Understanding the ecological and genetic factors that shape variation in life history traits is a central challenge in life history studies. This challenge is particularly acute in populations structured by age and sex. Although numerous studies receive their theoretical rationale from a life history perspective, only a few empirical studies show how life history traits are shaped by biotic and abiotic factors, and how these traits, in turn, affect population growth and evolutionary dynamics. At present, insights into the proximate mechanisms that shape life history schedules within populations come from a handful of easily manipulated and/or captive organisms that have relatively "fast" life history schedules. Understanding of which life history traits are key determinants of fitness remains poorly characterized for long-lived, slowly sexually maturing species such as most primates. The research aims of this proposal are: (1) To determine how life history traits are influenced by genetic, sex-specific, and ecological factors in a species with a slow life history schedule; (2) To determine which of these life history traits are currently under selection; (3) To determine patterns of phenotypic variation/covariation in these life history traits (4) To determine patterns of additive genetic, environmental, maternal, and cohort variation/covariation in these life history traits.
The data for this project come from a continuously monitored population of wild lemur, Verreaux's sifaka (Propithecus verreauxi verreauxi). Molecular, demographic, and quantitative genetic techniques will be employed to characterize the ecological and genetic factors that shape variation and covariation in life history traits. Results generated from this project will contribute to the development of demographic models that are capable of making realistic projections of population persistence by incorporating relevant ecological and genetic data. Results from this project will also be used to inform conservation decisions and management strategies by ascertaining which demographic traits influence population growth. As such, results from this proposal will have both theoretical and applied implications for conservation biology. Madagascar citizens will be trained in capture methods, and project results will be incorporated into the academic curriculum at Boston University.
On average, the human life course follows a predictable pattern. We are dependent on our parents when born. We rely on our parents and older siblings for social instructions, protection, and food when we are young. We attain puberty at around age 13 or 14 and we are fully sexually mature soon after. When we have children we often have singletons (not a litter). And finally, we live for a long time, up to 80 years or more on average. This pattern of our life course is called a life history strategy in evolutionary biology and anthropology. It refers to the general species-wide schedule of growth, reproduction, and death. In this regard, life history "traits" refer to things like average age at sexual maturity, generation time, average number of offspring produced, and average lifespan. The term "strategy" refers to the fact that, over evolutionary time, natural selection and other evolutionary factors have shaped the human life course so as to create a "strategy" of survival that allows humans to exist within their ecological niche. Other primates, apart from humans, are characterized by different life history strategies. The goal of this project is to investigate the factors that have shaped the life history strategy of a wild lemur species, Verreaux’s sifaka (sifaka hereafter). The life history strategy of the sifaka has some remarkable convergences with humans. For example, when accounting for differences in body size, both species are marked by slow growth and late age at sexual maturity and both have very long lifespans. These life history traits are likely governed by both genes and environment. Using funding from the National Science Foundation, we are investigating the genetic and ecological basis of life history traits in sifaka. Our research involves behavioral observations in the wild, laboratory research, and computational methods. Our field component is part of a larger, on-going project concerning community conservation and ecological monitoring at Beza Mahafaly Special Reserve in southwest Madagascar. So far, we have been able to get good estimates on key life history traits. Generation time in sifaka is very long, averaging 17 years. The population growth rate, which is a key determinant of the population’s ability to resist extinction is 0.98, which suggests a slight decrease in population numbers from year to year. Using rainfall data, sifaka life history data, and computer simulations we show that as annual rainfall increases, so does population growth rate, fertility rates, and life expectancy. We also found that as variation in annual rainfall increases the population growth rate is depressed. We have also used genetic and demographic data in order to understand if and how this population has changed in size in the past. Using a process called coalescent theory, we found that the sifaka population began to decline around 2300 years ago, which corresponds with the arrival of humans to Madagascar. Over the long-term, we suspect that the population has been declining due to a unpredictable pattern of rainfall and possibly human hunting more recently. Both of these factors can cause the population to have a slightly negative growth rate. The negative growth rate, in turn, can cause selection for delayed reproduction, long reproductive careers, and long life span. This is exactly the pattern we see in sifaka. As of the writing of this report, we are continuing to analyze data generated from this project, and our next goal is to look at the genetic underpinnings of life history traits. Funds from the project have allowed the PI to develop (and share with students and colleagues) expertise in computational and statistical theory, as well as in new genetic and demographic techniques. So far, this project has resulted in 3 primary articles, 3 review papers, and 6 conference presentations at national meetings. The PI has also been able to contribute to other related publications based on expertise gained during the course of this project. The project has also led to new course development at the PI’s host institution, including courses pertaining to primate ecology, reproduction, and conservation. The PI and colleagues are also involved in training and assisting Malagasy citizens in ecological monitoring techniques, as well as training graduate students in the USA in genetic techniques. Sifaka and humans have similar components in their respective life history strategies. We are ultimately interested in determining if parts of the human life history strategy are unique to humans or if components of this strategy are homologous extensions built from a basic primate pattern. Understanding the ecological, demographic, and genetic factors that shape sifaka lifespans and other life history traits can provide insight into the factors that have shaped human life history traits. Such a discovery could have applied consequences, for example, with respect to understanding the genetic and demographic factors that contribute to aging and lifespan in humans.
