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

Abstract

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.

Gelada feeding ecology in an intact ecosystem at Guassa, Ethiopia: Variability over time and implications for theropith and hominin dietary evolution

“…The bulk feeding strategy imposed by the geladas' large body size and occupancy of a graminoid-dominated habitat (Dunbar, 1983;Dunbar and Bose, 1991), however, probably prevents forbs from being the primary component of gelada diets for more than short periods. Intriguingly, the diets of immature geladas, which according to the Jarman-Bell Principle require less food overall affording them the opportunity to subsist on higher-quality foods than adults (Gaulin, 1979;P erez-Barber ıa et al, 2008;M€ uller et al, 2013), consist of a higher percentage of forbs and a lower percentage of graminoids than the diets of adult geladas at Guassa (Fashing et al, in prep. ).…”

High red deer density depresses body mass of roe deer fawns

“…This corresponds to the Jarman-Bell principle developed to explain the coexistence among African ungulates by a partitioning of food resources that differ in fibre content (Perez-Barberia et al 2008). Therefore, while an intermediate feeder (sensu Hofmann 1989) of large size can consume both highly calorific and poor-quality food, a small-sized concentrate selector (sensu Hofmann 1989) only consumes highly digestible forage (Storms et al 2008;van Soest 1996).…”

Winter Food Habits of Lagopus lagopus (Willow Ptarmigan) as a Mechanism to Explain Winter Sexual Segregation

“…Finally, extending the Jarman-Bell principle (a lower ratio of metabolic requirement to gut capacity allows larger herbivores to subsist on lower-quality diets compared with smaller herbivores) to the intraspecific level, we expected larger males to have a lower-quality diet (Bell 1970, Jarman 1974, Demment and Van Soest 1985. Although this prediction has been verified in herbivorous mammals, where males seem to have a simpler, lowerquality diet than females (Staines et al 1982, Bowyer 1984, 2004, Beier 1987, Ruckstuhl and Neuhaus 2006, Pérez-Barbería et al 2008, only a few studies with raptors and fish-eating species have explored sexual divergence in the diets of birds (Boal and Mannan 1996, Webster 1997, González-Solís et al 2000, Ishikawa and Watanuki 2002, Lee and Severinghaus 2004, Xavier et al 2006. In herbivorous birds, body size has been suggested as an important factor driving foraging behavior and diet (Durant et al 2003, Mini 2012.…”