Physiological regulation of growth during social ascension in a group living fish.

In social groups, dominant animals typically are larger and have better access to resources than subordinates. When subordinates are given the opportunity to ascend to a dominant position, they will elevate their rates of growth to help secure dominance. This study investigated the physiological mechanisms facilitating this increased growth. Using the group-living cichlid, Neolamprolo- gus pulcher, we investigated whether the insulin-like growth fac- tor (IGF) system—a key regulator of growth—is involved in the regulation of growth during social ascension. We also assessed differences in energy storage and expenditure among dominant, subordinate, and ascending males to determine the energetic costs associated with ascension. Daily growth rates tripled during as- cension, and ascending males expended more energy after ascen- sion, owing to higher rates of energetically costly social behaviors, increased locomotor activity, and larger home ranges. Ascenders did not increase food intake to offset increasing energetic costs but had half the liver glycogen energy stores of subordinates. To- gether, these results indicate a reliance on stockpiled energy re- serves to fuel the high energetic demands associated with ascen- sion. Transcript abundance of IGF binding proteins 1 (ig f bp1) and 2a (ig f bp2a) were low in ascenders relative to subordinates, sug- gesting a higher capacity for growth during ascension through increased bioavailability of circulating IGF-1. Our findings provide clear evidence of the energetic costs of social ascension and offer novel insight into the physiological mechanisms modulat- ing the social regulation of growth.