A comparison of paternity data and relative testes size as measures of level of sperm competition in the Hominoidea.

OBJECTIVES: The phrase "level of sperm competition" is used only vaguely in the primate literature. There is also little distinction between the important elements of frequency and intensity of sperm competition, largely because the two current forms of measurement (socio-sexual system and relative testes size) are both proxies which allow neither precision nor fine distinctions. Both measures have critics, socio-sexual system in particular being branded subjective, misleading, and changeable. Testes size is considered the more reliable despite its validation resting on correlations with the other, less reliable, proxy. Recently, genetic paternity studies have been mooted to provide a potentially superior third measure of sperm competition but so far lack a formal interpretive framework. Here we use the published and relatively comprehensive genetic field studies of the Hominoidea to develop such a framework. MATERIALS AND METHODS: Formulae are derived to convert paternity data into a direct measure of the frequency, intensity, and overall level of sperm competition. We then compare these measures with relative testes size at the study, species, and phylogenetic levels. RESULTS: A significant correlation between level of sperm competition and relative testes size was obtained at each level. These correlations provide independent support for the continuing use of testes size as a proxy measure when such a measure is sufficient. However, they also suggest that paternity data and our formulae yield a viable alternative measure. DISCUSSION: This alternative measure based on paternity data has a number of advantages. Not only is it a potentially direct measure of the level of sperm competition but it also allows the roles of frequency and intensity to be studied separately when of interest.