Advertisement Bertrand Desjardins, a researcher in the demography department of the University of Montreal, explains. Men dying sooner than women makes sense biologically: But even though women showed a longer life expectancy in almost every human society in the last decade of the 20th century, the size of the advantage varied greatly. For example, in the U.
Why is the mating game almost always a dance of two, whether the scene is a swank Manhattan watering hole or the shallows of an average pond? Why not three or four possibilities, or even more?
Binary mating began to evolve way back when, and over time became entrenched, especially in more complex creatures. Biologists inevitably disagree about what, exactly, constitutes a sexand therefore how "sexes" are to be counted.
They also wonder why sexual reproduction came to exist at all. But if the working definition focuses on the type of sex cell being produced, we can say that sperm-producers are males and egg-producers are females. And it becomes clear that no third sex cell—and so no third sex—has appeared in multicellular animals.
There are oddball critters, like clam shrimp and some harvester ants, in which three- or even four-sex scenarios might be said to exist. Advertisement Why do most sexual creatures do binary mating? Consider the plight of single-celled green algae trying to hook up in a pond. At first glance, the lack of more than one mating choice among these algae seems strange.
After all, if there were 10 equally prevalent types of mates, and a cell could fuse with any type other than its own, it would have nine shots in 10 of bumping into a potential partner just floating around blindly. If there were three mating types, its chances would drop to two in three. And with only two, they fall to one in two.
In other words, assuming that it takes some effort to find a mate, the two-type system is "the least efficient solution" for this population, says Laurence Hurst, an evolutionary geneticist at the University of Bath.
The best-known answer to this apparent puzzle involves the inheritance of DNA outside the nucleus of a cell.
Hurst focuses on crucial cellular structures called mitochondria. Their job is to generate energy for the cell, and they contain their own DNA, separate from the nuclear genome. When two parent cells fuse, we might expect both to contribute mitochondria to the offspring.
Because genetic mutations can make mitochondrial DNA replicate faster, at the expense of some genes that are good for the cell, normal mitochondria would be driven out if both parent cells contributed.
To avoid this problem, Hurst argues, natural selection may have favored divvying up the roles, so that one mating type would usually pass on mitochondria and other goodies and the other usually would not.
For mathematical modeling of these ideas, click here and here. Hurst has some evidence for his theory, albeit indirectly. In some single-celled ciliates, mating occurs by swapping nuclei rather than by fusing.
Once binary mating systems were in place, they probably helped set the stage for the familiar sperm-and-egg odd couple. In most animals, some individuals males produce lots of small, motile sperm, while others females invest in a smaller number of larger eggs and try to position them to get fertilized.
These approaches are evolutionary winners. Mathematical models suggest that "anything in between would be at a selective disadvantage," says Brian Charlesworth of the University of Edinburgh.
This may explain why no third type of sex cell has evolved in multicellular animals—and therefore, no third sex. Still, nookie can get weird out there.
Consider clam shrimp, for instance, which are crustaceans with folded carapaces and antennae that spin around like helicopter blades while the creatures swim, rather inefficiently. According to Stephen Weeks of the University of Akron, clam shrimp seem to come in three reproductive varieties: In other words, there are three sexual genotypes created by three possible pairings of sex chromosomes.
Does this count as three sexes? Weeks says he prefers to say clam shrimp have three "breeding types" to get around the question of whether the hermaphrodites ZW and WW should be considered distinctive sexes. Certain harvester ants of the genus Pogonomyrmex, if you really want to know could also be said to have three sexes, or even four.For example, in the U.S.
male life expectancy was years for males and years for females, a difference of years, whereas in France it was years and in the U.K., years. Battles of the Sexes illuminates how couples can recognize chemical dangers to their bonds and gives singles valuable insights for dating, empowering loving, lasting, committed romance between men and women that will benefit not only individuals, but also our entire species.5/5(16).
Men and women must realize their respective equal roles and strive to complement each other in their shared struggle to improve life.
In order to correct the abuse of male dominance, men must concentrate on using their dominant qualities for the good.
For instance, some men are born with two or three X chromosomes, just as some women are born with a Y chromosome. In some cases, a child is born with a mix between female and male genitalia. The Difference Between the Male and Female Brain.
Anne Moir and David Jessel, in their book Brain Sex: The Real Difference Between Men and Women, write: When women experience sex not as a ten-nine-eight countdown to climax, not as quest or test, but in terms of sensing, knowing, and feeling what one poet calls "the song of life singing. The moral and political implications of sex vary greatly from that their sex life is most robust when they are in their 30s and 40s, but sex life is often most deeply rewarding for older.