Guarded resources: The effect of intruder number on the tactics and success of defenders and intruders

To quantify the success rates of defenders and intruders at a guarded resource and to examine how competitive tactics vary in relation to the number of potential intruders, individual giant danio fish, Danio aequipinnatus, were observed defending a concentrated, renewing food source from 1-20 smaller zebrafish, Danio (= Brachydanio) rerio. With increasing zebrafish numbers, the foraging success of giant danios continuously decreased, the foraging success of zebrafish first increased then decreased, and the proportion of time spent close to the food source sharply increased for both zebrafish and giant danios, reaching an asymptote at intermediate numbers of zebrafish. Defender chase rate per intruder was high when zebrafish numbers were low and continuously declined with increasing numbers of zebrafish; the total number of chases per minute peaked at intermediate zebrafish numbers. The median distance and speed of chases decreased as zebrafish numbers increased. Chases by giant danios and flights by zebrafish were similar in distance and speed, but the relative cost of flights was higher than that of chases because of the asymmetry in size between intruders and defenders. These observations support the hypotheses that resource-guarding success decreases and that the cost of guarding increases as intruder numbers increase. They also indicate that both defenders and intruders adjust their behaviour patterns to these benefits and costs, and that the quantitative relationship between intruder numbers and the success of defence is a result of these interactions. (C) 1996 The Association for the Study of Animal Behaviour.