Physiological Costs of Bearing Weaponry in New Zealand Harvestmen (Arachnida, Opiliones)

Erin Powell *1, Chrissie Painting 1, Greg Holwell 1, Tony Hickey 1

1 University of Auckland, 3a Symonds Street, Auckland Central 1010

Eligible for student prize

Across animal taxa, many males brandish exaggerated structures which are used in male-male competition to secure access to mates. Within a single species, male weapons may exhibit variation in size and shape. To explain the evolution and maintenance of weapon polymorphism, studies have focused on exploring the morphological and behavioural fitness costs and compensatory traits that allow males to bear the most extreme weapons, yet few have attempted to quantify the costs of bearing exaggerated weaponry utilizing a physiological approach. We predicted that differences in metabolic processes (indicating aerobic or anaerobic respiration) would reflect morphological differences and accompanying behavioural strategies (fighting style). Males of the harvestmen Forsteropsalis pureora bear exaggerated chelicerae which vary in size and shape, forming three discrete morphs. Using this uniquely trimorphic species, we tested for differences between male morphs using multiple physiological approaches. A combination of respirometry, assays of metabolic enzyme activity, and treadmill performance have provided insight into the relative physiological costs involved with bearing extreme weapons. Our comprehensive approach reveals physiological costs of bearing weaponry rarely considered in the pursuit to understand the evolution of exaggerated structures.

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