Your mother-in-law. And the need she feels to help you raise your kids. Blame menopause. And evolution.
Humans are a cooperative breeding society. We live in extended family-groups in which both "breeders" and "non-breeders" contribute in rearing the offspring. However, humans are one of the select few species (including pilot whales and killer whales) that are known to stop reproducing long before we die. This means that a significant proportion of this cooperation includes non-breeding helpers in the form of post-menopausal women. As with most things in science, there are hypotheses that to try to explain this structure. Two of these are the Mother and Grandmother Hypotheses. Basically, they say that reproducing late in life is costly but that there are indirect benefits to increasing the reproductive success of your offspring. That makes sense when looking for reasons for the evolution of menopause, if you are going to die if you have late-in-life babies then it is probably beneficial to remove the ability to do so. But the reality is that, in humans, the death risk is only 3 percent before menopause, and that's in societies lacking modern medicine. So maybe there is another reason. Enter the inclusive fitness theory. This is often used to describe social insect societies and non-sterile helpers in cooperative breeding invertebrates. However, this theory typically ignores age-specific reproductive overlap between generations of females. Mix a little game-theory modeling in there and you get the Reproductive Conflict Hypothesis. This postulates that the conflict between mothers and their adult offspring selects against late-life reproduction which may, in turn, select against increasing ages of fertility. Perhaps now would time to point out that in this case we are not talking about the shouting-match kind of conflict, but rather the reproductive conflict between generations.
A new study, published this week in Ecology Letters, tests the hypothesis that this conflict between genetically unrelated generations of women (which I will name and heretofore call the mother-in-law conflict or MILC <--that's a good one right?!) could have promoted the evolution of menopause. Are you are causing your mother-in-law to become sterile? Evolutionarily speaking, of course. The authors of this study used a large data set from Finland that cataloged birth, death and marriage records kept by the Lutheran church between 1702 and 1908. They wanted to know if MILC really exists, what the consequences of it are, and which hypothesis best explained the evolution of menopause.
The researchers found that MILC is an important component. They found that the chances of children dying increased when mothers-in-law and daughters-in-law gave birth around the same time. The simultaneous reproduction of these generations was associated with a decline of 66 percent in offspring survival. Additionally, the children of the older women had 50 percent lower survival. Those are pretty large numbers which suggests that it is highly beneficial for the mothers-in-law not to have children. When using the inclusive fitness model, they found that when the in-law generations reproduced at the same time, there was a strong selection against women remaining fertile after age 51. So which hypothesis explains these changes in inclusive fitness? As it turns out, the Reproductive Conflict Hypothesis and the Grandmother Hypothesis contributed about the same while the Mother Hypothesis had no real effect. Essentially, there are more benefits to being a grandmother (and presumably helping raising your grandchildren) than there are in continuing to have children late in life. However, this study does not address the topic of plasticity in maintaining the ability to conceive late in life or the physiological explanations for the evolution of menopause. As always, more study is needed.
Mirkka Lahdenpera, Duncan O.S. Gillespie, Virpi Lummaa, & Andrew F. Russell (2012). Severe intergenerational reproductive conflict and the evolution of menopause Ecology Letters, Article first published online: 22 AUG 2012 DOI: 10.1111/j.1461-0248.2012.01851.x
Also:
Nature News article "In-law infighting boosted evolution of menopause"
