Fertility, parental investment, and the early adoption of modern contraception in rural Ethiopia.

OBJECTIVES: What triggers initial shifts to fertility limitation as populations undergo socioeconomic development remains poorly understood. Alternative models emphasize the social contagion of low fertility ideals, or the individual perception of economic and/or fitness benefits to fertility limitation. Few micro-level studies in communities experiencing the earliest stages of the demographic transition are available. In a previous study, we found little support for the role of social transmission through friendships and spatial networks in explaining contraceptive uptake in rural Ethiopia, where contraceptive prevalence is low (<20%). Here, using data from the same population, we investigate the possibility that early contraceptive uptake is best understood as a manipulation of parental investment in response to local environmental change. METHODS: We used data on >800 women which recorded fertility, birth spacing and offspring survivorship. We first investigated whether ever-users and non-users differ in their reproductive behavior and success prior to contraception use. We then conducted a within-women analysis to investigate the impact of contraceptive uptake on reproduction and child survivorship. RESULTS: Women who have experienced higher fertility and higher child survival adopt modern contraception sooner rather than later, and contraceptive use among early adopters is predictive of greater birth spacing. However, contraceptive uptake does not have an impact on offspring survivorship. CONCLUSIONS: Our data provide support for the idea that preferences for low fertility emerge in response to increasing competition between offspring. The study has implications for our understanding of the emergence of local fertility norms and the spread of modern birth control.

Association between hybrid status and reproductive success of captive male and female rhesus macaques (Macaca mulatta) at the California National Primate Research Center (CNPRC).

The California National Primate Research Center (CNPRC) houses more than 1,000 rhesus macaques (Macaca mulatta) of mixed Chinese-Indian ancestry. Most of these animals are kept in outdoor field cages, the colony's long term breeding resource. Since 2001, hybrids comprised between 4 and 49% of the field cage populations, but in most cases have represented a maximum of 10% of those populations. The increasing prevalence of hybrids is partly due to management efforts to distribute genetic diversity effectively and minimize genetic subdivisions. However, other factors may also contribute to the spread of hybrids within the colony, most notably variance in socio-sexual behaviors and physical attributes. It is known that hybrids of some species exhibit heterosis, such as early maturation, that can enhance reproductive success, and anecdotal observations of mixed groups of hybrid, Indian and Chinese animals at the CNPRC suggest that hybrids are more sexually active. To determine whether hybrids experienced a reproductive advantage, a study was conducted using birth records of 5,611 offspring born in the CNPRC colony between 2003 and 2009. We found that while the degree of Chinese ancestry (DCA) appeared to influence the maturational schedule of both males and females (maturation was inversely related to proportion of Chinese ancestry), DCA had no independent effect on either male or female RS or rank. Therefore, we have found no evidence that a hybrid phenotype confers an absolute reproductive advantage in our colony.

What role do olfactory cues play in chacma baboon mating?

The probability of ovulation in a number of primate species is associated with both visual and auditory cues. We use 18-month behavioral data from two chacma baboon troops to provide the first systematic assessment of the possibility that olfactory cues are also involved. Using variance in the rate of olfactory inspection by males as a proxy for changes in the intensity of female vaginal odor, we found that rates of inspection were broadly correlated with changes in female fertility. Males inspected cycling females significantly more than anovulatory, noncycling females and swelling females significantly more than nonswollen cycling females. Rates of inspection peaked around the time at which males first started guarding females and were sustained until the detumescence of the female's sexual skin. We conclude, therefore, that olfactory cues represent one component of a multimodal signal of ovulation in chacma baboons. The possible reasons for such a multimodal signal are discussed.

The evolution of bequeathal in stable habitats.

Adults sometimes disperse, while philopatric offspring inherit the natal site, a pattern known as bequeathal. Despite a decades-old empirical literature, little theoretical work has explored when natural selection may favor bequeathal. We present a simple mathematical model of the evolution of bequeathal in a stable environment, under both global and local dispersal. We find that natural selection favors bequeathal when adults are competitively advantaged over juveniles, baseline mortality is high, the environment is unsaturated, and when juveniles experience high dispersal mortality. However, frequently bequeathal may not evolve, because the fitness cost for the adult is too large relative to inclusive fitness benefits. Additionally, there are many situations for which bequeathal is an ESS, yet cannot invade the population. As bequeathal in real populations appears to be facultative, yet-to-be-modeled factors like timing of birth in the breeding season may strongly influence the patterns seen in natural populations.