Type of intruder and reproductive phase influence male territorial defence in wild-caught Siamese fighting fish

This study investigated how parental care increases with offspring age by examining the level of male aggressiveness toward potential intruders during egg guarding in a natural population of Siamese fighting fish (Betta splendens Regan). The degree of aggressiveness was measured at two reproductive phases in response to three types of intruders: male, female and females that have laid eggs. The nest-holding males became more aggressive after their eggs hatched than after the eggs were laid. Nest-holding males displayed gill cover erection, biting, tail beating and attacking at the highest rate towards male intruders, intermediate towards female intruders and the least aggressive towards females, which have laid eggs. (C) 2003 Elsevier B.V. All rights reserved.