UCB Moorea Class: Biology and Geomorphology of Tropical Islands

Defensive animal behavior is often a tradeoff between the energetic costs of defense and predator avoidance, with most animals choosing actions with the lowest energetic expense to evade their predators.Pholcus ancoralisis an introduced spider to Mo’orea, French Polynesia. I studied its defensive behaviors, with an emphasis on whirling, an energetically costly behavior which involves continual gyrations of the body. Different instars as well as females carrying egg sacs were disturbed with tactile, vibratory, and air movement stimuli to determine if larger instars and females with eggs have a higher whirling duration and frequency of defensive behaviors.P. ancoralisalso faced the potential spider predatorsThorelliola ensifera(Salticidae),Tangaroa tahitiensis(Uloboridae), andLeucauge granulata(Tetragnathidae) in a laboratory setting to determine if a higher whirling duration and higher frequency of defensive behaviors would lead to increased survivorship. The tactile stimulus elicited the greatest frequency and duration of whirling fromP. ancoralis. Adult pholcids had the highest whirling frequency and duration out of all the instars, with egg-carrying females displaying an even greater response than other adults. Third and younger instars chose to retreat significantly more than adults, suggesting a change in defensive strategies during development. Whirling was not associated with a higher survivorship ofP. ancoralisin the presence of potential predators, although repelling the predator and dropping away from it was. Thus, whirling is not an appropriate response toT. ensiferabecause it is conspicuous, energetically expensive, and does not promote survivorship. The inefficacy of whirling in this study suggests that this behavior may have arisen to defend against other salticids or other native natural enemies whichP. ancoralisescaped upon its introduction to Mo’orea.