Abstract
The body shapes of both wild‐caught and laboratory‐reared male and female Trinidadian guppies Poecilia reticulata from two low‐predation and two high‐predation populations were studied, but predation regime did not seem to be the most important factor affecting body shape. Instead, complicated patterns of plasticity in body shape among populations and the sexes were found. In particular, populations differed in the depth of the caudal peduncle, which is the muscular region just anterior to the tail fin rays and from which most swimming power is generated. Strikingly, the direction of population differences in caudal peduncle depth observed in wild‐caught individuals was reversed when P. reticulata were raised in a common laboratory environment.