Scientists from Australia and the UK have found evidence that smaller, heavier poeciliids are bolder.
The study by Culum Brown, Felicity Jones and Victoria Braithwaite is published in the most recent issue of the Journal of Fish Biology.
The authors studied populations of the poeciliid Brachyrhaphis episcopi from eight sites in three rivers running into the Panama Canal.
The study sites were of two kinds: high- and low-predation areas. In the first experiment, fish caught from the study sites were placed in a study enclosure, where the time taken for individual fish to emerge from shelter (in the form of a small box) was measured.
The second experiment involved measuring the time spent by individual fish placed in a small transparent plastic aquarium in investigating a novel object (a bright pink and yellow ball).
The authors found a strong correlation between body size and boldness, i.e. smaller fish tend to be bolder (taking less time to leave shelter and more time in investigating a novel object), and that bolder fish at any given size tended to be heavier.
They also found males to be bolder than females, and that fish from high-predation areas were bolder than those from low-predation areas.
The authors write: Leaving the safety of a shoal and approaching novel objects, or early emergence from cover following a disturbance, may seem like a dangerous occupation in high-predation areas, but it is likely to be adaptive if competition for food and mates is fierce.
These fish must emerge from shelter and ~get on with their lives even though the risk of doing so is potentially high.
The fact that fish from high-predation areas were slightly more inclined to leave the safety of the shoal and inspect the novel object is even more surprising given that fish in high-predation areas tend to school very strongly.
Fish in these areas must first overcome the need to school and then overcome their shyness before approaching the novel object.
For more information, see the paper: Brown, C, F Jones and VA Braithwaite (2007) Correlation between boldness and body mass in natural populations of the poeciliid Brachyrhaphis episcopi. Journal of Fish Biology 71, pp. 1590"1601.