Parent–offspring cannibalism throughout the animal kingdom: a review of adaptive hypotheses

Parents that kill and consume their offspring often appear to be acting against their own reproductive interests. Yet parent-offspring cannibalism is common and taxonomically widespread across the animal kingdom. In this review, I provide an overview of our current understanding of parent-offspring cannibalism, which has seen a proliferation in adaptive hypotheses over the past 20 years for why parents consume their own young. I review over four decades of research into this perplexing behaviour, drawing from work conducted on fishes, reptiles, insects, birds, and mammals among other taxa. Many factors have been hypothesised to explain parent-offspring cannibalism in nature, including poor parental energy reserves, small or large brood sizes, low or uncertain parentage, and high brood densities, and additional factors are still being uncovered. Parent-offspring cannibalism does not appear to have a single predominant explanation; rather, the factor, or set of factors, that govern its expression is largely taxon specific. Parents may either consume all offspring under their care (full-brood cannibalism) or consume a fraction of their offspring (partial brood cannibalism). These forms of cannibalism are thought to provide adaptive benefits to cannibals under a range of circumstances, primarily by allowing parents to allocate parental efforts more optimally - energy from eating (some of) one's current offspring can be redirected to other offspring, or to parental growth, survival, and ultimately to other future reproductive endeavours. Thus, parent-offspring cannibalism is a phenotypically plastic trait that responds to changing environmental, social, and physiological conditions. The expression of parent-offspring cannibalism in any given system is intimately linked to the reproductive value of current young relative to parents' expectations for future reproduction, and also to whether parental care is predominantly depreciable or non-depreciable. Furthermore, parent-offspring cannibalism has the potential to generate conflict between the sexes, and I briefly discuss some consequences of this conflict on patterns of mate choice. Finally, there still remain many aspects of this behaviour where our understanding is poor, and I highlight these topics to help guide future research.