Effet maternels et compromis évolutifs chez une espèce à reproduction coopérative, le Républicain social (Philetairus socius)

Maximizing of the number copies of genes that are transmitted to the next generations involves a series of tradeoffs. In cooperatively breeding species some sexually mature individuals do not breed but instead help other individuals to raise their offspring. These helpers are particularly interesting in a life history context as they create a predictably favorable breeding environment and their presence can thus influence evolutionary trade-offs. A major evolutionary trade-off that is often neglected in studies on cooperative breeding is maternal allocation, notably through maternal effects that are epigenetic modifications of offspring phenotype. Here we investigate whether there are maternal effects induced by the presence of helpers and their possible consequences on females and their offspring in a colonial cooperative breeder of southern Africa, the sociable weaver Philetairus socius. Our results show that females lay smaller eggs in the presence of helpers and in addition these eggs have lower corticosterone and testosterone concentrations. Our results also show a higher survival probability of females breeding in groups, which may be partially due to their lower investment in eggs. In addition, a study of roosting chamber temperatures in relation to group size suggests further benefits for parents and helpers, particularly through lower costs of thermoregulation that could also allow energy savings for survival. To start understanding the consequences of helpers presence and differential maternal allocation for offspring we conducted a cross fostering experiment. Our results show that eggs produced by females breeding in larger groups produce chicks that beg at a lower rate, showing that maternal effects may influence chicks’ behavior. Finally, we investigated post-fledging survival through capture-recapture analyses and, surprisingly, found that fledglings have a lower survival probability when raised with helpers. Taken together, these results demonstrate the importance of studying maternal effects on cooperative breeders and open several research prospects on family conflicts and life history trade-offs according to the presence of helpers.Maximiser le nombre de copies de gènes transmises aux générations suivantes implique une série de compromis. Chez les espèces à reproduction coopérative, des individus ne se reproduisent pas mais participent aux soins des jeunes d’autres individus reproducteurs. Ces assistants sont particulièrement intéressants dans le contexte des traits d’histoire de vie car ils forment un environnement prédictible favorable pour la reproduction, et leur présence peut aussi influencer les compromis évolutifs chez les reproducteurs. Un compromis évolutif majeur mais sous-étudié dans le cadre de la reproduction coopérative est l’allocation maternelle notamment via des effets maternels qui sont des modifications épigénétiques du phénotype de la descendance. Nous avons étudié l’existence d’effets maternels associés à la présence d’assistants et leurs possibles conséquences sur les femelles et leurs descendants chez un oiseau colonial et coopératif du sud de l’Afrique, le Républicain social Philetairus socius. Nos résultats montrent que les femelles pondent des oeufs plus légers en présence d’assistants et que ces oeufs sont moins concentrés en corticostérone et testostérone. Nos résultats montent aussi une plus grande probabilité de survie pour les femelles se reproduisant en groupe pouvant être en partie due à leur plus faible investissement dans les oeufs. De plus, l’étude de la température dans les nids en fonction de la taille des groupes a permis de suggérer d’autres bénéfices pour les parents et assistants, en particulier via une réduction des coûts de thermorégulation qui pourrait aussi permettre de garder de l’énergie pour la survie. Pour comprendre les conséquences de la présence d’assistants et de l’allocation différentielle pour les poussins, une expérience d’adoption croisée a été réalisée. Elle a révélé que les oeufs pondus par les femelles avec plus d’assistants produisent des poussins qui quémandent moins, montrant que des effets maternels pourraient influencer le comportement des poussins. Enfin nous avons étudié la survie des poussins après l’envol à l’aide d’analyses de captures recaptures et avons trouvé de manière surprenante que les poussins à l’envol ont une probabilité de survie plus faible lorsqu’ils sont élevés présence d’assistants. Ces résultats dans leur ensemble démontrent l’importance d’étudier les effets maternels chez les espèces coopératives et ouvrent de nombreuses perspectives de recherche sur les conflits familiaux et de compromis évolutifs associés à la présence d’assistants

Partitioning variance in population growth for models with environmental and demographic stochasticity

How demographic factors lead to variation or change in growth rates can be investigated using life table response experiments (LTRE) based on structured population models. Traditionally, LTREs focused on decomposing the asymptotic growth rate, but more recently decompositions of annual 'realized' growth rates using ' transient' LTREs have gained in popularity.Transient LTREs have been used particularly to understand how variation in vital rates translate into variation in growth for populations under long-term study. For these, complete population models may be constructed to investigate how temporal variation in environmental drivers affect vital rates. Such investigations have usually come down to estimating covariate coefficients for the effects of environmental variables on vital rates, but formal ways of assessing how they lead to variation in growth rates have been lacking.We extend transient LTREs to further partition the contributions from vital rates into contributions from temporally varying factors that affect them. The decomposition allows one to compare the resultant effect on the growth rate of different environmental factors, as well as density dependence, which may each act via multiple vital rates. We also show how realized growth rates can be decomposed into separate components from environmental and demographic stochasticity. The latter is typically omitted in LTRE analyses.We illustrate these extensions with an integrated population model (IPM) for data from a 26 years study on northern wheatears (Oenanthe oenanthe), a migratory passerine bird breeding in an agricultural landscape. For this population, consisting of around 50-120 breeding pairs per year, we partition variation in realized growth rates into environmental contributions from temperature, rainfall, population density and unexplained random variation via multiple vital rates, and from demographic stochasticity.The case study suggests that variation in first year survival via the unexplained random component, and adult survival via temperature are two main factors behind environmental variation in growth rates. More than half of the variation i

Antagonistic effect of helpers on breeding male and female survival in a cooperatively breeding bird.

1. Cooperatively breeding species are typically long lived and hence, according to theory, are expected to maximize their lifetime reproductive success through maximizing survival. Under these circumstances, the presence of helpers could be used to lighten the effort of current reproduction for parents to achieve higher survival. 2. In addition, individuals of different sexes and ages may follow different strategies, but whether male and female breeders and individuals of different ages benefit differently from the presence of helpers has often been overlooked. Moreover, only one study that investigated the relationship between parental survival and the presence of helpers used capture-mark-recapture analyses (CMR). These methods are important since they allow us to account for the non-detection of individuals that are alive in the population but not detected, and thus, the effects on survival and recapture probability to be disentangled. 3. Here, we used multi-event CMR methods to investigate whether the number of helpers was associated with an increase in survival probability for male and female breeders of different ages in the sociable weaver Philetairus socius. In this species, both sexes reduce their feeding rate in the presence of helpers. We therefore predicted that the presence of helpers should increase the breeders' survival in both sexes, especially early in life when individuals potentially have more future breeding opportunities. In addition, sociable weaver females reduce their investment in eggs in the presence of helpers, so we predicted a stronger effect of helpers on female than male survival. 4. As expected we found that females had a higher survival probability when breeding with more helpers. Unexpectedly, however, male survival probability decreased with increasing number of helpers. This antagonistic effect diminished as the breeders grew older. 5. These results illustrate the complexity of fitness costs and benefits underlying cooperative behaviours and how these may vary with the individuals' sex and age. They also highlight the need for further studies on the sex-specific effects of helpers on survival.Our research has received funding from the DST-NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology (South Africa), FEDER (Operational Programme for Competitiveness Factors – COMPETE) and Portuguese Foundation for Science and Technology (FCT; PTDC/BIA-BEC/103818/2008), Project “Biodiversity, Ecology and Global Change” co-financed by North Portugal Regional Operational Programme 2007/2013 (ON.2), under the NSRF, ERDF. to RC, the region Languedoc Roussillon to CD, the Natural Environment Research Council (UK) to BJH, St John's College, Cambridge and the University of Cape Town to CNS , and the European programme Marie Curie-IRSES (FP7-PEOPLE-2012-IRSES; ‘Cooperation’ 318994). This research was conducted within the International Associate Laboratory LIA “Biodiversity and Evolution”.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1111/1365-2656.12377 Data are available from Dryad digital repository http://dx.doi.org/10.5061/dryad.mk44

Why we should care about movements: Using spatially explicit integrated population models to assess habitat source-sink dynamics

Assessing the source-sink status of populations and habitats is of major importance for understanding population dynamics and for the management of natural populations. Sources produce a net surplus of individuals (per capita contribution to the metapopulation > 1) and will be the main contributors for self-sustaining populations, whereas sinks produce a deficit (contribution < 1). However, making these types of assessments is generally hindered by the problem of separating mortality from permanent emigration, especially when survival probabilities as well as moved distances are habitat-specific. To address this long-standing issue, we propose a spatial multi-event integrated population model (IPM) that incorporates habitat-specific dispersal distances of individuals. Using information about local movements, this IPM adjusts survival estimates for emigration outside the study area. Analysing 24 years of data on a farmland passerine (the northern wheatearOenanthe oenanthe), we assessed habitat-specific contributions, and hence the source-sink status and temporal variation of two key breeding habitats, while accounting for habitat- and sex-specific local dispersal distances of juveniles and adults. We then examined the sensitivity of the source-sink analysis by comparing results with and without accounting for these local movements. Estimates of first-year survival, and consequently habitat-specific contributions, were higher when local movement data were included. The consequences from including movement data were sex specific, with contribution shifting from sink to likely source in one habitat for males, and previously noted habitat differences for females disappearing. Assessing the source-sink status of habitats is extremely challenging. We show that our spatial IPM accounting for local movements can reduce biases in estimates of the contribution by different habitats, and thus reduce the overestimation of the occurrence of sink habitats. This approach allows combining all available data on demographic rates and movements, which will allow better assessment of source-sink dynamics and better informed conservation interventions

Unclear relationships between mean survival rate and its environmental variance in vertebrates

Current environmental changes may increase temporal variability of life history traits of species thus affecting their long-term population growth rate and extinction risk. If there is a general relationship between environmental variances (EVs) and mean annual survival rates of species, that relationship could be used as a guideline for analyses of population growth and extinction risk for populations, where data on EVs are missing. For this purpose, we present a comprehensive compilation of 252 EV estimates from 89 species belonging to five vertebrate taxa (birds, mammals, reptiles, amphibians and fish) covering mean annual survival rates from 0.01 to 0.98. Since variances of survival rates are constrained by their means, particularly for low and high mean survival rates, we assessed whether any observed relationship persisted after applying two types of commonly used variance stabilizing transformations: relativized EVs (observed/mathematical maximum) and logit-scaled EVs. With raw EVs at the arithmetic scale, mean-variance relationships of annual survival rates were hump-shaped with small EVs at low and high mean survival rates and higher (and widely variable) EVs at intermediate mean survival rates. When mean annual survival rates were related to relativized EVs the hump-shaped pattern was less distinct than for raw EVs. When transforming EVs to logit scale the relationship between mean annual survival rates and EVs largely disappeared. The within-species juvenile-adult slopes were mainly positive at low (0.5) mean survival rates for raw and relativized variances while these patterns disappeared when EVs were logit transformed. Uncertainties in how to interpret the results of relativized and logit-scaled EVs, and the observed high variation in EV's for similar mean annual survival rates illustrates that extrapolations of observed EVs and tests of life history drivers of survival-EV relationships need to also acknowledge the large variation in these parameters