In animal species in sex differences in life span are seen, it is the male that is typically the shorter-lived gender. For example, in 1983-85 in the U.K. the mean life span of men was 6.1 years less than that of women. Likewise, in C. elegans, males are shorter-lived than hermaphrodites (1). The factors determining gender differences in life span have been divided into three categories: constitutional endowment in longevity, reproductive biology and sex-related behaviour (2). Observed gender-specific life spans result from an underlying constitutional longevity minus the deleterious effects of reproductive biology (e.g., progeny production and effects of reproductive hormones) and sex-related behaviour (e.g., seeking and competing for mates or territorial defense). We have investigated the gender-specific constitutional longevity of C. elegans by excluding key elements of reproductive biology and behaviour. (This did not include hermaphrodite reproduction by self-fertilisation, which does not affect life span (3).) Single sex groups of males, but not hermaphrodites, were observed to congregate into clumps of animals attempting to mate with one another. If interactions between animals are prevented by isolation, median and maximum male life span exceeds that of the hermaphrodite by approximately 20% and 70%, respectively. We have obtained similar results using C. briggsae, and in the dioecious species C. remanei ssp. vulgaris and Caenorhabditis species (CB5161) males outlived females by an even greater extent. C. elegans males are more active than hermaphrodites. To exclude possible effects of male behaviour on male longevity, the life spans of three unc mutants,
unc-4(
e120),
unc-13(
e51) and
unc-32(
e189), were measured. Males were maintained in isolation. While these mutations had little effect on hermaphrodite life span, that of males was greatly extended. Unc males showed increases in life span of 50-90% relative to solitary N2 males. Despite the range in reduction of movement in the unc mutants from slight (
unc-4) to extreme (
unc-13), life spans of males of the three unc strains were similar. Thus, life span in solitary N2 males is reduced by some aspect of male behaviour blocked by the unc mutations. Interestingly, while a similar ratio of male: hermaphrodite life span was observed at 15C, 20C and 25C, the ratio of
unc-32:N2 male life span increased as temperature was reduced. Thus, whatever it is that N2 males but not
unc-32 males do that shortens their life span, its effects are more deleterious at lower temperatures. These results show that if reproduction and sex-related behaviour (including the elevated motility of males) is prevented in C. elegans, males are the longer-lived gender, by a factor of 1.7-2.2. Although greater female longevity is seen in many species, in most cases it is unclear whether this reflects a greater female constitutional longevity, or higher mortality in males resulting from reproductive activity or sex-related behaviour. In very few species have attempts been made to measure gender differences in life span in the absence of gross differences in behaviour or physiology. One means of reducing both male-specific reproductive activity and behaviour in vertebrates is castration. This has been shown to greatly increase male life span in marsupial mice, domestic cats and kokanee salmon. In one human study, eunuchs were found to live on average 13.5 years longer than control, intact males, and also to live significantly longer than control females. Understanding the biological basis of the difference in constitutional longevity between C. elegans males and hermaphrodites should provide insights into the fundamental mechanisms of life span determination. (1) Johnson and Wood (1982) PNAS 79: 6603; Gems and Riddle (1996) Nature 379: 723. (2) Carey et al. (1995) J. Anim. Ecol. 64: 107. (3) Friedman and Johnson (1988) Genetics 118: 75; Kenyon et al. (1993) Nature 366: 461.