They could certainly imagine investing: they have been invaded by cattle people over the last half century and they see husbandry all around. And they certainly could have afforded to keep their animals. But they just didn't (seem to) have it in them to "delay gratification". I think that our ability to invest and save resources must be new and different in our evolution.
My impression is that hunter-gatherers have a huge amount of social pressure towards short-term sharing.
You mentioned "Aunt Nettie getting sick" as a reason to slaughter cattle. Was it food for her? Expensive medical care or rituals? Something else?
Edit: Q&A is now closed. Thanks to everyone for participating, and thanks very much to Harpending and Cochran for their responses.
In response to Kaj's review, Henry Harpending and Gregory Cochran, the authors of the The 10,000 Year Explosion, have agreed to a Q&A session with the Less Wrong community.
If you have any questions for either Harpending or Cochran, please reply to this post with a question addressed to one or both of them. Material for questions might be derived from their blog for the book which includes stories about hunting animals in Africa with an eye towards evolutionary implications (which rose to Jennifer's attention based on Steve Sailer's prior attention).
Please do not kibitz in this Q&A... instead go to the kibitzing area to talk about the Q&A session itself. Eventually, this post will be edited to note that the process has been closed, at which time there should be no new questions.