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 th e number of potential intruders, individual giant danio fish, Danio ae quipinnatus, were observed defending a concentrated, renewing food sou rce from 1-20 smaller zebrafish, Danio (= Brachydanio) rerio. With inc reasing zebrafish numbers, the foraging success of giant danios contin uously decreased, the foraging success of zebrafish first increased th en decreased, and the proportion of time spent close to the food sourc e sharply increased for both zebrafish and giant danios, reaching an a symptote at intermediate numbers of zebrafish. Defender chase rate per intruder was high when zebrafish numbers were low and continuously de clined with increasing numbers of zebrafish; the total number of chase s per minute peaked at intermediate zebrafish numbers. The median dist ance and speed of chases decreased as zebrafish numbers increased. Cha ses by giant danios and flights by zebrafish were similar in distance and speed, but the relative cost of flights was higher than that of ch ases because of the asymmetry in size between intruders and defenders. These observations support the hypotheses that resource-guarding succ ess decreases and that the cost of guarding increases as intruder numb ers increase. They also indicate that both defenders and intruders adj ust 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 f or the Study of Animal Behaviour.