The Male and Female
Both males and females are affected by the same diseases and parasites. Even though this is true, on average women live about 5 years more than males . Males tend to have higher mortality rates than females for almost all causes of death across the lifespan. The major difference peaks in young adulthood when males reach reproductive maturity and begin to compete for mates. There is not a parasite that is specific to just males or females, but parasites kill twice as many males in developed countries and four times as many males in underdeveloped countries as compared to females . It seems that not only do males tend to get sicker than females, but they also tend to be more susceptible to parasites. If parasites and diseases aren't specific to gender, what is the cause for males' mortality rates?
Why Males are the Sicker Sex?
Physiologically: Testosterone Levels
Vertebrate males have different levels of sex hormones than that of females. Testosterone is the major sex hormone in males that improves their secondary sex characteristics and allows them to produce sperm. Although this hormone is very important to males, it seems to be a main reason of why males are the sicker sex . Testosterone depresses immune cells, tissues, and organs. Not only does testosterone decrease the immune system, it also increases other hormone levels such as cortisol, which is a stress hormone that suppresses the immune system even further. The more testosterone a male has, the weaker his immune system becomes . Not only does it decrease the immune system but it can give males something called the "Testosterone Storm". This is a surge in testosterone levels that seems to make men act reckless .
Males have to search for females to mate with which involves a good bit of roaming. Doing this expends a lot of energy which has a negative impact on the immune system. Roaming will also increase their exposure to parasties and pathogens. Sometimes mating rituals or habits cause males to become more exposed to parasites because they must stay in a certain area for a long period of time trying to seduce females .
Some males must fight or compete with other males in order to mate with females. Male to male aggression is highest when only a proportion of males are able to be considered a desirable mate or marriage partner. Cultures of species with polygamous cultures or species increase competition among males . Polygamous species favor males with higher testosterone levels because they will compete more, but this also decreases the immune system as mentioned earlier. Competition among males in a species fighting for a mate can receive fatal wounds which also contributes to a higher mortality rate than that of females. Males that must compete are usually much larger than the females in their species. Moore and Wilson found that the greater the difference in size between males and females, the higher the levels of parasitism in males. They also found the more intense the competition among the males, the sicker they were .
The Antechinus is a rodent-like marsupial mouse that lives in Australia. Some antechinus species have short term breeding periods. These animals are usually solitary except for during the breeding periods. During a breeding period, both male and female antechinus are ready to mate. This means that the male antechinus must fight for his females. They spend the majority of rut competing with other males for females. Competing with other males causes a lot of stress on the antechinus males. They are also constantly roaming as they try to find as many females as possible to mate with. Stress produced by the environment or social interactions have been recognized to increase the levels of corticosteroids which also affects the immune system. During this time, the male's digestive system will begin to break down due to stress and lack of eating. The large amount of corticosteroids released during this time cause immune system to failure. This makes the males unable to defend themselves against disease and parasitism. After the breeding period, the females are pregnant and all of the males die. The females can live up to two more years after their first mating season .
The example of the Antechinus is a dramatic example of how intensely males can be affected by competition, constant roaming, and the release of a large amount of stress hormones during the mating season.
The Spadefoot Toad
The Spadefoot toad is an amphibian that lives in the desert. They spend the majority of their time borrowed underground to keep cool. They come out during periods of rainfall to find a mate. They will travel to where the rain has made a pool and sit in the water calling for a mate. A female toad will visit a pond once a year and mate. They then lay their eggs and go back to a life of being alone. The longer a male toad is immersed in the water, the more susceptible he is to trematodes that live in the water. They will get more trematodes than females simply because they are exposed in the water pools for a longer period of time .
Do Females have an advantage?
Well, it is obvious that males will be sicker because of the previously stated reasons, but do females have advantages that make them healthier? Indeed, they do! Females have a sex hormone called Estrogen. This sex hormone is known to do just the opposite of Testosterone. It increases the immune system by stimulating T cells . T cells function by recognizing self from non-self and help stimulate an immune response . Most females, with the exception of some species, do not roam or search for their mates. They let their mates find them which decreases exposure to parasites. Females can also pick and choose a mate from the males that they encounter. This means that they are not competing for their mates.They can pick the healthier male to mate with. This will decrease the chance that the male she chooses will have parasites that can be passed to her.
|When estrogen is present, it increases the ability of a T cell to respond to an antigen and mediate an immune response .|
4. http://beheco.oxfordjournals.org/content/10/3/345.full11. http://www.niaid.nih.gov/topics/immunesystem/immunecells/pages/tcells.aspx
Very interesting and informative blog post! Your reasoning as to why women might live longer than men contained physiological, ecological, and sociological aspects. However, I wonder if females having two x chromosomes and males only having one x chromosome could be another possible reason for the female health advantage. Females receive one x chromosome from the their father and one x chromosome from their mother. If one of the chromosomes should contain a mutation, but the other is wild-type and "healthy," then the healthy chromosome can compensate and prevent expression of the mutation. However, if a male receives a mutation on his one x-chromosome, there is not another to "back it up" and thus, the male will be affected. It then makes sense that diseases that are caused by mutations on the x chromosome mostly affect only men. Some of these diseases could then cause the male's immune system to be vulnerable to outside pathogens, including parasites.ReplyDelete
Good points, but what about the countries in which women have a lower life expectancy than men? Although it's not the norm, it shouldn't be ignored, and some may show these countries as counterevidence to your points. However, I do support what you've written, and I believe that these outlying countries can be ignored in the data. I found in a Scientific American article that the only countries in which the life expectancy of men exceeds that of women (such as India or Bangladesh) stand out because of sexual discrimination. These countries practice things such as infanticide and bride burning (literally the burning of a bride because the groom and/or his family are unhappy with the dowry presented), which result in female losses that can account for the differences in life expectancy.ReplyDelete