BackgroundIn species with parental care, there is striking variation in offspring dependence at birth, ranging from feeding independence to complete dependency on parents for nutrition. Frequently, highly dependent offspring further evolve reductions or alterations of morphological traits that would otherwise promote self‐sufficiency. Here, we examine evidence for morphological evolution associated with dependence in burying beetles (Nicrophorus spp.), in which dependence upon parents appears to have several independent origins. In many species, precocial first instar larvae can survive without parenting, but several altricial species die at this stage on their own. We focused specifically on the mandibles, which are expected to be related to feeding ability and therefore independence from parents.ResultsWe find no evidence that the size of the mandible is related to dependence on parents. However, we do find a developmental and phylogenetic correlation between independence and the presence of serrations on the inner edge of the mandible. Mandibles of independent species bear serrations at hatching, whereas dependent species hatch with smooth mandibles, only developing serrations in the second instar when these larvae gain the ability to survive on their own. Phylogenetic evidence suggests that serrations coincide with independence repeatedly. We note a single exception to this trend, a beetle with a serrated mandible that cannot survive without parents. However, this exception occurs in a species that has recently evolved the loss of independence.ConclusionsWe argue that the absence of mandible serrations occurs due to alternative selection pressures incurred in larvae dependent upon parents to survive. We suggest that this may have led to a variable function for mandibles, perhaps related to increased competitive ability among siblings or increased efficiency in receiving nutrition from parents. Furthermore, we propose that the phylogenetic pattern we see is consistent with the long‐held evolutionary hypothesis that evolutionary change in behavior and physiology precede morphological change.

The variation in degree of offspring dependence in parents where parental care has evolved is striking, from feeding independence at birth to complete dependence on parents for all nutritional resources. This presents an evolutionary puzzle. Why lose the ability to feed as a contingency when parents may die or abandon broods? Comparisons of altricial and precocial vertebrates suggest that there may be life-history and developmental costs to early independence1-3. The generality of this beyond vertebrates is unclear, but we can extend the comparison as invertebrate species also vary in the level of independence in early life-history stages. For example, larvae of several burying beetle species (Nicrophorus), a genus in which parents regurgitate pre-digested food to begging larvae, have lost the ability to self-feed thus creating complete parental dependency for first instars4. Here, we ask whether variation in dependency amongst burying beetles is related to heterochrony in development of a more complex morphological structures. We show that the rate of development and allometry of mandibles of precocial larvae that can self-feed from birth are the same as those in altricial larvae that cannot survive without parenting. Instead, self-feeding is associated with shape variation in mandibles. In altricial species first instar larvae have smooth mandibles, whereas in precocial species mandibles are serrated. Later instars, which can self-feed in all species, have serrated mandibles. Serrations on teeth generally function to “grip and rip”5, whereas smooth blades function more to puncture6, and broods of altricial but not precocial Nicrophorus larvae show evidence for siblicide. We therefore suggest that altricial first-instar mandibles function more as weapons than feeding tools when released from self-feeding. This study presents a novel coevolution between developmental timing and parenting potentially mediated by sibling competition.