Offspring desertion by parents generally occurs at an early stage of parental care, which is considered to minimize the costs of parental care prior to desertion. This study investigated the effects of endocrinological constraint on early total filial cannibalism by male Rhabdoblennius nitidus in the field, a paternal brooding blennid fish with androgen-dependent brood cycling. In brood reduction experiments, cannibal males showed low levels of plasma 11-ketotestosterone (KT) relative to non-cannibals, and also similar levels of 11-KT to males in the parental care phase. Since 11-KT regulates male courtship intensity, males with decreased courtship activity would exhibit total filial cannibalism. However, there is a possibility that a transient increase in 11-KT levels at the early stage of parental care delays total filial cannibalism. In contrast, total filial cannibalism could occur before a decline to the lowest 11-KT levels, at which males might still be able to exhibit courtships, probably to reduce the costs of parental care. To understand how much and when caregiving males exhibit mating and parental care behaviors, it is important to consider not only the presence of endocrinological constraint but also its intensity and flexibility.

All statistical analyses were conducted using R version 3.5.2 (R Development Core Team, 2018).