Group size elicits specific physiological response in herbivores

Area of Science:

• Animal behavior
• Stress physiology
• Ecology

Background:

• Predation risk influences animal behavior and stress responses.
• Group size is known to affect vigilance and feeding behaviors.
• The processing of predation risk without direct cues remains unclear.

Purpose of the Study:

• To investigate how predation risk is perceived by animals as group size decreases.
• To test the hypothesis that group size itself acts as a stressor.
• To measure individual cortisol and vigilance responses in relation to group size.

Main Methods:

• Studied undisturbed sheep groups under controlled pasture conditions.
• Measured in situ cortisol concentrations and vigilance (head-up scanning) in individual sheep.
• Observed groups ranging in size from 2 to 100 individuals.

Main Results:

• Vigilance and average cortisol concentration decreased significantly with increasing group size.
• Individual cortisol responses showed high variability in small groups, decoupling from vigilance.
• A convergence of stress responses was observed in larger groups.

Conclusions:

• Group size alone can function as a stressor for animals, particularly in smaller groups.
• The mechanisms behind stress decay and convergence in larger groups require further investigation.
• Understanding these mechanisms is crucial for assessing adaptive anti-predator strategies.