Temporal but not spatial environmental variation drives adaptive offspring sex allocation in a plural cooperative breeder

Cooperatively breeding birds have been used frequently to study sex allocation because the adaptive value of the sexes partly depends upon the costs and benefits for parents of receiving help. I examined patterns of directional sex allocation in relation to maternal condition (Trivers‐Willard hypothesis), territory quality (helper competition hypothesis), and the number of available helpers (helper repayment hypothesis) in the superb starling, Lamprotornis superbus, a plural cooperative breeder with helpers of both sexes. Superb starlings biased their offspring sex ratio in relation to prebreeding rainfall, which was correlated with maternal condition. Mothers produced relatively more female offspring in wetter years, when they were in better condition, and more male offspring in drier years, when they were in poorer condition. There was no relationship between offspring sex ratio and territory quality or the number of available helpers. Although helping was male biased, females had a greater variance in reproductive success than males. These results are consistent with the Trivers‐Willard hypothesis and suggest that although females in most cooperatively breeding species make sex allocation decisions to increase their future direct reproductive success, female superb starlings appear to base this decision on their current body condition to increase their own inclusive fitness

Group Size and Social Conflict in Complex Societies

Conflicts of interest over resources or reproduction among individuals in a social group have long been considered to result in automatic and universal costs to group living. However, exploring how social conflict varies with group size has produced mixed empirical results. Here we develop a model that generates alternative predictions for how social conflict should vary with group size depending on the type of benefits gained from being in a social group. We show that a positive relationship between social conflict and group size is favored when groups form primarily for the benefits of sociality but not when groups form mainly for accessing group-defended resources. Thus, increased social conflict in animal societies should not be viewed as an automatic cost of larger social groups. Instead, studying the relationship between social conflict and the types of grouping benefits will be crucial for understanding the evolution of complex societies

Discrete but variable structure of animal societies leads to the false perception of a social continuum

Animal societies are typically divided into those in which reproduction within a group is monopolized by a single female versus those in which it is shared among multiple females. It remains controversial, however, whether these two forms of social structure represent distinct evolutionary outcomes or endpoints along a continuum of reproductive options. To address this issue and to determine whether vertebrates and insects exhibit the same patterns of variation in social structure, we examined the demographic and reproductive structures of 293 species of wasps, ants, birds and mammals. Using phylogenetically informed comparative analyses, we found strong evidence indicating that not all reproductive arrangements within social groups are viable in nature and that in societies with multiple reproductives, selection favours instead taxon-specific patterns of decrease in the proportion of breeders as a function of group size. These outcomes suggest that the selective routes to sociality differ depending upon whether monopolization of reproduction by one individual is possible and that variation within and among taxonomic groups may lead to the false perception of a continuum of social structures. Thus, the occurrence of very large societies may require either complete reproductive monopolization (monogyny/singular breeding) or the maintenance of a taxon-specific range of values for the proportional decrease in the number of breeders within a group (polygyny/plural breeding), both of which may reduce reproductive conflict among females

Seasonal changes in food quality: a proximate cue for reproductive timing in marine iguanas

We investigated the proximate environmental cues that influence the timing of reproduction in the seasonally and synchronously breeding Galápagos marine iguana (Amblyrhynchus cristatus). Marine iguana foraging patterns are closely linked to the tidal cycle, and they feed exclusively on macroalgae. The Galápagos Islands are characterized by seasonal currents that impact water temperature and ultimately algal abundance, which is known to affect iguana body condition and survival. In our study, marine iguanas preferred Gelidium sp., which had the highest nutritional quality (nitrogen content, carbon content, and C:N ratio) of the three species of macroalgae on which they feed. Nutritional quality in Gelidium changed predictably with increasing sea surface temperature, peaking around the median iguana copulation date. Similarly, foraging behavior changed around the time copulations began. To test if consistent relationships existed among food quality, water temperature, and breeding life history, we used seven years of data from this population and found that breeding occurred earlier in years when the mean December sea surface temperature was warmer. Although iguanas might cue directly on changes in sea surface temperature, given the frequency and intensity of El Niño events in the Galápagos, this hypothesis is unlikely. Rather, our results suggest that marine iguanas may use subtle changes in food quality (particularly energy content) associated with seasonal, but annually variable, changes in water temperature as a cue to initiate breeding. The excess available energy may be used to withstand the costly breeding period, as well as to provision yolk to improve hatchling growth and survival

Reproductive conflict and the costs of social status in cooperatively breeding vertebrates

Conflict over reproduction is an inherent part of group living. In many social vertebrates, conflict may be reflected as allostatic load, or the costs of social status and dominance rank, which may be quantified by measuring glucocorticoid stress hormones. Here, we develop the first quantitative model of allostatic load based on the tug‐of‐war model of reproductive skew to generate insights into the mechanisms underlying reproductive conflict in cooperative breeders and to determine whether glucocorticoids can be used to assess conflict levels in group‐living vertebrates. It predicts that subordinates have higher allostatic loads than dominants under most conditions, but when body condition is lower in dominants than in subordinates, dominants experience higher allostatic load. Group structure is also important, as dominants generally have higher allostatic loads than subordinates when there is a large number of subordinates in the group, but this cost can be reduced by increasing the number of dominants, as in plural breeding societies. Using glucocorticoid data from cooperatively breeding superb starlings Lamprotornis superbus, we found empirical support for both predictions. Our model is useful for understanding how the costs of social status influence reproductive sharing, and it suggests that glucocorticoids can be used to examine reproductive conflict and cooperation in social species

The role of species abundance in determining breeding origins of migratory birds with stable isotopes

Understanding the breeding origins of migratory birds captured on their wintering grounds has important management and conservation implications for declining populations of songbirds. Stable isotopes have recently been used to infer origins for species where application of conventional markers fails. A natural method of linking wintering birds to breeding populations, and one that has not been previously applied to stable isotopes, is based on likelihood. Using a likelihood assignment rule, birds are associated with the breeding population under which their realized isotope signature is most likely to have been generated. We report the first illustration of using likelihood-based assignment with stable isotope data. Moreover, within a probability-based framework for assignment, we argue that a more natural formulation of the assignment problem should be based on the probability of origin given the observed data, or the posterior probability of origin. The relationship between posterior assignment and that based on simple likelihood is embodied in Bayes Rule, which establishes a clear linkage between the distribution of the breeding population (i.e., relative abundance) and probability of origin. We demonstrate likelihood and posterior assignment using a large data set on Black-throated Blue Warblers (Dendroica caerulescens). Our results suggest that relative abundance is likely to be a more crucial consideration in the presence of less geographically structured isotopes, when the distribution of abundance is highly non-uniform, or when the range of the species is geographically restricted

Decline and Local Extinction of Caribbean Eusocial Shrimp

The tropical shrimp genus Synalpheus includes the only eusocial marine animals. In much of the Caribbean, eusocial species have dominated the diverse fauna of sponge-dwelling shrimp in coral rubble for at least the past two decades. Here we document a recent, dramatic decline and apparent local extinction of eusocial shrimp species on the Belize Barrier Reef. Our collections from shallow reefs in central Belize in 2012 failed to locate three of the four eusocial species formerly abundant in the area, and showed steep declines in colony size and increases in frequency of queenless colonies prior to their disappearance. Concordant with these declines, several nonsocial, pair-forming Synalpheus species increased in frequency. The decline in eusocial shrimp is explained in part by disappearance of two sponge species on which they specialize. Eusocial shrimp collections from Jamaica in 2012 showed similar patterns of decline in colony size and increased queenlessness compared with prior Jamaican collections. The decline and local extinction of eusocial shrimp happened against a backdrop of changes in coral assemblages during recent decades, and may reflect changes in abundance and quality of dead coral substratum and succession of the diverse cryptic organisms living within it. These changes document potentially worrisome declines in a unique taxon of eusocial marine animals

Seasonal Variation in Female Mate Choice and Operational Sex Ratio in Wild Populations of an Annual Fish, Austrolebias reicherti

The intensity of mating competition and the potential benefits for female of mating with certain males can be influenced by several extrinsic factors, such that behavioral decisions can be highly context-dependent. Short-lived species with a single reproductive season are a unique model to study context-sensitive mating decisions. Through exhaustive sampling in the field and simultaneous choice tests in the laboratory, we evaluated operational sex ratio (OSR) and female mate choice at the beginning and end of the reproductive season in the annual killifish Austrolebias reicherti. We found seasonal change in both OSR and female mate choice. At the start of the reproductive season the OSR did not deviate from parity, and females preferred larger males. Later in the reproductive season, while the proportion of males in the ponds decreased, females became unselective with respect to male size. The particular biological cycle of annual killifish, where both life expectancy and mating opportunities decline sharply over a short timescale, could account for the seasonal change in female choice. Reduction in choosiness could arise from diminished reproductive prospects due to a decline in male availability. Moreover, as the end of the season approaches, any benefits of choosiness are presumably reduced: a female's fitness will be higher if she mates with any male than if she forgoes reproduction and dies. Future work will disentangle the mechanisms underlying seasonal changes in mating preferences, notably direct responses to demographic factors, environmental cues, or intrinsic changes during development