Field energetics of free-living, lactating and non-lactating echidnas ( Tachyglossus aculeatus)

We measured daily energy expenditure (DEE) and water turnover rates in lactating and non-lactating short beaked echidnas ( Tachyglossus aculeatus) using the doubly labelled water technique during the lactation period in spring. Reproductively inactive echidnas were on average significantly heavier (median: 3354 g; range: 2929–3780 g; N=4) than lactating females (median: 2695 g; range: 2690–2715 g; N=3) during the equivalent time period. The median water flux rate of lactating echidnas (152 ml day −1; range: 120–198 ml day −1) did not differ significantly from that of non-lactating females (170 ml day −1; range: 128–227 ml day −1). The median DEE of echidnas that were lactating was 645 kJ day −1 (range: 581–850 kJ day −1), which was not different from the median DEE of non-reproductive control females (763 kJ day −1; range: 720–766 kJ day −1). Lactating females somehow compensate for the energy costs of milk production, resulting in a daily energy budget that is not different from that of non-reproductive females. At least part of their energy minimising strategy could involve the use of moderate heterothermy, allowing a greater proportion of daily energy expenditure to diverted to milk production.