From Habitat to Hormones: Year-around territorial behavior in rock-dwelling but not in forest and grassland lagomorphs and the role of DHEA

Year-around defense of extremely patchy habitat may require hormones that drive territorial behavior, but have no other costly physiological effects. The American pika (Ochotona princeps), but not the snowshoe hare (Lepus americanus) nor the eastern cottontail rabbit (Sylvilagus floridanus), exhibits this behavior. The former engages in contest competition against all individuals independent of sex to protect its territory in highly fragmented patches on mountain talus slopes; the latter in scramble competition in more continuous forests, grasslands, and shrublands. The hormonal basis for this difference in lagomorphs is unknown. Dehydroepiandrosterone is a prohormone produced by the zona reticularis of the adrenal cortex. It has no effect on aggressive behavior until converted in the brain to estrogen. We assessed levels (DHEA-S plus DHEA) in all species collected in the wild. In nonbreeding pikas, levels were 256 times higher than in hares and 22 times higher than in rabbits. Within species, females and males had similar levels. The proportion of the adrenal cortex devoted to the zona reticularis was significantly larger in pikas than in hares or rabbits. Our evidence is consistent with the hypothesis that dehydroepiandrosterone drives this individual-based, year-around territoriality in pikas. We propose a definitive experiment to determine this and recommend comparative studies in central Asia where there is high diversity of pika species whose behavior ranges from individual-based territoriality to colonial. Thus, we speculate that the wild American pika has the adrenal-brain nexus for all seasons and is an excellent model to understand how habitat drives the hormonal control of spacing behavior.