We measured whether males of five species of poeciliid fish made detou
rs to the right or left of a vertical-bar obstacle in order to approac
h a group of females. Three of these species, Gambusia holbrooki, Gamb
usia nicaraguensis and Poecilia reticulata showed a significant bias t
o the left, whereas Brachyrhaphis roseni and Girardinus falcatus showe
d a significant bias to the right. When tested for direction of turnin
g in front of an opaque barrier, or when a dummy predator was used as
a target in a detour test, G. holbrooki and G. falcatus showed similar
biases to the right (opaque barrier) and left (predator), thus sugges
ting that the difference observed when females were used as a target c
ould arise from species differences in the degree of sexual motivation
in a novel environment. The two species that showed bias to the right
with the females were less likely to exhibit sexual behaviour when pl
aced in a novel environment. Moreover, manipulation of the factors aff
ecting the relative strength of sexual motivation and of fear of a nov
el environment, such as how long fish were maintained in captivity or
in the test apparatus before being tested, caused shifts in the direct
ion of the lateral asymmetries. These results suggest that the presenc
e of functional asymmetries in behaviour could be widespread among ver
tebrates and that the direction of such asymmetries tends to be striki
ngly similar in closely related species, thus supporting the hypothesi
s of an; early evolution of laterality in brain and behaviour. (C) 199
7 The Association for the Study of Animal Behaviour.