Department of Anthropology, Texas A&M University
Saturday All day, Plaza Level
Among primates, body mass differences between males and females associates with male-male competition and polygynous mating systems. However, degree of sexual dimorphism negatively correlates with lower quality environments. Resource-poor environments such as high altitude forest may constrain the evolution of sexually selected traits. For instance, while increased sperm competition is important in mating systems with multiple breeding males, environmental constraints may limit the development of larger testes. Under this hypothesis, both sexual dimorphism and testes size are expected to decline with increasing altitude. This study examines whether two sexually selected traits, sexual dimorphism of body mass and testes volume, match allometric and behavioral expectations for pygmy tarsiers (Tarsius pumilus), a high-altitude tarsier species.
Pygmy tarsiers have been observed to sleep in groups with more than one adult male, indicating they should experience more sperm competition than lowland populations where adult males do not associate. Data on primate body size and relative testes size were compared from a combination of published literature and unpublished data. Pygmy tarsiers do not show significant body mass differences between the sexes (n=15; Kruskal-Wallis: α=0.05; Z=0.7578; p=0.4486). Compared to lowland tarsiers, pygmy tarsiers have relatively smaller testes volume, indicating less sperm competition. These results suggest a monogamous mating system among pygmy tarsiers, and may indicate these tarsiers experience less sexual selection pressures than lowland tarsier species in association with high altitude constraints. These results may be counfounded by temporal changes in testes size, since mating season and seasonal effects are unknown in pygmy tarsiers.
This study was funded by National Science Foundation Doctoral Dissertation Improvement grant number BCS-1028885, Primate Conservation, Inc. grant number 99-425691-00001, Conservation International Primate Action Fund, American Society of Primatologists Conservation Small Grant, Explorers Club Exploration Fund Grant, Texas A&M University Department of Anthropology, and the National Geographic Society.