Hypotheses proposed to explain ecological segregation of the sexes

(aka: reproductive strategy-predation risk hypothesis)

Explanation: Sexual segregation occurs in response to different selective pressures that influence the reproductive success of each sex.

Females: The principal factors influencing habitat selection by females are those selective pressures that influence offspring survival, such as risk of predation and the availability of a reliable supply of food and water. Consequently, females do not necessarily occupy habitats that provide the best foraging opportunities when offspring are most vulnerable, but may do so when predation is not important and other needs are met. In addition to environmental factors, differences in social behaviors influence the manner that protective behaviors are displayed (e.g., gregarious species form female-young groups to reduce predation risk in open habitats). Habitat selection and behavior of barren females may resemble that of segregated females because of strong site fidelity or social bonds with other females.

Males: The principal factor influencing reproductive success among males is size and body condition. Habitat choice and behavioral patterns of males are designed to maximize energy reserves in preparation for rut. When high quality forages are abundant, males exploit the best foraging opportunities available, even when doing so increases risks of predation. When high quality forage is scarce, energy conservation strategies are more important and male diets may include greater amounts of high fiber forages than found in diets of females.

Mixed-sex groups: Mixed-sex groups may form outside of rut to exploit high quality forage patches. Conversely, mixed-sex groups may also form in response to harsh environmental conditions when forage quality is universally poor and aggregation provides benefits associated with thermoregulation or increased security from predators.

Predictions:

1. The quantity of forage in habitats used by males is similar or greater to that of females during all seasons.
2. The quality of forage in habitats used by males is similar or greater to that of females when high quality forage is widely available and security or other offspring requirements restrict habitat choice by females or result in overutilization of preferred forages in localized areas.
3. The quality of male diets is similar or greater to that of females during the growing season when high quality forage is abundant.
4. When high quality forage is scarce due to seasonal conditions, poor habitat, or overutilization and obtaining high quality diets is energetically expensive, males consume more high fiber forages and lower quality diets than females.

Key References:                                                                                                                                                             

Bleich et al. (1997), Clutton-Brock et al. (1987), Main and Coblentz (1990, 1996), Main et al. (1996), Main and du Toit (2005), Mooring et al. (2003, 2005). See Appendix C for full citations.

Forage strategy hypothesis (FSH)

(aka: sexual dimorphism-body size, gastrocentric, and nutritional needs hypotheses)

Explanation: Sexual segregation occurs among sexually dimorphic ungulates in response to sex differences in dietary requirements and digestive efficiency.

Females: Smaller females, with their smaller rumens and gastrointestinal tracts, are less efficient at digesting high fiber forages than males and the energetic demands of reproduction, particularly lactation, require females consume high quality diets. Consequently, females select habitats with abundant high quality forage and consume higher quality diets than males.

Males: Larger body size increases both the absolute energy needs of males and the efficiency with which males are able to digest high fiber forages. Consequently, males segregate to habitats with abundant, high fiber forages and consume lower quality diets than females.

Mixed-sex groups: The FSH provides no prediction regarding the formation of mixed-sex groups outside of rut.

Predictions:

1. The quantity of forage in habitats used by males is similar or greater to that of females during all seasons.
2. The quality of forage in habitats used by males is lower than the quality of forage found in habitats used by females.
3. The quality of male diets is lower (include more high fiber forages) than the quality of female diets.

Key References:

Barboza and Bowyer (2000, 2001), Bowyer (2004), Main et al. (1996), Ruckstuhl and Neuhaus (2002). See Appendix C for full citations.