Small is beautiful for at least one species of livebearer, according to research by Finnish scientists published in a recent issue of the journal Ethology.
Male livebearers typically prefer larger females to smaller ones, because larger females are typically more fecund (produce more offspring).
The study by Outi Ala-Honkola, Laura Säilä and Kai Lindström on the Least killifish (Heterandria formosa) found the opposite to be true: that males preferred smaller females.
The authors conducted a series of mate choice experiments in tanks that had been divided equally into three by two removable opaque partitions. Two females were placed in small plastic containers at both end compartments, and a male was introduced into the central compartment.
The partitions were removed and the behaviour of the male was observed for 15 minutes, after which the partitions were replaced, the females were switched to opposite ends of the tank, the partitions removed and the males observed for another 15 minutes (this was done to offset any bias the males might have for one end of the tank).
The authors then conducted a free-swimming choice test, in which two females and a male were placed in the tank, and the behaviour of the male was again observed (this time for 30 minutes).
The authors recorded the time the male spent following one or both of the females, as well as the number of copulations and copulation attempts performed by the male. Both tests were conducted with one large and one small female, or with one virgin and one mated female of approximately the same size.
The authors found that the males significantly spent more time with the small female in the mate-choice experiment, but showed no significant preference for virgin or mated females. In the free-swimming experiment, the males did not discriminate between females based on either their size or their virginity.
The authors suggest that this unique preference for small females, or the lack of preference for large females, results from strong first male sperm precedence in the least killifish (ie. the first male to mate with a virgin female is most likely to sire most or all of the brood).
The males are likely to be using female size as a proxy to assess their virginity (smaller females are more likely to be virgin), and do not appear to use pheromone cues to do so (as the results of the virgin/mated choice experiments indicate).
For more information, see the paper: Ala-Honkola, O, L Säilä and K Lindström (2010) Males prefer small females in a dichotomous choice test in the poeciliid fish Heterandria formosa. Ethology 116, pp. 736–743.