Does the Jarman-Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep.

The Jarman-Bell principle states that large-bodied mammalian herbivores can subsist on lower quality diets because of their lower metabolism requirement/gut capacity ratio. Two major hypotheses for sexual segregation (the behaviour in which animals of the same species aggregate by sex) base their foundations on extending this principle to the intraspecific level, despite the lack of experimental evidence to support this. The first proposes that the larger males can process fibre (low-quality diet) more efficiently than the smaller females, leading to sexual segregation by habitat partitioning due to selection of different food quality and/or quantity (sexual dimorphism-body size hypothesis). The second suggests that the longer time and extra rumination required to digest low-quality food will cause asynchrony of behaviour between males and females, which then leads to sexual segregation (activity budget hypothesis). To provide experimental evidence for the Jarman-Bell principle at the intraspecific level we carried out a set of digestibility trials in Soay sheep (Ovis aries) using grass hay as the diet to test whether sexual dimorphism in body mass can produce significant sexual differences in the efficiency of food digestion. Males were slightly more efficient in digesting forage than females that were at least 30% smaller than the males. Overall, there was a decrease in faecal output of 1 g/kg body mass in favour of males. These differences were not due to differences in food selection, passage rates or faecal particle size and it was not clear why males were more efficient in digesting forage. Although these results do not directly support arguments for either the sexual dimorphism-body size or activity budget hypotheses, they do indicate that the physiological argument upon which the Jarman-Bell principle is founded also operates at the intraspecific level and may be an important factor influencing sexual segregation.