1. Echidnas are egg-laying mammals found across Australia and in Tasmania they hibernate resulting in a most unusual mating system: males enter hibernation in late summer-early autumn and arouse in late autumn-early winter to mate, although females are still hibernating. Groups of males compete for matings and both males and females mate with multiple partners. Females that mate early return to hibernation even when pregnant, and males continue to mate with pregnant females. We asked to what extent to can the bizarre combination of behavioural and physiological features that characterize reproduction of Tasmanian echidnas be attributed to their phylogeny, and how much is a consequence of their ecology? 2. To understand the interaction between energetics and the echidna mating system in determining the timing of echidna hibernation we analysed data from an 18-year study of a wild population of Tasmanian echidnas 3. Males with high fat reserves arouse earliest and seek out suitable females, and females that mate early in the mating season re-enter hibernation while pregnant. 4. Competition between males drives early mating and while mating with males in the best condition could be advantageous for females and their young, egg-laying in winter is potentially disadvantageous, and post-mating hibernation by females is a means of delaying hatching of young until environmental conditions are more favourable. This post-mating hibernation by females is usually disrupted by males which mate with them although they are already pregnant. 5. Comparisons with other echidna populations suggests that a decreased activity period due to hibernation has not increased male-male competition. 6. Similar competition between groups of males for access to females is seen in chlamyphorid armadillos, which occupy a similar ecological niche to echidnas.