An investigation of the alarm response in anuran amphibian larvae

Graduation date: 1985The results of my research suggest that tadpoles of the Western\ud toad (Bufo boreas) and the Cascades frog (Rana cascadae) display an\ud alarm response to chemicals from injured conspecific tadpoles.\ud Furthermore, results suggest that the Bufo boreas alarm response can\ud be elicited by a natural predator when it feeds on a conspecific\ud tadpole and that this response decreases the capture efficiency of\ud predators.\ud Initially, the alarm responses of B. boreas and R. cascadae were\ud elicited by extracts containing chemical cues from injured\ud conspecifics. B. boreas tadpoles increased their activity and avoided\ud areas into which conspecific extracts were added. The activity and\ud avoidance responses were not elicited by extracts of non-conspecifics\ud or by a control solution of water. R. cascadae tadpoles did not\ud exhibit the avoidance response but did increase activity when a\ud conspecific extract was added. An extract of non-conspecifics or a\ud control solution of water did not elicit these reactions.\ud In the second part of my study, I tested the hypothesis that a\ud natural predator could injure tadpoles and release the alarm\ud substance. Results suggest that while capturing and eating a Bufo\ud tadpole, the giant waterbug ( Lethocerus americanus) caused enough\ud damage to elicit the alarm reaction in conspecific tadpoles. Test\ud tadpoles significantly increased their activity over control levels\ud and avoided the side of the tank where a feeding predator was located\ud in a visually isolated but interconnected container. When a nonconspecific\ud tadpole (Pacific treefrog, Hyla regilla) was used as prey,\ud B. boreas tadpoles did not avoid the side where the predator fed,\ud although activity did increase. This increase in activity may have\ud been due to agitation caused by the presence of predators.\ud I then examined two possible functions of the alarm substance:\ud direct predator deterrence and warning of conspecifics. I found that\ud dragonfly naiads (Aeshna umbrosa), another natural predator, neither\ud moved away from an area into which a Bufo extract was introduced nor\ud moved away when a control solution was introduced, suggesting that the\ud alarm substance in extract form may not directly deter predators.\ud If, however, the alarm substance warns conspecifics, the resulting\ud alarm response should affect the capturing efficiency of a predator. In fact, Aeshna naiads took significantly longer to catch B. boreas\ud tadpoles after the introduction of a Bufo alarm substance extract than\ud after the introduction of a control solution. For aquatic systems,\ud this is the first evidence that suggests an alarm reaction elicited by\ud a chemical substance may protect individuals from predation