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Vigilance while feeding by female Dall’s sheep (Ovis dalli dalli Nelson, 1884): interactions among predation risk factors

University of British Columbia

I proposed and tested the Interactive Risk Factors Hypothesis (IRFH), in which predation risk factors have an interactive (multiplicative) rather than an independent (additive) effect on vigilance while feeding. Four predictions were tested on adult female Dall's sheep (Ovis dalli dalli) in the wild: 1) Vigilance increases with decreasing group size, but the magnitude of this response becomes smaller as (a) distance to cliffs decreases, and/or (b) distance to obstructive cover increases. 2) Vigilance increases with increasing distance to cliffs, but the magnitude of this response becomes smaller as (a) group size increases, and/or (b) distance to obstructive cover increases. 3) Vigilance increases with decreasing distance to obstructive cover, but the magnitude of this response becomes smaller as (a) group size increases, and/or (b) distance to cliffs decreases. 4) The magnitude of any of the above responses will be greater for mothers with neonates than for adult females within 2 months prior to the lambing season. Predictions 1a, 2a, and 4 were supported, implying that when risk is low due to other conditions, animals may increase vigilance very little or not at all in response to a particular factor that would otherwise have a strong effect. A priori analysis showed that distance to obstructive cover had no effect on vigilance (predictions 1 b, 2b and 3), which neither rejects nor supports the hypothesis. This result, however, was probably due to incomplete field measurements and a posteriori analysis suggested that distance to obstructive cover does have an interactive effect on vigilance. The IRFH is a general hypothesis that should be able to handle any other factors that affect risk, and is potentially applicable to any terrestrial prey. Testing for interactions among predation risk factors may provide a more realistic approach to understanding vigilance than merely assuming that such factors have independent effects.

Thesis/Dissertation

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2009-03-02

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http://hdl.handle.net/2429/5325

Animal Science

University of British Columbia

UBCV

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