I said I'd go to bed, but then I read something interesting. Lately there has been some discussion in the comments here about what level of body fat is healthy for women. Is it healthy for women to be as lean as men should be (below 18% body fat)? Some authors say yes. After all, don't women in third world refugee camps have babies all the time?
Turns out it's more complex than that. Ancient Bodies, Modern Lives is my go-to reference on this sort of thing and lo and behold it had some answers on this matter.
To a great extent, reproductive hormone levels are set during development and reflect resource availability while a girl is growing.As we will see with regard to fetal development, a maturing system (the reproductive system in this case) reads cues about the environment to assess future conditions and adjusts levels of hormones and other components of the system to match the expected conditions. If a girl develops in a health-rich environment, her system "expects" that environment to be stable, but if she experiences short-term deprivation fo food, for example, her reproductive system may down regulate to wait for the expected better times.
Wenda mentions some rare examples of famines in first world countries, like the Dutch famine in WWII, where fertility levels completely crashed. So it's theorized that each woman has a "set point" established while growing up that determines her body's response to food intake.
Ammenorhea is a complex condition. Lots of long-distance runners suffer from it(which makes me doubt that women participating in persistence hunts was ever part of our evolutionary history)...but others don't. It sounds like a cliche, but everyone is different. For exercise-induced ammenorhea, it's not just body fat that's a factor, but cortisol and probably quite a few other hormones.
I would note that many advocates of low body fat for women are men. My own opinion is that this is foolish and will lead women to disordered eating. The only body fat that you want to avoid is visceral fat. I guess if you don't want to have children and care mostly about being super lean you can do that...but don't pretend it's healthy or natural.
Lassek and Gaulin also argue that hip and buttocks fat are the primary sources of fatty acids that are passed from the mother to the fetus during gestation and the infant during lactation. These long-chain polyunsaturated fatty acids play a major role in brain development. Thus, they argue, hip and buttocks body fat does not just signal overall energy availability for pregnancy, but also signals that the essential fatty acids for brain development are in sufficient supply. Does hip and buttock fat correlate with cognitive abilities? IN an examination of the third NHANES study database, Lassak and Gaulin found that high amounts of hip and buttocks fat relative to waist size (a low waist-hip ratio) was predictive of women's own and their offspring's cognitive performance. They refer to this fat as "a privileged store of neurodevelopmental resources.
I think other people refer to it as "my humps"
Edit: some commenters got the idea that this is about being "fat", which is it not. I mention that visceral fat is a highly unhealthy possession, and it is. Ideally a woman should have a relatively flat belly (unless she is pregnant), but not worry about ample hips, buttocks, or breasts.