Living Longer: Variability in Infection-Fighting Genes Can Be a Boon for Male Survival

Female mammals tend to always outlive the males.  This can be because males are usually more aggressive and this causes them to deplete their energy compared to females.  Because of the aggression, males are more likely to get sick from parasites.  If males were able to have a higher variability of immune genes, they would be able to increase their lifetime.  The mammals that most that follow these qualities are wolves.   

http://www.sciencedaily.com/releases/2012/05/120511104157.htm