The Structure of Guppy Life Histories: The Tradeoff between Growth and Reproduction

Abstract

This study reports on two experiments characterizing the relationship between growth and reproduction in guppies. The first experiment compares the growth of reproducing and nonreproducing siblings raised on controlled levels of food availability. Nonreproducing females have significantly more energy in somatic tissues. The bulk of this energy is in fat tissues. Standard length and "protein" do not differ significantly. The extra energy in nonreproducers is therefore devoted predominantly to energy storage, as opposed to somatic growth. A comparison of total production, however, reveals that most of the energy "saved" by not reproducing does not appear in somatic tissues. The cause of this loss is unknown. The second experiment compares the somatic growth of different stocks of Trinidadian guppies which had previously been demonstrated to devote different quantities of energy to reproduction (Reznick 1980). Increases in the amount of energy devoted to reproduction were matched by decreases in the energy devoted to somatic tissues. Furthermore, all of the stocks have equal total production. These fish are therefore equally efficient in converting food into biomass but differ in how this biomass is allocated to growth and reproduction. A key assumption of the theory predicting the impact of certain forms of selection on life history evolution is that an increase in the resources devoted to current reproduction reduces the future capacity to reproduce. Because fecundity is directly proportional to size in guppies, the reported tradeoff between growth and reproduction satisfies this assumption. However, such theoretical treatments concentrate solely on the evolution of reproductive effort. Because growth and reproduction are complementary, it is equally plausible, especially for the guppies used in these experiments, that growth rate evolved, rather than reproductive effort.