Females of mammals (including humans) tend to outlive males, a circumstance that is usually attributed to males´ more aggressive and hence energy-depleting behaviour, especially when they compete for females. This might also explain why males of many species usually show a higher parasite burden than females. Therefore, high variability of immune genes, supposed to reduce susceptibility to pathogens, may be more important for males.

Scientists at the Research Institute of Wildlife Ecology (FIWI) of the University of Veterinary Medicine, Vienna have now found that male Alpine chamois heterozygous at a particular immune gene locus (i.e. who possess two different forms of that gene) indeed survive significantly longer than homozygous individuals (i.e. those with two identical copies of the gene) but they found no such effect for female chamois.

The results are published in the current issue of the journal BMC Evolutionary Biology.

A heavy price to pay for love

It comes as no surprise to learn that males devote considerable efforts in wooing females. To understand just how costly their exertions are, Helmut Schaschl, Franz Suchentrunk, David L. Morris, Hichem Ben Slimen, Steve Smith, and Walter Arnold have undertaken a long-term study of survival patterns and their possible immune genetic basis in free-living populations of Alpine chamois (Rupicapra rupicapra).

Alpine chamois may live for 16 to 20 years in the wild, although male survival may decrease appreciably after the age of 11. This was the case in areas affected by scabies, a highly contagious disease that occurs in regular waves across large parts of the Eastern Alps. In these populations, reproductively active males have a higher death rate than females and younger males, presumably because the energetic challenge of the winter rut renders them more susceptible to the disease. "We found that the older chamois males depleted their body fat stores at the end of the harsh alpine winter about six weeks before females and younger males," says Walter Arnold, one of the authors. "With lower body fat reserves they have less energy to put into their immune systems. This might explain why the genetic component of immune defence is more important for males than for females."

Genetic diversity to the rescue

The scientists looked for features of the immune genes supposed to reduce susceptibility to pathogens. They analysed the variability (i.e. homo- vs. heterozygosity) of a gene of the major histocompatibility complex (MHC), which has an important role in recognizing infectious agents. In areas affected by scabies, the proportion of males heterozygous at that MHC gene increased with age, suggesting higher mortality of homozygous individuals. There was no such trend among females. Importantly, the survival of male chamois did not appear to be related to heterozygosity in general but only to the heterozygosity of the MHC gene. This makes sense, as different immune gene variants within an individual potentially recognise a broader spectrum of pathogens and thus confer an enhanced immunity on the animal.

"Apparently, the higher energy expenditure of rutting males only becomes a problem when their immune system is heavily challenged. In this situation the males with heterozygous immune genes seem to have a distinct survival advantage," explains Arnold. The research results thus point to a sex-specific fitness benefit of the variability of MHC genes.