Graduate Thesis Or Dissertation
 

An investigation of the alarm response in anuran amphibian larvae

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  • The results of my research suggest that tadpoles of the Western toad (Bufo boreas) and the Cascades frog (Rana cascadae) display an alarm response to chemicals from injured conspecific tadpoles. Furthermore, results suggest that the Bufo boreas alarm response can be elicited by a natural predator when it feeds on a conspecific tadpole and that this response decreases the capture efficiency of predators. Initially, the alarm responses of B. boreas and R. cascadae were elicited by extracts containing chemical cues from injured conspecifics. B. boreas tadpoles increased their activity and avoided areas into which conspecific extracts were added. The activity and avoidance responses were not elicited by extracts of non-conspecifics or by a control solution of water. R. cascadae tadpoles did not exhibit the avoidance response but did increase activity when a conspecific extract was added. An extract of non-conspecifics or a control solution of water did not elicit these reactions. In the second part of my study, I tested the hypothesis that a natural predator could injure tadpoles and release the alarm substance. Results suggest that while capturing and eating a Bufo tadpole, the giant waterbug ( Lethocerus americanus) caused enough damage to elicit the alarm reaction in conspecific tadpoles. Test tadpoles significantly increased their activity over control levels and avoided the side of the tank where a feeding predator was located in a visually isolated but interconnected container. When a nonconspecific tadpole (Pacific treefrog, Hyla regilla) was used as prey, B. boreas tadpoles did not avoid the side where the predator fed, although activity did increase. This increase in activity may have been due to agitation caused by the presence of predators. I then examined two possible functions of the alarm substance: direct predator deterrence and warning of conspecifics. I found that dragonfly naiads (Aeshna umbrosa), another natural predator, neither moved away from an area into which a Bufo extract was introduced nor moved away when a control solution was introduced, suggesting that the alarm substance in extract form may not directly deter predators. If, however, the alarm substance warns conspecifics, the resulting alarm response should affect the capturing efficiency of a predator. In fact, Aeshna naiads took significantly longer to catch B. boreas tadpoles after the introduction of a Bufo alarm substance extract than after the introduction of a control solution. For aquatic systems, this is the first evidence that suggests an alarm reaction elicited by a chemical substance may protect individuals from predation.
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