Why do male cichlids have to be small to be sneaky? This is the question that Japanese scientists Kazutaka Ota, Masanori Kohda and Tetsu Sato sought to answer in their study on the shell-dwelling cichlid Lamprologus callipterus published in the most recent issue of the journal Naturwissenschaften.
Male L. callipterus come in two forms: the 'regular' version (referred to as bourgeois males by scientists) that is several times larger than the female and defends a harem consisting of several females; and the parasitic version, which matures rapidly, remains small for life (they are smaller than females) and employ a sneaking tactic in mating with females.
Because the dwarf, parasitic males mate by wriggling past the female into her shell, and staying there to release sperm close to her when she spawns, the authors hypothesise that the parasitic males are small because of the need to pass through the inner shell space.
The authors test their hypothesis by analysing the natural variation found in female size and shell size, taking into account the fact that the size of the inner shell space is dictated by the size of the shells the females choose.
The authors found that female size increased faster than shell size. This meant that larger females utilise relatively smaller shells so that dwarf males entering the shells have to be smaller relative to females in the populations where larger shells are available. Given that shell size availability is limited in each population, their results suggest that dwarf males are depressed in their growth by the available size of the inner shell spaces.
For more information, see the paper: Ota, K, M Kohda and T Sato (2010) Why are reproductively parasitic fish males so small? — influence of tactic-specific selection. Naturwissenschaften 97, pp. 1113–1116.