Size increase with altitude in the Rufous-collared Sparrow (Zonotrichia capensis)

Schimmel/Conrades Science Center 163

Heat is generated by the body volume and lost across its surface. Therefore larger homeotherms with their proportionately larger volume and smaller surface area will withstand cold better than small homeotherms, which has been addressed in some basic ecological principles. Bergmann’s Rule indicates that organisms at higher latitudes (and therefore lower temperatures) will be larger than those at lower latitudes, and Allen’s Rule indicates that appendage size is generally smaller in cooler temperatures. Both Bergmann’s and Allen’s rules relate body size to latitude and/or temperature, but environmental temperature also changes with altitude. We tested the possible relationship between body size and altitude in Rufous-collared Sparrows (Zonotrichia capensis), which are abundant and range from sea level to the snow line in the Andes in a narrow latitudinal range in Peru. Due to the range of temperatures, we expected a similar change in body size with altitude as described for latitude by Bergmann’s Rule and decreased appendage size at higher altitudes, consistent with Allen’s Rule. To estimate the change in size, we measured the tarsometatarsus and took the weight of 198 museum specimens from the Peruvian Andes. Our results supported Bergmann’s Rule: an ANCOVA demonstrated that tarsometatarsus length increased significantly with altitude, but did not differ significantly between males and females, nor was there an interaction between sex and altitude. However, we did not find support for Allen’s Rule because a similar analysis of the ratio of tarsometatarsus to body mass did not yield any significant relationships. Thus, while the size of birds may be affected by altitude, and thus temperature, size is also affected by other competing selective pressures.