Devoted fathers or selfish lovers? Conflict between mating effort and parental care in a harem-defending arachnid

Partial paternity does not always select for female-biased care

The theoretical literature predicts that parentage differences between the sexes, due to females mating with multiple males, select males to provide less parental care and females to care more for the offspring. We formulate simple evolutionary games to question the generality of this prediction. We find that the relationship between paternal care and fitness gained from extra-pair matings is important. A trade-off between these two quantities is required for partial paternity and complete maternity to bias the evolutionary stable strategy (ESS) toward more female care. We argue that this trade-off has been implicitly or explicitly assumed in most previous theories. However, if there is no trade-off between paternal care and extra-pair matings, parentage differences do not influence the ESS sex roles. Moreover, it is also possible for these two quantities to have a positive relationship, in which case we predict selection for male care is possible. We support these predictions using agent-based simulations. We also consider the possibility that caring males have greater opportunities to guard their paternity, and find that this mechanism can also select for male-biased care. Hence, we derive the conditions under which male care may be selected despite partial paternity and complete maternity.

The evolutionary genetics of paternal care: How good genes and extrapair copulation affect the trade-off between paternal care and mating success

We investigate the evolution of a gene for paternal care, with pleiotropic effects on male mating fitness and offspring viability, with and without extrapair copulations (EPCs). We develop a population genetic model to examine how pleiotropic effects of a male mating advantage and paternal care are affected by "good genes" and EPCs. Using this approach, we show that the relative effects of each on fitness do not always predict the evolutionary change. We then find the line of combinations of mating success and paternal care that bisects the plane of possible values into regions of positive or negative gene frequency change. This line shifts when either good genes or EPCs are introduced, thereby expanding or contracting the region of positive gene frequency change and significantly affecting the evolution of paternal care. Predictably, a direct viability effect of "good genes" that enhances offspring viability constrains or expands the parameter space over which paternal care can evolve, depending on whether the viability effect is associated with the paternal care allele or not. In either case, the effect of a "good gene" that enhances offspring viability is substantial; when strong enough, it can even facilitate the evolution of poor paternal care, where males harm their young. When nonrandom mating is followed by random EPCs, the genetic regression between sire and offspring is reduced and, consequently, the relative strengths of selection are skewed away from paternal care and toward the male mating advantage. However, when random mating is followed by nonrandom EPCs, a situation called "trading up" by females, we show that selection is skewed in the opposite direction, away from male mating advantage and toward paternal care across the natural range of EPC frequencies.

The costs of being a good dad: egg-carrying and clutch size impair locomotor performance in male midwife toads (Alytes obstetricans)

Parental care is widespread across the animal kingdom. Parental behaviours are beneficial by increasing offspring survival but induce significant costs to the parents. Because parental care is far more common in females, the associated reproductive costs have been largely studied in this sex. Although male parental care is likely to involve significant costs, it has been markedly less well investigated. We studied the costs of egg-carrying on locomotor performance in an amphibian species (Alytes obstetricans) with male parental care. We examined complementary parameters including hopping performance, righting response, hindleg muscle response to egg burden, and homing time in males carrying or not carrying eggs. We found that carrying males showed altered locomotor performance for most traits. In addition, alteration of performance was closely related to relative clutch size. Clutch desertion occurred in smaller individuals carrying larger relative clutch mass, and performance after desertion was similar to that of non-reproductive individuals. Overall, our study demonstrates that carrying eggs significantly alters male mobility and that performance–clutch size trade-offs are relevant in understanding the evolution of paternal care.