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Sexual segregation is the behavior in which animals of different sex in a species live in separate groups outside the mating season. Recently a new concept, namely, the “activity‐budget hypothesis,” has claimed to be the ultimate explanation of this behavior. The new hypothesis explains not only sexual segregation, but also segregation between animals of different size within sex (i.e., social segregation). The hypothesis states that the activity patterns of animals will differ when big differences in body mass exist between them, because of the associated difficulties of the synchronization in behavior making it costly to form groups, leading to segregation by size. Here we tested the assumptions and predictions of the activity‐budget hypothesis using 40 Soay sheep (Ovis aries) as the model species in a 2.3‐ha experimental arena. Sheep were divided into treatment groups to test the effect of sex, body mass, and food supplementation in their activity budgets, behavioral synchronization, diet composition, intake, food digestibility, and spatial segregation. Our animals segregated by sex but not by size, and food supplementation did not affect the spatial distribution of any sex, which is all against the predictions of the hypothesis. We also found sexual differences in dry‐matter digestibility independent of body mass, which questions the Jarman‐Bell principle at the intra‐specific level, which is the foundation for some other hypotheses of sexual segregation. Increasing behavioral synchronization led to segregation, but at the same time forming groups facilitates synchronization, so it is unclear which (i.e., synchronization or segregation) is the cause and which the consequence. Our results do not support the activity‐budget hypothesis and clearly indicate that there is no strong association between behavioral synchronization and segregation.

The Pyrenean rock ptarmigan (Lagopus muta pyrenaica) lives at one of the southernmost limits of the ptarmigan range. Their small population sizes and the impacts of global changes are limiting factors in the conservation of this threatened subspecies. An effective conservation policy requires precise basic knowledge of a species' food and habitat requirements, information that is practically non-existent for this Pyrenean population. Here, we describe the diet of a ptarmigan population in the Eastern Pyrenees, the environmental factors influencing its variability and the relationship between diet floristic composition and quality. Diet composition was determined by microhistological analysis of faeces and diet quality was estimated from free-urate faecal N content. Our results show that grouse diet is based mainly on arctic-alpine shrubs of the Ericaceae family, as well as dwarf willows (Salix spp.) and Dryas octopetala. The most frequently consumed plant species was Rhododendron ferrugineum, but its abundance in the diet was negatively related to the diet nitrogen content. Conversely, the abundance of Salix spp., grass leaves and arthropods increased the nitrogen content of the diet. Seasonality associated with snow-melting contributed the most to variability in the Pyrenean ptarmigan diet, differentiating winter from spring/summer diets. The latter was characterised by a high consumption of dwarf willows, flowers, arthropods and tender forb leaves. Geographic area and sex-age class influenced diet variability to a lesser extent. Current temperature increases in the Pyrenees due to global warming may reduce the persistence and surface area of snow-packs where preferred plants for rock ptarmigan usually grow, thus reducing food availability. The high consumption of Rh. ferrugineum characterised the diet of the Pyrenean population. Given the toxicity of this plant for most herbivores, its potential negative effect on Pyrenean ptarmigan populations should be evaluated.