It is well established that hermaphrodites live longer than males, either in monoxenic liquid culture (mean hermaphrodite and male life spans: 19.9 and 17.7 days, respectively; Johnson and Wood, 1982 P.N.A.S. 79 6603), or on plates at 20C (median hermaphrodite and male life spans, 16.7 and 10.1 days, respectively; Gems and Riddle, 1996 Nature 379 723). In both studies, animals were maintained in groups of 15-60. However, two new lines of evidence indicate that interactions between males greatly reduce N2 male life span. If such interactions are prevented a much longer male life span is revealed. Firstly, male life span is strongly dependent on population density. Males maintained at densities of 1, 2 and 10 animals/plate had median life spans of 22.7+3.4 (SE.), 15.9+0.4 and 10.7+0.4 days, respectively (2 trials, total sample: 280). Conversely, hermaphrodite life span is not population density dependent. Hermaphrodites maintained at 1 and 40 animals/plate had median life spans of 17.8+1.0 and 17.3+0.1 days, respectively (2 trials, total sample: 150). It was also found that, unlike hermaphrodites, male life span was greatly increased by more frequent transfer to freshly spread plates. Thus, it appears that some property of older bacterial lawns decreases male life span. This was circumvented by using UV-killed bacterial lawns, which had no apparent detrimental effects on males. On UV-killed lawns, median but not maximum life span of solitary males was greatly increased, whereas hermaphrodites showed only a slight increase in both median and maximum life span. Solitary males on UV-killed food had a median life span of 26.5+1.5 days (2 trails, sample: 74), compared to 20.8+0.5 days for hermaphrodites, solitary or grouped (2 trails, sample, 63). Thus, in the absence of male-male interaction and food effects, median male life span is 27% longer than that of hermaphrodites. Secondly, many unc mutations greatly extend the life spans of males but not hermaphrodites. For example, unc-4(e120), unc-13(e51) and unc-32(e189) males, maintained at 15-30 animals/plate (non-killed food), had average median life spans of 43.7+2.7, 30.6+0.6 and 28.0+0.8 days, respectively (two trials, sample sizes: 29-55/trial), representing life span increases of 299%, 180% and 155%, respectively, relative to N2 male controls maintained at similar population densities. Thus, males kill each other, and this effect depends on movement. Why do solitary N2 males and Unc males live longer than grouped N2 males? Since populations of N2 males but not Unc males congregate into clumps, probing one another with their copulatory bursa, it is possible that the reduction in life span seen in grouped males represents a homosexual mating cost. Similar homosexual mating costs have been seen in male houseflies (Ragland and Sohal, 1973 Exp. Geront. 8 135). Although the increased life spans of Unc males may be explained in part by their inability to mate, their life spans are considerably longer than those of solitary N2 males. Perhaps the high motility of N2 males reduces life span relative to more quiescent animals. To test whether mating with hermaphrodites in the absence of other males shortens male life span, lone males were incubated with 6 hermaphrodites (1 trial, sample: 25 males), and median male life span was 9.5 days, compared to 19.3 days for unmated controls maintained at 1 male/plate (sample: 46). We conclude that a) interactions between males and either hermaphrodites or other males - possibly mating behaviour - shortens life span; and b) shortening of life span of males maintained in groups with hermaphrodites may be due to heterosexual or homosexual interaction, or both. Thus, our prior belief that male life span is not reduced by exposure to hermaphrodites (Gems and Riddle, ibid.), which was based entirely on comparisons of males exposed to hermaphrodites with males exposed to other males, is incorrect. Preliminary results also indicate that the life spans of solitary tra-1(e1099) XX males resemble those of solitary N2 males rather than hermaphrodites. Thus, sexual dimorphism in life span is not directly dependent on X chromosome number, but rather is a function of genes regulated by tra-1. What is it about males that results in their living a third again as long as hermaphrodites (in the absence of mating costs)? Knowing this would provide insights into the genetic basis of life span.

Affiliation:
- Division of Biological Sciences, University of Missouri, Columbia MO 65211