution of primate intelligence
relates it to solving
the problems of life in a social network. As the name of the
"Machiavellian intelligence hypothesis" suggests, primates might have
evolved their characteristic intelligence to steer through a maze of
social bonds that are used to influence competitive outcomes through
asymmetries in power. This hypothesis, however, is based
largely
on the behavior of a few relatively well-known species whose societies
are characterized by salient competition
and dominance relations. In these species, the formation of
coalitions
seems to improve individuals’ strategic position in competitive
situations.
In species like blue monkeys, in which competition and dominance are
not
salient features of social life, the effectiveness of using social
connections
to improve competitive power may be limited. However,
hypotheses
that
explain the evolution of primate-wide cognitive abilities must clearly
apply
to the entire order, not just a limited set of species. The
existence
of relatively non-competitive social systems, like those of blue
monkeys
and other primate species, thus seems problematical for this
hypothesis.