The link between body size and lifehistory characteristics has led to a large number of studies that have in-vestigated the nature of these relationships. [3][4][5][6][7][8] Life-history characteristics often scale with body weight in a predictable way so that body size information can be used to predict the life histories of species that cannot be observed directly because they are extinct or difficult to study. Despite this, descriptions of scaling relationships do not tell us why life histories vary. They do not explain variation in body size, the form of the scaling relationship, or life history variation that is not predicted by body size variation. Theoretical modeling, comparative studies limited to primates, and intraspecific work that concentrates on the variation of life histories within a species or population have all been used to try and understand life history variation. This review attempts to draw these studies together to give an overview of work on primate life histories.
COMPARATIVE LIFE HISTORIES AND ALLOMETRY
A large animal usually takes longer to grow to maturity than does a small animal. This simple observation may at first seem to be intuitively easy to explain. Few of us are surprised to hear that a small bushbaby can grow to maturity in less than a year, whereas a large orangutan female will not start breeding until she is more than 10 years old. One might expect that a large animal could take in more food than a small one and that a large baby could eat more and hence grow faster. This is in fact true. Orangutan infants grow at a rate of about 15 grams/day whereas dwarf bushbabies grow only 0.9 grams/day. But although the orangutan is 230 times heavier at birth than the bushbaby, it does not grow 230 times as fast. In addition, the bushbaby has a head start on the orangutan: it is 12% of its adult size at birth whereas the orangutan baby is only 5% of its adult weight. This, together with the bushbaby's proportionately higher growth rate, means that it pulls ahead and reaches its adult size long before the orangutan does.
This example illustrates the allometric relationship between many life-An animal's life history can be summarized by key variables that account for its life course from conception to death. Biological parameters that are of interest relate to reproductive effort and developmental rates (e.g., gestation length, neonatal weight, prenatal and postnatal growth rates, weaning age, and weaning weight) and the rate of reproduction (e.g., age at first and last reproduction, interbirth interval, the number of offspring per litter, birth rate, and the intrinsic rate of natural increase [r max ]). The rather obvious fact that such variables differ from species to species and from individual to individual has been the subject of much interest since the late 1960s, following the observation that species seem to be arranged in a spectrum that ranges from small animals that breed rapidly and develop early, have many young per litter, and have short lives, to large animals that breed slowly and develop late, have few young per litter, and have long lives. 1,2 Caroline Ross is a member of the Ecology and Evolutionary Biology Research Group and a lecturer at Roehampton Institute London. She has carried out research into primate life histories using a comparative perspective and is presently concentrating on the relationship between life histories and patterns of infant care in primates and other mammals. She has also carried out field work in India, looking at the social structure and ecology of Hanuman langurs. She is currently collaborating with Dr. Mike Lawes of Natal University on a research project looking at the socioecology of blue monkeys (Cercopithecus mitis) at Cape Vidal, South Africa.