The age at which male California sea lions (Zalophus califomianus) have been thought to reach reproductive maturity ranges anywhere from three to five years old. As a result of ongoing research and a successful breeding management program made possible through a zoological setting, we have discovered that male California sea lions can reach reproductive maturity as early as two years of age. This discovery has led us to begin our breeding management program much earlier than was previously thought necessary.

Variation in breeding age among mammal species can occur under a variety of circumstances. Abundant food resources, increased access to reproductive females, and reduced competition among reproductive males are just a few factors that affect the reproductive potential of a population. In polygynous species, such as California sea lions, reproductive maturity in males occurs early, but due to a highly evolved breeding system which selects for older sexually dimorphic males, reproductive success does not occur until a bull is large enough to defend a harem.

We have found that when there are abundant food resources and decreased competition among males of reproductive age due to castration of all males in the exhibit over three years of age, two year old male California sea lions successfully copulated with and impregnated reproductively mature females. This finding was based on several cases where this occurred and by histological evidence of spermatogenesis in a two year old male California sea lion. These findings suggest that if your institution's goal is zero population growth, castration or separation must take place prior to the breeding season for two year old males.