Fat cycling in the mosquitofish (Gambusia affinis): fat storage as a reproductive adaptation

Summary

We argue, based on reviewed literature covering reptiles, amphibians, birds, and fish, that fat storage may represent a life history adaptation because it enables an organism to shift in time when resources are allocated to reproduction. We applied these arguments to fat and population cycles in three populations of the mosquito fish, Gambusia affinis. For males, there appeared to be a constant size at maturation during the reproductive season. Mature males became scarce late in the summer. At the same time, immature males delayed maturity and attained much larger sizes; they matured in large numbers in the fall. The amount of stored fat tended to be equal for immature and mature males at all times except in the late summer. In the August samples, when mature males were relatively rare, they also had the lowest level of fat reserves. It appears that the older generation of mature males did not store fat and did not overwinter. At the same time, immature males registered a two to three fold increase in fat reserves. These differences in fat content between mature and immature males disappeared by September, probably because of the recruitment of a new generation of mature males. The reserves were gradually utilized during the winter. Females reproduced from the late spring through mid- to late-summer. They stopped reproducing in the late summer, when there was ample time to produce an additional litter of young. There was an inverse relationship between resources devoted to reproduction and fat reserves. As reproductive allotment decreased in the late summer, fat reserves increased. The magnitude of the change in fat reserves was similar to that displayed by males. The reserves were depleted over the winter. Significant reserves remained at the beginning of the reproductive season the following spring. Reproducing females utilized the remaining reserves significantly more rapidly than non-reproducing females. An analysis of resource availability revealed an overall decrease in food availability in the late summer, coincident with the increase in fat reserves. These cycles are therefore not attributable to changes in resource availability. They instead indicate a change in how resources are allocated by the fish. The trends in the data indicate that fat reserves are used to shift investment in reproduction from the late summer to the following spring. In males, deferring maturity, rather than maturing in August, allows them to store the necessary reserves to survive the winter so that they can mate the following spring. In females, a subset of the fat reserves is intended for producing the first clutch of eggs the following spring. The female pattern corresponds to those reported for a diversity of organisms. The possible advantages of shifting reproductive effort from the fall to the following spring include higher fecundity and higher offspring fitness. The limitations of the methodology and potential directions for future research are discussed.