Male-biased predation of a fiddler crab

Great-tailed grackles, Quiscalus mexicanus, prey on fiddler crabs, Uca beeb ei, either by running straight at them or by running past them then angling sharply back to strike. Grackles that used angled runs caught only males a nd were twice as successful as birds that used straight runs and caught equ al numbers of males and females. Why do grackles that use angled runs catch only males? Males were not differentially available, nor did grackles pref er them to females in choice tests. Fewer crabs entered their burrows when we moved a model predator past them than when we moved it directly towards them and crabs allowed the model to get closer when it passed then returned to strike. Although crabs may take more risks when birds use angled runs, the sexes did not differ in their escape responses. Hence, differential ris k taking cannot explain male-biased predation by birds using angled runs. W e suggest that males, with their large claws and lighter colours, are more conspicuous than cryptic females. When a bird runs past then turns to dash back at a crab it may be able to keep track of a male much better than a fe male. Indeed, grackles missed all females they struck at using angled runs. Hence, males may be conspicuous and preferred prey to birds using angled r uns. Males' enlarged claws, bright colours and other sexually selected trai ts may increase male predation rate in this context. (C) 2001 The Associati on for the Study of Animal Behaviour.