Public interest in social and economic equality is burgeoning. In a recent paper in PNAS that examines both nonhuman primates and humans, we measured a related phenomenon, lifespan equality – a measure of whether lifespans in a population tend to vary a lot or be similar to each other. We used data from six well-studied primate populations – our own Amboseli baboon population, and also mountain gorillas in Rwanda, chimpanzees in Gombe Stream National Park in Tanzania, sifaka in Beza Mahafaly in Madagascar, muriqui monkeys in the RPPN Feliciano Miguel Abdala in Brazil, and capuchin monkeys in Santa Rosa National Park, Costa Rica. Read More
Surviving infancy is challenging for wild primates, as decades of research on species ranging from lemurs to gorillas have revealed. For an infant baboon, surviving the first year of life requires learning to identify and successfully consume more than 250 types of food, avoiding fatal disease and predation, and perhaps most importantly identifying and avoiding dangers from other baboons both inside and outside their social group. In Amboseli, first-year mortality has averaged about 30 percent over the four decades of our study, but has climbed as high as 50 percent during difficult times. Just getting through infancy represents a huge piece of the Darwinian gauntlet that every animal must run.
In humans and other animals, harsh circumstances in early life are linked to poor health and high mortality rates in adulthood. It is thought that these effects are greatest when multiple adverse conditions occur at the same, but this hypothesis has rarely been tested. We used prospective data on 196 wild female baboons in Amboseli to show that the number of adverse circumstances that a female experiences during her juvenile years predicts how long she lives as an adult. Specifically, we examined the effects of six different adverse circumstances: (i) being born in a drought, (ii) having a low ranking mother, (iii) having a socially isolated mother, (iv) having your mother die before you reach 4 years of age (the approximate age at sexual maturity for females), (v) having a younger sibling born when you yourself are still quite young (1.5 years of age or less), and (vi) living a very large social group. Females who experience 3 or more of these adverse circumstances tend to die 10 years earlier than females who experience no adverse circumstances. For comparison, the average female, once she reaches adulthood, lives to about 18.5 years of age, so 10 years is a very large difference in the life of a female baboon.
What are the costs and benefits for animals living in groups of different sizes? Balancing the trade-offs between within-group competition (which favors smaller groups) and between-group competition (which favors larger groups) suggests that intermediate sized groups may be best, yet empirical support for this prediction has largely been lacking. Using long-term data on wild baboons, we provide novel evidence that individuals living in intermediate-sized groups have energetically optimal space-use strategies and lower glucocorticoid (stress hormone) concentrations than individuals in either large or small groups. Our results offer new insight into the costs and benefits of group living. Read More
Many social animals form linear dominance hierarchies, with a clear rank order among group members. Such hierarchies can profoundly influence health and access to resources, but the mechanisms underlying hierarchy formation and maintenance remain unclear. Do individual dominance ranks simply emerge from individual attributes – such as fighting ability? Or are linear hierarchies the product of social self-organization processes such as winner and loser effects – i.e. the phenomenon in which winners become more likely to win in subsequent encounters, and losers become more likely to lose? Here, we present the first evidence for social self-organization processes in a wild animal population. Read More
The exaggerated estrous swelling displayed by females of many Old World primates has often been thought of as a trait that functions as a reliable indicator of individual female quality. Indeed, baboon swellings are regularly held up as the text-book example. This idea is based on Pagel (1994), which put forward the Reliable Indicator Hypothesis. This hypothesis posited that the exaggerated swellings of some females are consistently larger than others (i.e. inter-individual variation in size), that this swelling size is an indicator of heritable female ‘quality’ (quality that translates into variation in lifetime reproductive success), and that males therefore prefer those females that have larger swellings. Previous work in the Amboseli baboons provided support for the first part of the reliable indicator hypothesis; swellings of some females are indeed larger than others (Fitzpatrick et al. 2014). However, until the recent study by Amboseli researchers, a complete test of the Reliable Indicator Hypothesis had not been conducted in any primate population. Read More
Traditionally, testosterone is thought as a male hormone and estrogens as female hormones. However, estrogens and testosterone are produced by both sexes and appear to have important roles in both males and females. Measuring hormone concentrations in the wild without disturbing the animals is now feasible using non-invasive methods like fecal determination. These methods, however, need careful validation as the circulating hormone is usually degraded into several metabolites and it is necessary to verify that the antibody used in the immunoassay only recognized metabolites of the original hormone. Because the metabolites excreted in feces may be different for males and females, it is essential to validate immunoassays in both sexes. In this study, we determined whether the radioimmunoassays we previously used to measure fecal testosterone in male baboons and fecal estrogens in female baboons were suitable to measure these hormones in the opposite sex. Read More
There's a new ABRP paper out on predictors of female dominance rank. Dominance rank influences many fitness-related traits in the Amboseli baboons and in other mammalian populations. Therefore, understanding how and why animals attain a given dominance rank is an important goal in behavioral ecology. For female baboons, dominance rank is maternally inherited and generally follows predictable patterns. Specifically, adult daughters attain a rank immediately below their mother, and in reverse age order (a phenomenon known as ‘youngest ascendency’). This pattern of rank inheritance is assumed to be widespread and generally stable in cercopithecine primates; however, few studies have systematically examined whether the rules of female rank inheritance always hold true, and under what circumstances they are violated. Read More
Although females of many taxa display sexual signals as they become ready to be fertilized, little is known about what other types information, if any, may be contained in those signals. Female baboons display one of the most conspicuous signals of fertility in the animal world; exaggerated estrous swellings. These swellings appear on the animal’s hind end during the follicular phase of the sexual cycle, are largest around the period of ovulation, and quickly disappear during the luteal phase. By characterizing several different sources of variance in swelling size, our new study reveals the potential information content in this charismatic trait. Read More
Our recent paper addresses the question of reproductive cessation in humans and other primates in PNAS. Reproductive aging in human females – characterized by declining fertility in the 30’s, followed by complete reproductive cessation in mid-life – is an evolutionary puzzle because, all else being equal, mid-life reproductive cessation is inherently disadvantageous. One important question regarding the evolution of early reproductive aging is whether it is unique to humans or shared with other species. This has been the topic of much speculation but no definitive answer to this question has previously been available; comparative data from nonhuman primates are particularly important in answering this question because of the close phylogenetic relationship between humans and nonhuman primates. Read More