Menopause in chimps? Or not?

I've blogged about The Grandmother Hypothesis. Roughly, the question is why do women go through a "change" which rapidly shifts them from being able to become pregnant, though at sharply reduced rates by the time that menopause occurs, to a state of infertility where they may survive for up to three decades? Some argue that this is a peculiar human adaptation and that our social structures, where grandmothers may gain more in investing in their grandchildren than continuing to produce offspring in terms of long term reproductive fitness, are the cause. In contrast to women for males the pattern is one of gradual and consistent decline as the reproductive system is affected by the global breakdown of function characteristic of senescence. Now some data from chimps, Menopause in Chimps?:

At about 35 years of age, fertility steeply decreases in women as their "pool" of oocytes steadily shrinks. And at an average age of 51--when women have only about 1000 eggs left--they stop ovulating and enter menopause, typically living an additional 3 decades. Wild female chimpanzees have a much shorter life span: More than 90% die by the age of 40, and scant data exist about whether the ones that survive similarly experience what's known as "reproductive senescence."

Harvard University anthropologist Melissa Emery Thompson and colleagues--a who's who in chimpanzee research, including Richard Wrangham and Jane Goodall--followed 185 wild female chimpanzees for several decades. As they report online 13 December in Current Biology, only 34 mothers survived past the age of 40, but nearly half of them gave birth, and one had a baby at the age of 55. In contrast to humans, says Emery Thompson, fertility in wild chimpanzees seems to senesce at the same pace as the rest of the body. "It's a completely normal mammalian pattern, just like cardiac function will decline with age," she says.

In other words, in chimpanzees there isn't a strong asymmetry between the sexes in the trajectory of reproductive decline. The inference is that menopause is a derived character within our lineage since the separation of hominids from chimpanzees ~6 million years ago.