Men carry the seeds of their own destruction in the genes present in their sperm, research suggests. Scientists working on mice have highlighted a specific gene that, although carried by both sexes, appears to be active only in males. They believe it allows males to grow bigger bodies - but at the expense of their longevity. The study, by Tokyo University of Agriculture, appears in the journal Human Reproduction. Although the study was conducted on mice, the researchers believe it could apply to all mammals - including humans. They studied mice created with genetic material from two mothers, but no father. This was achieved by manipulating DNA in mouse eggs so the genes behaved like those in sperm. The altered genetic material was implanted into the eggs of adult female mice to create embryos. The resulting offspring, completely free of any genetic material inherited from a male, lived on average a third longer than mice with a normal genetic inheritance. Better immune function The mice with two mothers were significantly lighter and smaller at birth. But they appeared to have better functioning immune systems. The researchers believe the key is a gene called Rasgrf1. One copy of the gene is passed down from both parents, but only the version inherited from fathers is active - the version inherited from mothers is effectively silenced in a process known as imprinting. Lead researcher Professor Tomohiro Kono said: "We have known for some time that women tend to live longer than men in almost all countries worldwide, and that these sex-related differences in longevity also occur in many other mammalian species. "However, the reason for this difference was unclear and, in particular, it was not known whether longevity in mammals was controlled by the genome composition of only one or both parents. "Our results suggested sex differences in longevity originating at the genome level, implying that the sperm genome has a detrimental effect on longevity in mammals. "The study may give an answer to the fundamental questions: that is, whether longevity in mammals is controlled by the genome composition of only one or both parents, and just maybe, why women are at an advantage over men with regard to lifespan." In the UK the average lifespan for men is 77.4 and for women 81.6. The researchers said in nature males tended to concentrate resources on building a large body, because strength and bulk help them fight for mating opportunities with females. In contrast, females tended to conserve energy for breeding and providing for their offspring. Dr Allan Pacey, an expert in reproduction at the University of Sheffield, said: "The results of this study are intriguing, and this is a topic that clearly needs further investigation. "However, I would resist the temptation to fantasise about whether this may one day to a medical treatment to extend life through gene manipulation. "I think humans have a good innings on the whole and we should try and be content with that." Professor Kay-Tee Khaw, an expert in ageing at the University of Cambridge, said the findings were not necessarily applicable to humans. She said: "These are interesting findings but I think any sex differences in longevity - which in humans have changed over time and differ in different environments - may have more complex explanations than any single gene."