The Evolution of Male and Female Parental Care in Fishes

Abstract

In this paper we propose an explanation for (a) the predominance of male care in fishes, and (b) the phylogenies and transitions that occur among care states. We also provide a general evolutionary model for studying the conditions under which parental care evolves. Our conclusions are as follows: (i) Parental care has only one benefit, the increased survivorship of young. It may, however, have three costs: a “mating cost,” an “adult survivorship cost,” and a “future fertility cost.” (ii) On average, males and females will derive the same benefit from care. They probably also pay the same adult survivorship cost. However, their mating cost and future fertility costs may differ, (iii) A mating cost usually applies only to males. However, this cost may be reduced by male territoriality and, in some situations, be entirely removed. Under this condition, natural selection on present reproductive success is equivalent for males and females, (iv) When fecundity accelerates with body size in females, while male mating success follows a linear relationship with body size, future fertility costs of parental care are greater for females than males. Although further tests are needed, a preliminary analysis suggests this often may be the case in fishes. Thus, the predominance of male parental care in fishes is not explained by males deriving greater benefits from care, but by males paying smaller future costs. Males thus accrue a greater net fitness advantage from parental care (see expressions [6] and [12]). (v) The evolution of biparental care from uniparental male care may occur because male care selects for larger egg sizes and increased embryo investment by females. This increases the benefit to the female of parental care, (vi) By contrast, uniparental female care may originate from biparental care when males are selected to desert. This occurs when female care creates a mating cost to males. In some cases male desertion may “lock” females into uniparental care. However, in many other cases females may be selected to desert, giving rise to “no care.” (vii) The origin of uniparental female care from no care is rare in externally fertilizing fishes. This is because the benefits of care rarely outweigh a female's future fertility costs (expression [9]). For internally fertilizing species, however, the benefit of care is high whereas the cost is probably low. Most of these species have evolved embryo retention, (viii) When parental care begins with male care and moves to biparental care, our analysis suggests that care evolution will include cyclical dynamics. Parental care in some fishes may thus be seen as transitional and changing through evolutionary time rather than as an evolutionarily stable state. In theory, “no care” may be a phylogenetically advanced state.