Parental care results in a greater mutation load, for which it is also a phenotypic antidote

Benevolent social behaviours, such as parental care, are predicted to
relax selection against deleterious mutations, enabling them to persist.
We tested this prediction experimentally using burying beetles,
Nicrophorus vespilloides, which make an edible nest for their larvae, whom
they nourish and defend. For 20 generations, we allowed replicate
experimental burying beetle populations to evolve either with
post-hatching care (‘Full Care’ populations) or without it (‘No Care’
populations). Lineages were seeded from these experimental populations and
then inbred to expose differences in their mutation load. Outbred lineages
served as controls. Half the lineages received post-hatching care, half
did not. We found that inbred lineages derived from the Full Care
populations had lower breeding success and went extinct more quickly than
lineages derived from the No Care populations – but only when offspring
received no post-hatching care. We infer that Full Care lineages carried
more recessive deleterious mutations. When parents provided care, the
developmental environment was sufficiently benign that broods had higher
survival, whether the population had a high mutation load or not. We
suggest that the increased mutation load caused by parental care increases
a population’s dependence upon care. This could explain why care is seldom
lost once it has evolved.