HETEROSPECIFIC INTRUDERS, TERRITORIAL DEFENSE AND REPRODUCTIVE SUCCESS IN THE BEAUGREGORY DAMSELFISH

A territorial defender is expected to attack an intruder based on the intruder's ability to harvest the defender's crucial resources within the territory. This prediction was tested using the territorial beaugregory damselfish Stegastes leucostictus Müller & Troschel and three different species of intruders; a herbivore (Scarus iserti Bloch), an egg predator (Thalassoma bifasciatum Bloch), a conspecific male. The defender's attack behavior was expected to correlate with a species potential in eating the current clutch of eggs and/or influencing the total amount of eggs secured by the defender over the summer. Intruders were tested by placing individuals in clear bottles within the defender's territory. When eggs were absent, the defender's attack rate was highest against the herbivore. When guarding eggs the defender escalated its attack behavior, uniformly, against all species. The intensity of the attack was correlated to the size of his clutch only when the intruder was the egg predator. When intruders were presented in pairs, and one of the intruders was the conspecific male, the defender always positioned himself near the bottle containing the conspecific male. Only when guarding eggs did the defending male differentiate between the egg predator and the herbivore. The total number of eggs the defender secured over the summer was not significantly correlated to his aggressive level against conspecific intruders. Thus defending males appeared to use proximate cues for defense rather than the long term reproductive benefits gained from maintaining the territory. © 1990.