Thermal tolerance may cause sexual segregation in sexually dimorphic species living in hot environments

In hot environments, where many species seek shade to decrease their heat load at midday, sexual differences in heat sensitivity could lead to different patterns of shelter use in males and females, constituting a sexual segregation mechanism. In sexually size-dimorphic species living in these areas, the larger males have more difficulties in dissipating body heat and reducing evaporative water loss. This occurs in the great bustard (Otis tarda), the most sexually size-dimorphic bird. A previous study showed a correlation between summer temperature at breeding areas and northward migration to cooler areas only in males, providing strong evidence for their lower heat tolerance compared with females. However, not all males migrate, and here, we investigate the behavioral adaptations that help sedentary males to withstand summer heat. Males showed a stronger dependence on vegetation providing shade than females at midday. All males searched for shade after the morning foraging period, whereas only two-thirds of the females did it, entering shade 1 h later than males. Males foraged less intensively, moved shorter distances between foraging and resting locations, and flew less than females. The use of shade is the main cause of the observed sexual differences in microhabitat selection in summer, whereas lower food intake, and reduced locomotor and flight activity represent additive factors affecting spatial and social segregation. These patterns are probably of strong adaptive value for males, and can be interpreted as mechanisms to prevent overheating during summer, the season when males reach lowest body weights and highest mortality values.