Predation risk in relation to brain size in alternative prey of pygmy owls varies depending on the abundance of main prey.

Anders Pape Møller,Kari Hongisto,Erkki Korpimäki

doi:10.1371/journal.pone.0236155

Abstract

Large brains in prey may select for adoption of anti-predator behavior that facilitates escape. Prey species with relatively large brains have been shown to be less likely to fall prey to predators. This results in the prediction that individuals that have been captured by predators on average should have smaller brains than sympatric conspecifics. We exploited the fact that Eurasian pygmy owls Glaucidium passerinum hoard small mammals and birds in cavities and nest-boxes for over-winter survival, allowing for comparison of the phenotype of prey with that of live conspecifics. In Northern Europe, main prey of pygmy owls are voles of the genera Myodes and Microtus, while forest birds and shrews are the most important alternative prey. Large fluctuations (amplitude 100-200-fold) in vole populations induce rapid numerical responses of pygmy owls to main prey populations, which in turn results in varying predation pressure on small birds. We found, weighed and measured 153 birds in food-stores of pygmy owls and mist-netted, weighed and measured 333 live birds of 12 species in central-western Finland during two autumns with low (2017) and high (2018) pygmy owl predation risk. In two autumns, individuals with large brains were captured later compared to individuals with small brains, consistent with the hypothesis that such individuals survived for longer. Avian prey of pygmy owls had smaller heads than live birds in autumn 2018 when predation risk by pygmy owls was high. This difference in head size was not significant in 2017 when predation risk by pygmy owls was reduced. Finally, avian survivors were in better body condition than avian prey individuals. These findings are consistent with the hypothesis that pygmy owls differentially prey on birds in poor condition with small brains. These findings are consistent with the hypothesis that predation risk imposed by pygmy owls on small birds in boreal forests varies depending on the abundance of the main prey (voles).

Highlights

Predator-prey interactions often result in death of prey [1,2]
The marked increase in vole abundance induced rapid numerical response of pygmy owls: the number of individual pygmy owls encountered at food-stores of the study area in autumn 2018 was twice as high as that of conspecifics in autumn 2017 (Table 1)
Multiplying the number of owls present in the study area with the number of food-store sites, where owls were encountered, resulted in approximately five times more food-store sites being visited by pygmy owls in 2018 than in 2017 (98 vs. 21, in 2018 five foodstore sites were visited by two owls, while in 2017 none were visited twice; see Table 1)

Summary

Introduction

Predator-prey interactions often result in death of prey [1,2]. There is strong selection for behaviors that facilitate escape from predators. Efficient predator evasion may in many cases require sophisticated behavior, evasion behaviour can be improved. Brain size and predation risk through modifications to sensory-motor circuits. Predator evasion may in turn select for increased cognitive capacity and a larger brain. This may select for more efficient offense facilitating the capture and eventual killing of prey [3,4]. A range of different kinds of anti-predator behaviors that reduce or prevent successful predation have been described [3,4]

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Conclusion