Parental antagonism and parent-offspring co-adaptation interact to shape family life

The family is an arena for conflicts between offspring, mothers and fathers that need resolving to promote the evolution of parental care and the maintenance of family life. Co-adaptation is known to contribute to the resolution of parent-offspring conflict over parental care by selecting for combinations of offspring demand and parental supply that match to maximize the fitness of family members. However, multiple paternity and differences in the level of care provided by mothers and fathers can generate antagonistic selection on offspring demand (mediated, for example, by genomic imprinting) and possibly hamper co-adaptation. While parent-offspring co-adaptation and parental antagonism are commonly considered two major processes in the evolution of family life, their co-occurrence and the evolutionary consequences of their joint action are poorly understood. Here, we demonstrate the simultaneous and entangled effects of these two processes on outcomes of family interactions, using a series of breeding experiments in the European earwig, Forficula auricularia, an insect species with uniparental female care. As predicted from parental antagonism, we show that paternally inherited effects expressed in offspring influence both maternal care and maternal investment in future reproduction. However, and as expected from the entangled effects of parental antagonism and co-adaptation, these effects critically depended on postnatal interactions with caring females and maternally inherited effects expressed in offspring. Our results demonstrate that parent-offspring co-adaptation and parental antagonism are entangled key drivers in the evolution of family life that cannot be fully understood in isolation.     

Hormone levels predict individual differences in reproductive success in a passerine bird

Hormones mediate major physiological and behavioural components of the reproductive phenotype of individuals. To understand basic evolutionary processes in the hormonal regulation of reproductive traits, we need to know whether, and during which reproductive phases, individual variation in hormone concentrations relates to fitness in natural populations. We related circulating concentrations of prolactin and corticosterone to parental behaviour and reproductive success during both the pre-breeding and the chick-rearing stages in both individuals of pairs of free-living house sparrows, Passer domesticus. Prolactin and baseline corticosterone concentrations in pre-breeding females, and prolactin concentrations in pre-breeding males, predicted total number of fledglings. When the strong effect of lay date on total fledgling number was corrected for, only pre-breeding baseline corticosterone, but not prolactin, was negatively correlated with the reproductive success of females. During the breeding season, nestling provisioning rates of both sexes were negatively correlated with stress-induced corticosterone levels. Lastly, individuals of both sexes with low baseline corticosterone before and high baseline corticosterone during breeding raised the most offspring, suggesting that either the plasticity of this trait contributes to reproductive success or that high parental effort leads to increased hormone concentrations. Thus hormone concentrations both before and during breeding, as well as their seasonal dynamics, predict reproductive success, suggesting that individual variation in absolute concentrations and in plasticity is functionally significant, and, if heritable, may be a target of selection.     

Effects of testosterone on song, aggression, and nestling feeding behavior in male great tits, Parus major

As outlined in the trade-off hypothesis of testosterone (T) secretion, fluctuations in T during the breeding season might reflect how males allocate their time and energy to competitive behaviors for mates and territories, associated with high T levels, and parental activities, associated with low T levels. In the present study, great tit, Parus major, males were implanted with T-filled or empty silastic capsules at the start of the breeding season and the behavior of these two male categories was compared during the entire breeding season. As a measure of competitive behavior we looked at song behavior and territorial responsiveness to a male decoy, during the three main stages of the breeding period (the egg-laying, incubation, and nestling stages). As a measure of parental care we looked at feeding behavior during the nestling stage. Our results only partly supported the trade-off hypothesis. T implants increased plasma androgen levels and enhanced spontaneous song activity and the production of aggressive vocalizations in response to a decoy. However, our results suggest that the degree of physical aggression might be less than fully coupled with T. First, approach to the decoy was not affected by the treatment. Second, although T levels are known to vary from high during egg laying to low while feeding young, control and T-treated males spent similar amounts of time close to the decoy in the three breeding stages. Our results thus suggest that vocal and physical aggression might be regulated differently in the great tit. Furthermore, in contrast with most other studies on temperate bird species but in agreement with a previous study on the great tit, T treatment did not affect male feeding rates. As the dose of T we used was lower than that typically used in other studies, we cannot at present completely exclude the possibility that the latter result reflects this lower dose of T rather than the species used.     

Parental age influences developmental stability of the progeny in Drosophila

The stochastic nature of biochemical processes is a source of variability that influences developmental stability. Developmental instability (DI) is often estimated through fluctuating asymmetry (FA), a parameter that deals with within-individual variation in bilateral structures. A relevant goal is to shed light on how environment, physiology and genotype relate to DI, thus providing a more comprehensive view of organismal development. Using Drosophila melanogaster isogenic lines, we investigated the effect of parental age, parental diet and offspring heterozygosity on DI. In this work, we have uncovered a clear relationship between parental age and offspring asymmetry. We show that asymmetry of the progeny increases concomitantly with parental age. Moreover, we demonstrate that enriching the diet of parents mitigates the effect of age on offspring symmetry. We show as well that increasing the heterozygosity of the progeny eliminates the effect of parental age on offspring symmetry. Taken together, our results suggest that diet, genotype and age of the parents interact to determine offspring DI in wild populations. These findings provide us with an avenue to understand the mechanisms underlying DI.     

Post-hatching parental care masks the effects of egg size on offspring fitness: a removal experiment on burying beetles

Parents can increase the fitness of their offspring by allocating nutrients to eggs and/or providing care for eggs and offspring. Although we have a good understanding of the adaptive significance of both egg size and parental care, remarkably little is known about the co-evolution of these two mechanisms for increasing offspring fitness. Here, we report a parental removal experiment on the burying beetle Nicrophorus vespilloides in which we test whether post-hatching parental care masks the effect of egg size on offspring fitness. As predicted, we found that the parent's presence or absence had a strong main effect on larval body mass, whereas there was no detectable effect of egg size. Furthermore, egg size had a strong and positive effect on offspring body mass in the parent's absence, whereas it had no effect on offspring body mass in the parent's presence. These results support the suggestion that the stronger effect of post-hatching parental care on offspring growth masks the weaker effect of egg size. We found no correlation between the number and size of eggs. However, there was a negative correlation between larval body mass and brood size in the parent's presence, but not in its absence. These findings suggest that the trade-off between number and size of offspring is shifted from the egg stage towards the end of the parental care period and that post-hatching parental care somehow moderates this trade-off.     

Parental energy expenditure of the male three-spined stickleback

The energy expenditure of male three-spined sticklebacks performing parental care was 12·3 and 9·9 J g⁻¹ h⁻¹ estimated by respirometry and ration manipulation, respectively. The energy expenditure of caring male sticklebacks was significantly higher than for non-caring males.