Female biased sexual dimorphism is uncommon in mammals, and its ontogeny is poorly documented. Female biased sexual dimorphism is usually attributed to increased fecundity of large females. We studied a population of cliff chipmnks (Tamias dorsalis), a small mammal with female biased sexual dimorphism, to describe the development of juvenile sexual dimorphism and to measure adult sexual dimorphism and seasonal body mass. To test the fecundity hypothesis, we compared female body size to litter size and offspring body mass. Cliff chipmunks in southeastern Arizona, USA were trapped from March – October 2013-2014. Animal body mass was recorded for each trapping event and a subset of nursing females were radio-collared to track and observe their nest locations for offspring. Offspring of radio-collared females were weighed at initial nest emergence and repeatedly trapped and weighed throughout the year.
Results/Conclusions
Juveniles were not sexually dimorphic at initial nest emergence, and did not differ in body mass until 3 months after first leaving the nest. Adult chipmunks maintained a relatively stable body mass from March – October with females being consistently larger than males. Female body mass was stable throughout the year, whereas male body mass in August was slightly higher than other months. Large and small females did not differ in litter size, and juveniles of large and small females did not differ in body condition. Our results indicate that the female fecundity hypothesis does not explain sexual dimorphism in chipmunks. As females are consistently larger than males, the ontogeny of sexual dimorphism may provide insights into the selection pressures on males and females that lead to female biased sexual dimorphism.