The Evolution of Male and Female Parental Care in Fishes

In this paper we propose an explanation for (a) the predominanceof male care in fishes, and (b) the phylogenies and transitionsthat occur among care states. We also provide a general evolutionarymodel for studying the conditions under which parental careevolves. Our conclusions are as follows: (i) Parental care hasonly one benefit, the increased survivorship of young. It may,however, have three costs: a "mating cost," an "adult survivorshipcost," and a "future fertility cost." (ii) On average, malesand females will derive the same benefit from care. They probablyalso pay the same adult survivorship cost. However, their matingcost and future fertility costs may differ, (iii) A mating costusually applies only to males. However, this cost may be reducedby male territoriality and, in some situations, be entirelyremoved. Under this condition, natural selection on presentreproductive success is equivalent for males and females, (iv)When fecundity accelerates with body size in females, whilemale mating success follows a linear relationship with bodysize, future fertility costs of parental care are greater forfemales than males. Although further tests are needed, a preliminaryanalysis suggests this often may be the case in fishes. Thus,the predominance of male parental care in fishes is not explainedby males deriving greater benefits from care, but by males payingsmaller future costs. Males thus accrue a greater net fitnessadvantage from parental care (see expressions 6] and 12]).(v) The evolution of biparental care from uniparental male caremay occur because male care selects for larger egg sizes andincreased embryo investment by females. This increases the benefitto the female of parental care, (vi) By contrast, uniparentalfemale care may originate from biparental care when males areselected to desert. This occurs when female care creates a matingcost to males. In some cases male desertion may "lock" femalesinto uniparental care. However, in many other cases femalesmay be selected to desert, giving rise to "no care." (vii) Theorigin of uniparental female care from no care is rare in externallyfertilizing fishes. This is because the benefits of care rarelyoutweigh a female's future fertility costs (expression 9]).For internally fertilizing species, however, the benefit ofcare is high whereas the cost is probably low. Most of thesespecies have evolved embryo retention, (viii) When parentalcare begins with male care and moves to biparental care, ouranalysis suggests that care evolution will include cyclicaldynamics. Parental care in some fishes may thus be seen as transitionaland changing through evolutionary time rather than as an evolutionarilystable state. In theory, "no care" may be a phylogeneticallyadvanced state.