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 (median hermaphrodite and male life spans, 16.7 and 10.1 days, respectively; Gems and Riddle, 1996 Nature 379 723)(all studies at 20C). In both studies, animals were maintained in groups of 15-60. However, two new lines of evidence indicate that interactions between males greatly reduce 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 26.1, 15.5 and 11.0 days, respectively (sample sizes: 40-50/trail). Conversely, hermaphrodite life span is not population density dependent. Hermaphrodites maintained at 1 and 40 animals/plate had median life spans of 16.8 and 17.4 days, respectively. Thus, in the absence of male/male interaction, male life span is 55% longer than that of hermaphrodites. Secondly, many unc mutations greatly extend the life spans of males but not hermaphrodites. For example, unc-4
) and unc-32
) males, maintained at 15-30 animals/plate, had average median life spans of 43.7+/-2.7 (SE.), 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. We have also compared the manner in which daily mortality rate, m(x), changes with age in males which are solitary, grouped or mated with hermaphrodites. Solitary males, like unmated hermaphrodites, show an approximately exponential increase of mortality with age. However, grouped and mated males show a complex pattern of mortality, with an early exponential increase in m(x), which then peaks on day 11-12, and then levels off and even decreases. This resembles the pattern of aging seen in several fly species (Curtsinger et al, 1992 Science 258 461; Carey et al, 1992 ibid., 457). Interestingly, although m(x) increases with age at a similar rate in grouped and mated males, it reaches a plateau slightly earlier in mated populations, resulting in peak mortality rates of 44.9+/-3.6 (SE.) and 23.4+/-0.2% in grouped and mated populations, respectively (sample sizes: 2,383 and 1,605, respectively). This results in greater longevity among the mated males. 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 probable 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). Whether the shortening of life span seen in mated C. elegans males is due to heterosexual or homosexual interactions is under investigation. Possibly the earlier, lower peak in mortality in the males mated with hermaphrodites is the result of an earlier male menopause resulting from increased ejaculation. Preliminary results show that sperm exhaustion occurs at younger ages in males which are mated with hermaphrodites throughout life when compared to those mated only at later ages. We have also studied the effects of life-extending daf-2
mutations on male longevity. At 20C, two ts daf-2
, result in similar life spans in both sexes. Males showed a simple exponential increase of m(x) with age, despite the fact that male fertility was not obviously reduced, and clumps of males were observed engaging in homosexual activity. Thus, reduced DAF-2 either renders males resistant to homosexual mating costs, or subtly alters homosexual behavior. What is it about males that results in their living half again as long as hermaphrodites (in the absence of mating costs)? Knowing this would provide insights into the genetic basis of life span.