THE POPULATION BIOLOGY OF THE BUTTERFLY, EUPHYDRYAS EDITHA. IV. SPERM PRECEDENCE—A PRELIMINARY REPORT

The reproductive biology of the checkerspot butterfly, Euphydryas editha, is being investigated in order to integrate the details of the mating system of this nymphalid with other aspects of its population biology which have been elucidated (Ehrlich, 1961; Ehrlich and Mason, 1966; Labine, 1964). In the course of these investigations it was found that some females of E. editha in natural populations mate more than once during their reproductive life. For example, spermatophore counts showed that about a third of the females collected late in the 1964 season from the Silver Creek population, San Jose, California, had mated twice. Females have considerable capacity for sperm storage. In laboratory matings a single insemination is usually sufficient to fertilize all of a female's eggs, which can occasionally number 1200 or more. Clearly, most females mating for the second time in the field have, at the time of mating, sperm from their first mate stored in the spermatheca. Thus, they will subsequently lay eggs which theoretically could have been fertilized by either of the two males. The apportioning of paternity after second matings is obviously of great selective importance to individual males, and its specification seemed critical to an understanding of the mating system of E. editha. Two possible alternatives have been considered: either the two males mating with the same female contribute offspring to the the next generation in numbers directly proportional to the relative amounts of their sperm in the spermatheca, or one of the males fertilizes a disproportionate number of the eggs. In Tribolium (Schlager, 1960) and Drosophila (LeFevre and Jonsson, 1962) the latter alternative has been found. Females mating with several males lay eggs which have been predominantly fertilized by sperm of the last male to mate. This phenomenon has been described as "sperm precedence" or "sperm displacement. " The following experiments were undertaken to determine if sperm precedence occurred with multiple matings of E. editha. It should be emphasized at the start that Euphydryas editha is a difficult animal with which to work in the laboratory. Although much effort has been expended, techniques of continuous culture have yet to be devised. Until such efforts are successful, animals can be obtained only once a year, and then only for a short period. The work reported in this paper extended over two years. Techniques were refined during the spring of the first year, and during the spring of the second year, the experiments described below were performed. The results obtained are in some ways unsatisfactory. The number of animals involved was of necessity small, and the definitiveness of the results clearly could be improved by additional experimentation. On the other hand, the results do provide persuasive evidence of the operation of sperm precedence in Euphydryas editha. Since this information directs attention to a ph'enomenon which is probably of considerable importance in shaping reproductive strategies in Lepidoptera, and which also provides an explanation for some of the peculiarities of the reproductive biology of Euphydryas editha, presentation of these preliminary results seems justified. 1 This work was supported in part by Grant GB-1430 from the National Science Foundation and Grant 5T1-GM-365 from the Public Health Service.