REPRODUCTIVE COMPENSATION IN TRIBOLIUM CASTANEUM

The wide range of reproductive capacities among different kinds of organisms, the many striking occurrences of evolutionary novelties which pertain to mating success, and the variety of genetic systems have provided the basis for many current discussions on patterns of reproduction. Furthermore, the preservation of rare and desirable species, and the control of common and noxious ones, often depends on our understanding of those factors which determine reproductive rates. In these discussions, it is generally recognized that reproduction is usually the major component of Darwinian fitness, and that the act of reproduction, through interaction with other components of fitness, entails some cost to the parental organisms; therefore, one can speak of evolutionary tactics which maximize the probability of successful transmission of genes to future generations (Stearns, 1976). Reproduction is correlated with both extrinsic environmental factors (food supply, tidal cycles, daylength, etc.) and components within the population (density, social and age structure, etc.). Viewed from this perspective, there are similarities between the salmon and the cicada, the smuts and trematodes, aphids and rotifers, and so on. Of particular interest is the phenomenon of reproductive compensation, in which iteroparous, or continuous, breeders have age-specific reproductive rates which vary with different conditions experienced throughout their lifespan, and for which the total reproductive output is so constrained that there is an inverse dependence between reproductive rates at different times in the lifespan. Mertz (1971) discusses the demographic consequences of variable reproductive rates over an individual's lifetime. Sonleitner (1961) had measured the oviposition rates of Tribolium castaneum beetles in high and low density populations; the lowdensity cohort had a higher rate early in its lifespan, and a lower rate late in life, than the high-density cohort. Beetles cultured at high density also had a longer lifespan. Mertz cites this reproductive pattern (which he called "deferred reproduction") as being advantageous for animals with long life spans and argues that under favorable conditions (such as low density) reproductive effort should be expended early in life, and conversely, reproduction should be deferred to late in life when a population is declining in numbers (e.g. under high density conditions). Replication in Sonleitner's experiment was low and the results partially confounded by differences in survivorship, and because the experiment was terminated after 99 days of adult life, it was not possible to estimate the total reproduction over the beetles' lifespan. Most importantly, this experiment left unanswered the question of the effect of exposure to different densities at one stage of reproduction on subsequent oviposition rates when the beetles are brought into the same environment. To answer this last question, I (Boyer, 1976) measured the effects of different "prior environment" treatments (in which age-structure, nutritional content of the medium, density, and presence/absence of a competitor species were the variables) imposed on larvae or young adults of Tribolium castaneum. The beetles later were removed from these treatments and their oviposition rates measured under identical conditions. I found that old beetles tended to have higher reproductive rates if their earlier reproduction had been lower than