MEASURING THE COST OF REPRODUCTION. II. THE CORRELATION STRUCTURE OF THE LIFE TABLES OF FIVE FRESHWATER INVERTEBRATES

The cost hypothesis states that any increment in present reproduction is associated with a decrement in the expectation of future reproduction. This follows from the limited quantity of resources available to organisms, so that increased allocation to present reproduction implies a withdrawal of resources from growth or maintenance and a consequent reduction of future fecundity or survival (Cody, 1966; Williams, 1966). This causal sequence is in turn held to imply a negative correlation between the present reproduction of individuals and their expected future reproduction when they experience the same conditions of life. In the first paper of this series (Bell, 1983) I have shown that these negative correlations do not exist in the life table of the rotifer Platyias patulus, when cultured under favorable conditions in the laboratory. Instead, the correlations between present fecundity and future survival or fecundity are generally zero or positive, whether we compare clonal mean values or the values for individuals within clones. The work that I shall describe here was undertaken to investigate the generality of the Platyias result, in order to eliminate the possibility that it arose from some special feature of the experimental organism. It will therefore lead to a general statement of the correlation structure of life histories observed under favorable conditions in the laboratory. The basic experimental design follows Bell (1983), which is similar to that of Snell and King (1 977). A number of asexual females are isolated in appropriate culture vessels and allowed to reproduce. Their offspring are removed as soon as possible after birth and in turn isolated. The founding female herself plays no further part in the experiment; the results of the experiment comprise observations of the reproduction of her progeny (and, if necessary, more distant descendants), cultured under conditions made as nearly uniform as possible, from birth until death. Small metazoans which can be maintained in small volumes of liquid medium are particularly convenient to culture, manipulate and score, and often have asexual life cycles of a few weeks. Five such organisms were chosen for detailed study. A species of Aelosoma (Oligochaeta: Aelosomatidae), close to A. tenebrarum Vejdovsky, was isolated from tows through macrophytes in Lac Hertel, Mont St. Hilaire, southern Quebec. It reproduces vegetatively by paratomy: a fission zone forms about two-thirds the way down the body, with the head of the new worm differentiating before detachment, so that the product of fission is an individual somewhat smaller than its parent but morphologically almost completely differentiated. Sexual structures were never observed in my material. In an early experiment the worms were cultured on tissue-culture plates (Linbro 76-03305), using 3 ml of a stock suspension of the green alga Ankistrodesmus, which rapidly settles out and provides a green carpet on which the worms graze. This did not prove very satisfactory; many clones failed to reproduce, and it was often difficult to see the worms in the algal detritus. A single clone was allowed to proliferate and provided useful data. Most of the results described here, however, refer to a second experiment in which the worms were cultured in a soil-water me-