Interaction of hibernation and male reproductive function in wild Tasmanian echidnas Tachyglossus aculeatus setosus

The short-beaked echidna Tachyglossus aculeatus is a seasonally breeding mammal with a near ubiquitous distribution throughout Australia. In Tasmania, breeding follows a period of deep hibernation, and males begin mating approximately 30 days after the termination of hibernation. The echidna has exceptionally large testes, which may reach a maximum of 1% of body mass during the mating period. As involution of gonads is considered a prerequisite for entering hibernation and hibernation typically suppresses all reproductive function, this raises questions about the timing of testes recrudescence in the Tasmanian echidna. We measured plasma testosterone concentrations and used ultrasonography to measure testicular and crural gland volume through the annual cycle in wild Tasmanian echidnas. Testes were at their minimum size (0.06% of body mass) in December (early summer); testes recrudescence occurred prior to entry into hibernation when plasma testosterone concentrations were low; and testes were maintained at 75% of their maximum volume throughout the hibernation period. The crural glands, which are secondary reproductive structures in the echidna, also exhibited an annual pattern of recrudescence and involution, with recrudescence occurring after males emerged from hibernation, when plasma testosterone was rising. We suggest that the unusual strategy of testes recrudescence occurring prior to hibernation in the Tasmanian echidna is a consequence of extremely high competition between males.