Strategic growth decisions in helper cichlids

Abstract

Recently, it has been shown that group-living subordinate clownfish Amphiprion percula increase their growth rate after acquiring the dominant breeder male position in the group. Evidence was found for strategic growth adjustments of subordinate fishes depending on the threat of eviction, i.e. subordinates adjust their growth rates so they remain smaller than the dominant fish and thereby limit the threat of being expelled from the territory. However, it is impossible to exclude several alternative factors that potentially could have influenced the observed changes in growth, owing to the nature of that experiment (removing the second-ranking fish--the breeder male--caused the third-ranking fish to change sex to become breeder male and change rank). We studied strategic growth decisions in the group-living Lake Tanganyika cichlid Neolamprologus pulcher under controlled laboratory conditions with ad libitum food availability. First, we show that male breeders grow faster than subordinate male helpers of the same initial size and confirm that N. pulcher shows status-dependent growth. Second, we improved on the experimental design by not removing the dominant breeder male in the group; instead we replaced the breeder male with a new breeder male in a full factorial design and measured growth of the subordinate male helpers is a function of the size difference with the old and the new breeder male. As predicted, male helpers showed strategic growth adjustments, i.e. growing faster when the size difference with the breeder male is large. Strategic growth adjustments were less pronounced than status-dependent growth adjustments.

The relationship between social status, behaviour, growth and steroids in male helpers and breeders of a cooperatively breeding cichlid

“…Second, as under certain conditions fish can adapt their growth to their social situation (Buston, 2003), we expected a reduction in growth of helpers if the size difference to breeders is small. We indeed found such strategic growth adjustments, as male helpers grew more slowly when the size difference between them and the male breeder was small (Heg et al, 2004b).…”

“…Effects of the treatments on body measurements and behaviour have been reported elsewhere (Heg et al, 2004b;Hamilton et al, 2005). All families showed natural territory defence and breeding behaviour during the experiment.…”

“…It was divided into 8 equal sections, each containing 5 group compartments (for details of the setup see Heg et al, 2004b). The tank was located in a climate controlled room at the Ethologische Station Hasli, University of Bern.…”

“…Thirty-two experimentally created breeding groups (see Heg et al, 2004b;also Taborsky, 1984also Taborsky, , 1985 of four fish each were assembled in each compartment of the ring tank, consisting of a focal large helper male (standard length SL = 40-45 mm), a small helper male (SL = 30-35 mm), a breeding female (SL = 55-65 mm) and either a large (SL = 70-75 mm) (N = 16 groups) or a small (SL = 46-51 mm) breeder male (N = 16 groups). Eight breeding pairs without helpers were created in the edge compartments of each section (focal male breeders and female breeders of standard length SL = 40-45 mm, i.e.…”

“…N. pulcher is an excellent model system to study the complex relationships between social structure, behaviour and hormone levels, because it allows experimental manipulations of social structure under controlled conditions (Taborsky, 1984(Taborsky, , 1985Heg et al, 2004b;.…”