European fiddler crabs place mudballs around their burrow openings. Bo
th males and females placed mudballs, but there were major differences
between the sexes in mudballing behaviour, suggesting that the female
's mudballs were a by-product of digging out the burrow whereas the ma
le's may have additional functions. When the male's mudballs were remo
ved experimentally, the number and intensity of male-male agonistic in
teractions increased significantly; Experimentally visually isolated m
ales spent longer making mudballs and less time waving. In a binary ch
oice test, females were more likely to approach dummy males with mudba
lls, spent longer near these males and were more likely to enter their
burrows than dummy males without mudballs. The same pattern was appar
ent for males with 30 rather than 20 mudballs. These results are consi
stent with a dual function for mudballs in U. tangeri: to reduce the n
umber and intensity of aggressive interactions between neighbouring ma
les and to attract females. (C) 1998 The Association for the Study of
Animal Behaviour.