Female horses (mares) exhibit a very strange behaviour after they mate with a male. A mare will mate with every other male in the stable, almost immediately after mating with the first one. This strange post-coital promiscuity does not appear to make any sense from an evolutionary perspective, as one would think that pregnancy would lead to a decrease in libido so the mother can focus on caring for her embryo. However, this promiscuity is part of a strategy that helps protect the baby horses.
It is common behaviour in the animal kingdom for a male to kill a female’s young so that he can mate with her and produce babies with his own genetic material. This also happens in horses, where stallions engage in infanticide, kicking a foal to death if it is not his. For a mother horse, this is not only heartbreaking, but also a tremendous waste of the energy she put into pregnancy. So to protect her offspring and conserve energy, she ensures that no male knows who the father of her baby is. It has been shown that males who mated with a female who gave birth will not attack the foal, as there is a chance that it is his. Thus, a mother horse guarantees the protection of her child by prostituting herself to all of the males around.
Although the story of a mother willing to sacrifice everything, even her dignity, for the safety of her offspring may be inspiring, it has also been observed that if a mare cannot mate with all of the males in the stable, she will instead abort the pregnancy. For example, if a new horse is brought into the herd after the mare becomes pregnant, the mare senses the danger to her eventual foal and proceeds to abort it. In a study involving zebras, it was found that bringing in a new male made the foal’s chances of survival fall to less than 5 percent.
There are many physiological events that puzzle scientists. Menopause is one of these as it is very uncommon in other mammals. Why do human females stop having periods after aging? From an evolutionary point of view, an organism that has lost reproductive function cannot aid evolution and thus it is a mystery how a trait like menopause survived natural selection. The leading theory in how such a phenomenon happened is the grandmother hypothesis.
According to this hypothesis, as humans are social animals menopause can still be an evolutionary advantage despite not being able to produce offspring. This is because older women can invest the massive amount of energy and time required to upkeep childbearing in other places. For example, they can help their family and society grow by working or taking care of children instead. Furthermore, as the probability of miscarriages and congenital defects rise with aging (generally after a woman hits the age of 30, the chances of a healthy pregnancy decreases), menopause has the function of protecting the gene pool of the species. These facts combined lead to the conclusion that after an individual has reached a certain age, taking care of their children or grandchildren instead of birthing more offspring is more effective in propagating their own genes. Also, there is no one that can propagate massive amounts of wisdom and information to the next generation like the elderly.
In modern society, menopause has more significance than at any point in the history of human beings. As our average life span has surpassed 80 and heading towards 90, almost half of a woman’s life is post-menopause. In some ways, the grandmother hypothesis contains within it a certain philosophy regarding life. As we age, we give birth to children and raise them until they become independent, at which point we escape our basic biological duty of reproducing to lead our “own” lives. Senescence is like a second spring after one’s “biological” life. It is the start to a new life – a more “human” life of your own where you can focus on seeking pure happiness.