The role of lipid metabolism during parental care in two species of burying beetle (Nicrophorus spp.)

Despite the ubiquity of energetic costs as an explanation for behavioural evolution, the proximate basis of these costs is often unclear. One candidate that predictably regulates energetic expenditure in insects is the lipid metabolism pathway, which is used in behavioural activities such as flight. Here, we investigated lipid metabolism and its role in parental behaviour of males and females in two species of burying beetle, Nicrophorus orbicollis and Nicrophorus vespilloides, which provide pre-hatching care by burying and preparing vertebrate carcasses before larvae arrive, and then post-hatching care by directly regurgitating food to begging offspring. We took two approaches: first, we investigated changes in the lipid stores of parenting beetles during pre-hatching and post-hatching care by measuring triglyceride concentration, compared to beetles not involved in care. Second, we quantified expression of the lipid transporter gene apolipophorin-III (apoLp-III) in these three behavioural states. Females and males of both species were similar in triglyceride use, but there were species differences. We observed a loss of triglycerides during pre-hatching but not post-hatching care in N. vespilloides, but no change in triglycerides during either pre-or post-hatching care in N. orbicollis. There were both species and sex differences in apoLp-III expression. Expression decreased during caring in both male and female N. orbicollis, and male N. vespilloides, but only decreased in female N. vespilloides providing post-hatching care. This roughly corresponded to changes in triglyceride abundance. These results indicate that either parental care is not energetically costly or, more likely, that lipid metabolism plays only a partial role in regulating energetic expenditure during Nicrophorus parental care. We suggest that the energy source used varies, perhaps between species but certainly across behavioural states. Furthermore, our results suggest that ApoLp-III may provide an alternate behavioural or immune function during parental care besides its canonical role as a lipid transporter. (C) 2017 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.