The role of social environment on parental care: offspring benefit more from the presence of female than male helpers

Mothers in a cooperatively breeding bird increase investment per offspring at the pre-natal stage when they will have more help with post-natal care

In many cooperative societies, including our own, helpers assist with the post-natal care of breeders' young and may thereby benefit the post-natal development of offspring. Here, we present evidence of a novel mechanism by which such post-natal helping could also have beneficial effects on pre-natal development: By lightening post-natal maternal workloads, helpers may allow mothers to increase their pre-natal investment per offspring. We present the findings of a decade-long study of cooperatively breeding white-browed sparrow-weaver, Plocepasser mahali, societies. Within each social group, reproduction is monopolized by a dominant breeding pair, and non-breeding helpers assist with nestling feeding. Using a within-mother reaction norm approach to formally identify maternal plasticity, we demonstrate that when mothers have more female helpers, they decrease their own post-natal investment per offspring (feed their nestlings at lower rates) but increase their pre-natal investment per offspring (lay larger eggs, which yield heavier hatchlings). That these plastic maternal responses are predicted by female helper number, and not male helper number, implicates the availability of post-natal helping per se as the likely driver (rather than correlated effects of group size), because female helpers feed nestlings at substantially higher rates than males. We term this novel maternal strategy "maternal front-loading" and hypothesize that the expected availability of post-natal help either allows or incentivizes helped mothers to focus maternal investment on the pre-natal phase, to which helpers cannot contribute directly. The potential for post-natal helping to promote pre-natal development further complicates attempts to identify and quantify the fitness consequences of helping.

Facultative and persistent offspring sex-ratio bias in relation to the social environment in cooperatively breeding red-winged fairy-wrens (Malurus elegans)

Abstract

Females should facultatively bias offspring sex ratio when fitness returns vary among sexes. In cooperative breeders, where individuals help raise others’ young, overproducing the philopatric sex will be adaptive when helpers are absent, whereas overproducing the dispersive sex may be adaptive to reduce intrasexual competition. Thus, fitness returns are expected to vary with the social environment. However, any offspring sex-ratio biases may also result from consistent among-female differences (e.g. quality) and/or environmental variables (e.g. food availability). Yet, few studies have disentangled facultative from persistent biases. We investigated offspring sex-ratio biases in relation to the social environment in cooperatively breeding red-winged fairy-wrens (Malurus elegans). Repeated observations of the same females over nine years allowed for disentanglement of facultative from persistent biases. Females without help did not overproduce daughters, despite female helpers being associated with higher fledgling survival (resource enhancement hypothesis). Instead, females without helpers facultatively overproduced sons —the slower dispersing sex— thereby ensuring long-term helper availability. Furthermore, offspring sex ratio was not biased towards the rarer sex of helpers present in the group or population (resource competition hypothesis). However, females with sex-biased helping produced similarly skewed offspring sex ratios. This among-female association may not be surprising, because helpers are previous seasons’ offspring. Thus, in addition to facultative responses to prevailing social conditions, we found evidence for persistent biases among females. This could potentially explain previous evidence for resource competition/enhancement that have typically been interpreted as facultative responses, highlighting the need for a within-female approach to better understand the adaptiveness of sex-ratio biases.

Significance statement

Under certain conditions, females may benefit from producing a biased offspring sex ratio, but evidence for such effects in vertebrates is weak and inconsistent. Here, using observations of the same females under different social conditions, we show that cooperatively breeding red-winged fairy-wrens facultatively biased offspring sex ratio towards sons when living in pairs, thereby ensuring the availability of a workforce to assist in raising future offspring. However, biased offspring sex ratio patterns may also be the result of consistent differences among females. Indeed, we also found evidence for such patterns and suggest that this could be an explanation for previous findings which are often interpreted as facultative responses.

Life history and the evolutionary loss of parental care

Parental care has been gained and lost evolutionarily multiple times. While many studies have focused on the origin of care, few have explored the evolutionary loss of care. Understanding the loss of parental care is important as the conditions that favour its loss will not necessarily be the opposite of those that favour the evolution of care. Evolutionary hysteresis (the case in which evolution depends on the history of a system) could create a situation in which it is relatively challenging to lose care once it has evolved. Here, using a mathematical approach, we explore the evolutionary loss of parental care in relation to basic life-history conditions. Our results suggest that parental care is most likely to be lost when egg and adult death rates are low, eggs mature quickly, and the level of care provided is high. We also predict evolutionary hysteresis with respect to egg maturation rate: as egg maturation rate decreases, it becomes increasingly more costly to lose care than to gain it. This suggests that once care is present, it will be particularly challenging for it to be lost if eggs develop slowly.

Plasticity in social behaviour varies with reproductive status in an avian cooperative breeder

Cooperatively breeding vertebrates are common in unpredictable environments where the costs and benefits of providing offspring care fluctuate temporally. To balance these fitness outcomes, individuals of cooperatively breeding species often exhibit behavioural plasticity according to environmental conditions. Although individual variation in cooperative behaviours is well-studied, less is known about variation in plasticity of social behaviour. Here, we examine the fitness benefits, plasticity and repeatability of nest guarding behaviour in cooperatively breeding superb starlings (Lamprotornis superbus). After demonstrating that the cumulative nest guarding performed at a nest by all breeders and helpers combined is a significant predictor of reproductive success, we model breeder and helper behavioural reaction norms to test the hypothesis that individuals invest more in guarding in favourable seasons with high rainfall. Variation in nest guarding behaviour across seasons differed for individuals of different reproductive status: breeders showed plastic nest guarding behaviour in response to rainfall, whereas helpers did not. Similarly, we found that individual breeders show repeatability and consistency in their nest guarding behaviour while individual helpers did not. Thus, individuals with the potential to gain direct fitness benefits exhibit greater plasticity and individual-level repeatability in cooperative behaviour.

Integrating Fitness Components Reveals That Survival Costs Outweigh Other Benefits and Costs of Group Living in Two Closely Related Species

Group living can be beneficial when individuals reproduce or survive better in the presence of others, but, simultaneously, there might be costs due to competition for resources. Positive and negative effects on various fitness components might thus counteract each other, so integration is essential to determine their overall effect. Here, we investigated how an integrated fitness measure (reproductive values [RVs]) based on six fitness components varied with group size among group members in cooperatively breeding red-winged and superb fairy wrens (Malurus elegans and Malurus cyaneus, respectively). Despite life-history differences between the species, patterns of RVs were similar, suggesting that the same behavioral mechanisms are important. Group living reduced RVs for dominant males, but for other group members, this was true only in large groups. Decomposition analyses showed that our integrated fitness proxy was most strongly affected by group size effects on survival and was amplified through carryover effects between years. Our study shows that integrative consideration of fitness components and subsequent decomposition analysis provide much needed insights into the key behavioral mechanisms shaping the costs and benefits of group living. Such attribution is crucial if we are to synthesize the relative importance of the myriad group size costs and benefits currently reported in the literature.

Indirect fitness benefits through extra-pair mating are large for an inbred minority, but cannot explain widespread infidelity among red-winged fairy-wrens

Extra‐pair paternity (EPP) has been suggested to improve the genetic quality of offspring, but evidence has been equivocal. Benefits of EPP may be only available to specific individuals or under certain conditions. Red‐winged fairy‐wrens have extremely high levels of EPP, suggesting fitness benefits might be large and available to most individuals. Furthermore, extreme philopatry commonly leads to incestuous social pairings, so inbreeding avoidance may be an important selection pressure. Here, we quantified the fitness benefits of EPP under varying conditions and across life‐stages. Extra‐pair offspring (EPO) did not appear to have higher fitness than within‐pair offspring (WPO), neither in poor years nor in the absence of helpers‐at‐the‐nest. However, EPP was beneficial for closely related social pairs, because inbred WPO suffered an overall 75% reduction in fitness. Inbreeding depression was nonlinear and reduced nestling body condition, first year survival and reproductive success. Our comprehensive study indicates that EPP should be favored for the 17% of females paired incestuously, but cannot explain the widespread infidelity in this species. Furthermore, our finding that fitness benefits of EPP only become apparent for a small part of the population could potentially explain the apparent absence of fitness differences in population wide comparisons of EPO and WPO.

Integrating Ecological and Evolutionary Context in the Study of Maternal Stress

Maternal stress can prenatally influence offspring phenotypes and there are an increasing number of ecological studies that are bringing to bear biomedical findings to natural systems. This is resulting in a shift from the perspective that maternal stress is unanimously costly, to one in which maternal stress may be beneficial to offspring. However, this adaptive perspective is in its infancy with much progress to still be made in understanding the role of maternal stress in natural systems. Our aim is to emphasize the importance of the ecological and evolutionary context within which adaptive hypotheses of maternal stress can be evaluated. We present five primary research areas where we think future research can make substantial progress: (1) understanding maternal and offspring control mechanisms that modulate exposure between maternal stress and subsequent offspring phenotype response; (2) understanding the dynamic nature of the interaction between mothers and their environment; (3) integrating offspring phenotypic responses and measuring both maternal and offspring fitness outcomes under real-life (either free-living or semi-natural) conditions; (4) empirically testing these fitness outcomes across relevant spatial and temporal environmental contexts (both pre- and post-natal environments); (5) examining the role of maternal stress effects in human-altered environments-i.e., do they limit or enhance fitness. To make progress, it is critical to understand the role of maternal stress in an ecological context and to do that, we must integrate across physiology, behavior, genetics, and evolution.

Multiple benefits of alloparental care in a fluctuating environment

Although cooperatively breeding vertebrates occur disproportionately in unpredictable environments, the underlying mechanism shaping this biogeographic pattern remains unclear. Cooperative breeding may buffer against harsh conditions (hard life hypothesis), or additionally allow for sustained breeding under benign conditions (temporal variability hypothesis). To distinguish between the hard life and temporal variability hypotheses, we investigated whether the number of alloparents at a nest increased reproductive success or load-lightening in superb starlings (Lamprotornis superbus), and whether these two types of benefits varied in harsh and benign years. We found that mothers experienced both types of benefits consistent with the temporal variability hypothesis, as larger contingents of alloparents increased the number of young fledged while simultaneously allowing mothers to reduce their provisioning rates under both harsh and benign rainfall conditions. By contrast, fathers experienced load-lightening only under benign rainfall conditions, suggesting that cooperative breeding may serve to take advantage of unpredictable benign breeding seasons when they do occur. Cooperative breeding in unpredictable environments may thus promote flexibility in offspring care behaviour, which could mitigate variability in the cost of raising young. Our results highlight the importance of considering how offspring care decisions vary among breeding roles and across fluctuating environmental conditions.

Testosterone, social status and parental care in a cooperatively breeding bird

The steroid hormone testosterone not only plays an important role in gamete production, but also influences social and aggressive behavior. Testosterone varies seasonally, peaking when competition for mates is high and declining during parental care. Surprisingly, little is known about how testosterone mediates social conflict and parental care behavior in highly social species like cooperative breeders, where group members compete for breeding opportunities and provide parental or alloparental care. We examined how testosterone differs across breeding roles in the tropical cooperatively breeding superb starling, Lamprotornis superbus. We determined whether testosterone was elevated in larger groups, and whether testosterone was negatively related to total levels of parental and alloparental care. We found that male breeders had higher testosterone than male helpers and female breeders and helpers during incubation. However, breeding males exhibited a significant decline in testosterone from incubation to chick rearing, and all individuals had similar levels during the chick rearing stage. Additionally, helpers-but not breeders-in large social groups had higher testosterone than those in small groups. Finally, testosterone was not correlated with nestling provisioning rates during chick rearing, suggesting that natural variation in the low levels of testosterone observed during periods of high parental care does not affect nestling provisioning. Together, these results offer insight into how testosterone is related to breeding roles, intra-group conflict, and parental care in a highly social species.

Multidimensional environmental influences on timing of breeding in a tree swallow population facing climate change

Most phenological traits are extremely sensitive to current climate change, and advances in the timing of important life-history events have been observed in many species. In birds, phenotypic plasticity in response to temperature is thought to be the main mechanism underlying yearly adjustment in the timing of breeding. However, other factors could be important and interact to affect the levels of plastic responses between and/or within-individuals. Here, we use long-term individual-based data on tree swallow (Tachycineta bicolor) to identify the spatial and environmental drivers affecting plasticity in laying date and to assess their importance at both population and individual levels. We found that laying date has advanced by 4.2 days over 10 years, and that it was mainly influenced by latitude and an interaction between spring temperature and breeder density. Analyses of individual plasticity showed that increases in temperature, but not in breeder density, resulted in within-individual advances in laying date. Our results suggest that females can adjust their laying date as a function of temperature, but that this adjustment will be partly constrained in habitats with lower breeder densities. Such potential constraint is especially worrying for the broad array of species already declining as a result of climate change.