Male survival advantage on the Baja California peninsula

Sex bias in mortality risk changes over the lifespan of bottlenose dolphins

Research on sex biases in longevity in mammals often assumes that male investment in competition results in a female survival advantage that is constant throughout life. We use 35 years of longitudinal data on 1003 wild bottlenose dolphins (Tursiops aduncus) to examine age-specific mortality, demonstrating a time-varying effect of sex on mortality hazard over the five-decade lifespan of a social mammal. Males are at higher risk of mortality than females during the juvenile period, but the gap between male and female mortality hazard closes in the mid-teens, coincident with the onset of female reproduction. Female mortality hazard is non-significantly higher than male mortality hazard in adulthood, resulting in a moderate male bias in the oldest age class. Bottlenose dolphins have an intensely male-competitive mating system, and juvenile male mortality has been linked to social competition. Contrary to predictions from sexual selection theory, however, male-male competition does not result in sustained male-biased mortality. As female dolphins experience high costs of sexual coercion in addition to long and energetically expensive periods of gestation and lactation, this suggests that substantial female investment in reproduction can elevate female mortality risk and impact sex biases in lifespan.

Adult sex ratios: causes of variation and implications for animal and human societies

Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation.

Abandonment of the Middle Cumberland Region of Tennessee during the Mississippian period: Temporal and sex differences in survivorship

OBJECTIVES

The current research explores survivorship differences in the Middle Cumberland Region of Tennessee during the Mississippian period (ca. 1000-1500 AD). Using updated paleodemographic methods, this study investigates whether individuals from the Late Mississippian period had lower survivorship compared to individuals from the Early Mississippian period, foreshadowing groups on the brink of abandonment. Additionally, this study examines whether there were sex disparities in survivorship.

MATERIALS AND METHODS

Adult age-at-death estimates from human skeletal remains (n = 545) were calculated using Transition Analysis, a Bayesian maximum likelihood method. Survivorship was reconstructed using Gompertz and Gompertz-Makeham hazards models, with Akaike Information Criterion compared to determine the best fitting model.

RESULTS

For individuals surviving into adulthood, mean age-at-death decreased by 7 years from the Early Mississippian to Late Mississippian period. Marked differences between the sexes indicate lower survivorship of females compared to males. Male survivorship decreased sharply from the Early Mississippian to Late Mississippian period, from a mean age-at-death of 57.99 years to 44.45 years. Female survivorship remained constant throughout the Mississippian period (41 years).

DISCUSSION

Temporal differences in male survivorship may be the result of interactions between climate change, decreased maize harvests, and sociopolitical strife. Sustained maternal mortality may have destabilized small-scale communities that characterized the Mississippian MCR, thereby precipitating population decline.

NDVI predicts birth seasonality in historical Baja California Sur, Mexico: adaptive responses to arid ecosystems and the North American Monsoon

Birth seasonality is a phenomenon whereby populations can be characterized by a single month or season in which births peak. While non-human animal research suggests that seasonal birth-pulses are related to variation in climate and local energy availability, social scientists debate the mechanisms responsible for it in humans. Here we investigate the role of precipitation, temperature, and energy availability on seasonal conception and birth pulses using a historical dataset from the Baja California peninsula - a hot, arid desert that experiences seasonal climatic fluctuations associated with the North American Monsoon. Analyses suggest that 1) local energy availability had a negative relationship with conception pulses; and 2) birth pulses had a positive relationship with local energy availability and a negative relationship with temperature. Taken together, our analyses suggest that women timed conceptions when local energy availability was lowest (challenging expectations of conception rates as simply reflecting ecological influences on female fecundity), so that children were born during the seasonal "green-up" associated with the North American Monsoon. Given our results, we speculate that birth seasonality represents a form of traditional ecological knowledge to improve neonate health and wellbeing.