Androgens and carotenoids play a fundamental role in the expression of secondary sex traits in animals that communicate information on individual quality. In birds, androgens regulate song, aggression, and a variety of sexual ornaments and displays, whereas carotenoids are responsible for the red, yellow, and orange colors of the integument. Parallel, but independent, research lines suggest that the evolutionary stability of each signaling system stems from tradeoffs with immune function: androgens can be immunosuppressive, and carotenoids diverted to coloration prevent their use as immunostimulants. Despite strong similarities in the patterns of sex, age and seasonal variation, social function, and proximate control, there has been little success at integrating potential links between the two signaling systems. These parallel patterns led us to hypothesize that testosterone increases the bioavailability of circulating carotenoids. To test this hypothesis, we manipulated testosterone levels of red-legged partridgesAlectoris rufawhile monitoring carotenoids, color, and immune function. Testosterone treatment increased the concentration of carotenoids in plasma and liver by >20%. Plasma carotenoids were in turn responsible for individual differences in coloration and immune response. Our results provide experimental evidence for a link between testosterone levels and immunoenhancing carotenoids that (i) reconciles conflicting evidence for the immunosuppressive nature of androgens, (ii) provides physiological grounds for a connection between two of the main signaling systems in animals, (iii) explains how these signaling systems can be evolutionary stable and honest, and (iv) may explain the high prevalence of sexual dimorphism in carotenoid-based coloration in animals.

Aim: Anthropogenic food subsidies are increasingly present in ecosystems, but their impacts remain poorly understood. Big game hunting is a growing activity that annually subsidizes ecosystems with tonnes of carrion world‐wide. By feeding on carrion, scavengers support key ecosystem functions and services, becoming key vectors to transfer the impacts of human‐mediated food subsidies across ecosystems. We characterize and compare the structure of vertebrate communities feeding on these subsidies, namely big game hunting remains, at a global scale.Location: Global.Methods: We collected data from a countrywide field study in Spain and broadened it up to nine regions in four continents by reviewing scientific literature. We analysed the structure of the scavenger communities considering species composition, richness and scavenging frequency.Results: Seventy‐nine vertebrate species, 19% globally threatened, scavenged food subsidies from big game hunting world‐wide. Scavenger richness (2.0–11.0% of vertebrates/region) positively correlated with total vertebrate richness. Although scavenger communities at hunting remains varied among regions, we describe a general structural pattern. Birds and mammals dominate consumption, with birds scavenging twice more frequently than mammals – but more mammal species scavenge compared to birds. Generalists dominate scavenging globally, especially where the presence of obligate scavengers (vultures) and apex predators (e.g. wolves, hyenas, eagles) is low.Main conclusions: Anthropogenic food from hunting subsidizes many vertebrate species from different trophic levels and conservation status and thus is expected to affect from populations to ecosystems. Obligate scavengers and apex predators seem to play a key role structuring the scavenger community through top‐down mechanisms. The general structure of scavenger communities we describe here provides a benchmark for comparisons of subsidized and non‐subsidized communities. More data on the spatio‐temporal availability of anthropogenic food subsidies and their consumption by scavengers world‐wide are needed to efficiently preserve biodiversity, and the associated ecological functions and services, in increasingly subsidized ecosystems.

Whether predators can limit their prey has been a topic of scientific debate for decades. Traditionally it was believed that predators take only wounded, sick, old or otherwise low‐quality individuals, and thus have little impact on prey populations. However, there is increasing evidence that, at least under certain circumstances, vertebrate predators may indeed limit prey numbers. This potential role of predators as limiting factors of prey populations has created conflicts between predators and human hunters, because the hunters may see predators as competitors for the same resources. A particularly acute conflict has emerged over the past few decades between gamebird hunters and birds of prey in Europe. As a part of a European‐wide research project, we reviewed literature on the relationships between birds of prey and gamebirds. We start by analysing available data on the diets of 52 European raptor and owl species. There are some 32 species, mostly specialist predators feeding on small mammals, small passerine birds or insects, which never or very rarely include game animals (e.g. hares, rabbits, gamebirds) in their diet. A second group (20 species) consists of medium‐sized and large raptors which prey on game, but for which the proportion in the diet varies temporally and spatially. Only three raptor species can have rather large proportions of gamebirds in their diet, and another seven species may utilise gamebirds locally to a great extent. We point out that the percentage of a given prey species in the diet of an avian predator does not necessarily reflect the impact of that predator on densities of prey populations. Next, we summarise available data on the numerical responses of avian predators to changing gamebird numbers. In half of these studies, no numerical response was found, while in the remainder a response was detected such that either raptor density or breeding success increased with density of gamebirds. Data on the functional responses of raptors were scarce. Most studies of the interaction between raptors and gamebird populations give some estimate of the predation rate (per cent of prey population taken by predator), but less often do they evaluate the subsequent reduction in the pre‐harvest population or the potential limiting effect on breeding numbers. The few existing studies indicate that, under certain conditions, raptor predation may limit gamebird populations and reduce gamebird harvests. However, the number and extent of such studies are too modest to draw firm conclusions. Furthermore, their geographical bias to northern Europe, where predator–prey communities are typically simpler than in the south, precludes extrapolation to more diverse southern European ecosystems. There is an urgent need to develop further studies, particularly in southern Europe, to determine the functional and numerical responses of raptors to gamebird populations in species and environments other than those already evaluated in existing studies. Furthermore, additional field experiments are needed in which raptor and possibly also mammalian predator numbers are manipulated on a sufficiently large spatial and temporal scale. Other aspects that have been little studied are the role of predation by the non‐breeding part of the raptor population, or floaters, on the breeding success and survival of gamebirds, as well as the effect of intra‐guild predation. Finally there is a need for further research on practical methods to reduce raptor predation on gamebirds and thus reduce conflict between raptor conservation and gamebird management.

The oxidation handicap hypothesis proposes that testosterone mediates the trade‐off between the expression of secondary sexual traits and the fight against free radicals. Coloured traits controlled by testosterone can be produced by carotenoid pigments (yellow–orange–red traits), but carotenoids also help to quench free radicals. Recently, it has been shown that testosterone increases the amount of circulating carotenoids in birds. Here, a testosterone‐mediated trade‐off in the carotenoid allocation between colour expression and the fight against oxidative stress is proposed. Male red‐legged partridges were treated with testosterone, anti‐androgens or manipulated as controls. Testosterone‐treated males maintained the highest circulating carotenoid levels, but showed the palest red traits and no evidence of oxidative damage. Increased levels of a key intracellular antioxidant (i.e. glutathione) indicated that an oxidative challenge was in fact induced but controlled. The trade‐off was apparently solved by reducing redness, allowing increased carotenoid availability, which could have contributed to buffer oxidative stress.