Altitude decreases testis weight of a frog (Rana kukunoris) on the Tibetan plateau

Producing sperm is energetically inexpensive, and strong competition for mating partners can lead to increased size of the testes in an effort to enhance reproductive success. On the other hand, selection on testes size can also be imposed by environmental conditions. We studied altitudinal variation and directional asymmetry in testis weight in a high-altitude frog (Rana kukunoris) endemic to the Tibetan plateau (2300-3500 m altitude). Testis weight decreased with increasing altitude and body size. The left testis was significantly larger than the right testis for all populations, and relative asymmetry between testes was unrelated to altitude or body size. The harsh environmental conditions at high altitudes may constrain the ability of males to allocate energy towards increased testis size. They could also be associated with altered operational sex ratios, thus reducing the strength of male-male competition.