Temperature effects on the costs of inducible behavioural defences in prey

Vincent Van Uitregt (2011). Temperature effects on the costs of inducible behavioural defences in prey PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Vincent Van Uitregt
Thesis Title Temperature effects on the costs of inducible behavioural defences in prey
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2011-11
Thesis type PhD Thesis
Total pages 129
Total black and white pages 129
Language eng
Subjects 060399 Evolutionary Biology not elsewhere classified
060201 Behavioural Ecology
060203 Ecological Physiology
060604 Comparative Physiology
Abstract/Summary Predation is a ubiquitous selective pressure that drives the evolution of defences in prey. Such defences can be constitutive; a permanent part of the prey’s phenotype, or inducible; plastic traits that switch on when there is a perceived risk of predation. It follows that inducible defences should only evolve when the threat of predation varies either spatially or temporally and more importantly that defensive traits are beneficial in the presence of predators but costly in their absence. If defensive traits are not costly in a non-predator environment then selection could not act to maintain the non-defensive phenotype, and defences should become fixed (constitutive). Although many empirical studies find evidence of such costs of inducible defences, many other studies also fail to detect costs – indicating defensive traits are beneficial in both the presence and absence of predation. These results contradict theory and undermines either our understanding of how inducible defences persist in natural populations or our ability to detect such costs in experimental situations. My study aims to provide a deeper mechanistic understanding of the costs of defensive traits, which may reveal why these expected costs are so difficult to detect empirically. In this thesis I examine behavioural defences in the larvae of two species, the urban mosquito, Aedes notoscriptus, and the cane toad, Rhinella marina. Here, I focus on the costs of defences, their mechanisms and their sensitivity to environmental temperature. In a series of experiments, I first investigate the inducible behavioural defences in A. notoscriptus and the olfactory cues that induce them. Here I show that larval A. notoscriptus reduce activity in the presence of olfactory cues of predation. Their behavioural response is induced by both pheromones from crushed conspecifics as well as kairomones from predators. When exposed to either cue alone, larvae induce the same level of response as when exposed to both pheromones and kairomones from actual predation. Larvae induce behavioural defences throughout their ontogeny, but the behavioural response diminishes as development progresses. Larvae also induced behavioural defences appropriate to the level of predation in the environment. When exposed to cues from an increasing threat of predation (i.e. more fish preying on more larvae), the magnitude of the behavioural response (decrease in activity) also increased. Surprisingly, the olfactory assessment of predation risk does not transfer past metamorphosis. Ovipositing females actually show a preference for laying eggs in water that contained olfactory cues from crushed conspecifics. I also determined both the survival benefits of behavioural defences in A. notoscriptus, and the costs of their induction. I reared larvae in the absence (control) and presence (treatment) of olfactory predation cues and then competed these two behavioural phenotypes in the presence and absence of a real predation threat. Larvae raised in the presence of predation cues survived for longer in the presence of predatory fish, Hypseleotris galii, compared to larvae raised under control conditions. In the absence of predation, larvae inducing behavioural defences grew and developed slower, metamorphosed at a smaller size and were less resistant to starvation after metamorphosis. In the wild, these traits are likely to confer reduced survival and reproductive success in A. notoscriptus. I then determined how environmental temperature influences these costs of behavioural defences in A. notoscriptus. I present two alternate hypotheses for how temperature could influence the costs of defences. The first hypothesis predicts that the costs of reducing activity could be higher at warmer temperatures due to the higher energetic demands of routine metabolic function. Alternatively, the costs of reducing activity could be greater at cooler temperatures as the time exposed to predation cues due to the extended developmental period would be extended increasing the accrual of costs. In this experiment I reared larvae (as before) in the absence and presence of olfactory cues of predation at three experimental temperatures; 18, 23, and 28°C. Again, larvae exposed to predation cues grew and developed slower, were smaller at metamorphosis and less resistant to starvation as adults. These effects were greater for those at 18°C compared to the warmer temperatures suggesting that temperature primarily influences costs by altering the exposure time to the threat of predation and leads to the greater accrual of costs throughout development. To further examine how temperature influences the costs of behavioural defences in prey I then performed a similar experiment in cane toads, Rh. mairna. I reared tadpoles in the absence and presence of olfactory cues from macerated conspecifics, which induced behavioural defences (reduced activity) similar to that seen in A. notoscriptus. Tadpoles were reared in the absence and presence of olfactory cues at both 25 and 30°C. Tadpoles exposed to predation cues grew slower and were smaller at metamorphosis. Most importantly, these effects were again greater at the cooler temperature in a similar pattern to that observed for A. notoscriptus larvae. However, greater costs to growth in larvae at the cooler temperature were evident when larval mass was measured at the same timepoint in all treatments. This suggests that extended exposure to predation cues at cooler temperatures is not the only mechanisms driving a temperature effect on costs. Overall, this thesis provides good evidence that costs of inducible defensive traits are pervasive in prey, a key theoretical requirement for the maintenance of inducible defences. I also develop a good mechanistic understanding of how the costs of these defences manifest, which may help understand why so many other studies fail to detect these costs. My work also shows that the costs of inducible defences can be thermally sensitive. This thermal sensitivity could drive variation in plasticity between populations that experience different thermal environments, and may also suggest that large-scale changes in climate may alter predator prey dynamics via their ability to induce defences.
Keyword Adaptive phenotypic plasticity
inducible defences
behavioural defences
costs of defences
olfactory predation cues
Aedes notoscriptus
Rhinella marina (Bufo marinus)

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Created: Fri, 15 Jun 2012, 08:53:22 EST by Vincent Van Uitregt on behalf of Library - Information Access Service