Multiple paternity is related to adult sex ratio and sex determination system in reptiles

The adult sex ratio (ASR, the proportion of males in the adult population)
is an emerging predictor of reproductive behaviour, and recent studies in
birds and humans suggest it is a major driver of social mating systems and
parental care. ASR may also influence genetic mating systems. For
instance, male-skewed ASRs are expected to increase the frequency of
multiple paternity (defined here as a clutch or litter sired by two or
more males) due to higher rates of coercive copulations by males and/or
due to females exploiting the opportunity of copulation with multiple
males to increase genetic diversity of their offspring. Here we evaluate
this hypothesis in reptiles that often exhibit high frequency of multiple
paternity, although its ecological and life-history predictors have
remained controversial. Using a comprehensive dataset of 81 species
representing all four non-avian reptile orders, we show that increased
frequency of multiple paternity is predicted by more male-skewed ASR, and
this relationship is robust to simultaneous effects of several
life-history predictors. Additionally, we show that the frequency of
multiple paternity varies with the sex determination system: species with
female heterogamety (ZZ/ZW sex chromosomes) exhibit higher levels of
multiple paternity than species with male heterogamety (XY/XX) or
temperature-dependent sex determination. Thus, our across-species
comparative study provides the first evidence that genetic mating system
depends on ASR in reptiles. We call for further investigations to uncover
the complex evolutionary associations between mating systems, sex
determination systems and ASR.