Age at puberty in male African striped mice: the impact of food, population density and the presence of the father

P>1. The time at which animals enter puberty and become sexually mature is a significant life-history trait, influencing lifetime reproductive success. Great variation exists both between and within species. 2. The proximate mechanisms regulating the time at which a male enters puberty are not well-understood. Environmental cues are predicted to provide the relevant information on resource availability and opportunities for reproduction. When these are good the onset of puberty begins whereas at other times investment in survival becomes more important. 3. Male African striped mice (Rhabdomys pumilio) demonstrate large variation in the age at which they enter puberty, with grassland populations starting at 4 weeks old and semi-desert populations at over 10 weeks old. 4. We predicted that differences in the availability of food, social organization and population density could explain these differences. 5. Using data on 170 individual males from 4 years of field studies in a semi-desert population, we found that males became scrotal at a younger age when no breeding male was present in their group and when food was abundant, while population density had no effect. 6. In laboratory experiments we demonstrated that males fed with poor protein food, that regularly encounter larger unfamiliar males (mimicking high population density), and that live in family groups with their father present, become scrotal at a significantly later age, independent of their growth rate. 7. Males housed in family groups have lower testosterone but higher corticosterone levels than singly housed males, indicating they are sexually suppressed. When they become scrotal in their family group, their testes are only half as large as those of their singly housed brothers, and they contained significantly less sperm. 8. We conclude that male striped mice have a flexible response to the onset of puberty, and that the onset of sexual maturity is dependent on several environmental cues. Our results indicate that there is no threshold body mass, which, when reached, would automatically trigger puberty in male striped mice. 9. Male helpers in some species are reproductively suppressed, but ours is the first study that demonstrated the importance of different ecological factors in the timing of puberty in male helpers in a facultative cooperatively breeding species.