Having no use for them otherwise, presumably. From ScienceDaily:
By testing and interviewing dozens of members of the Twe and Tjimba tribes in northwest Namibia, the anthropologists showed that men who did better on a spatial task not only traveled farther than other men but also had children with more women, according to the study published this week in the journal Evolution and Human Behavior.
“These findings offer strong support for the relationship between sex differences in spatial ability and ranging behavior, and identify male mating competition as a possible selective pressure shaping this pattern,” the researchers conclude in their paper.
The Twe and Tjimba were good subjects for the study because they travel over distances of 120 miles during a year, “navigating on foot in a wide-open natural environment like many of our ancestors,” Vashro says.
The tribes “have a comparatively open sexual culture,” Vashro says. Cashdan adds, “They have a lot of affairs with people they’re not married to, and this is accepted in the culture.” Many men have children by women other than their wives.
That also made the tribes good for the study, because “in a culture where you don’t have mates outside of marriage, we’re not going to expect as tight a relationship between range size and reproductive success,” Cashdan says.
How does mating pressure favor navigation skills?
“Navigation ability facilitates traveling longer distances and exploring new environments,” Vashro says. “And the farther you travel, the more likely you are to encounter new mating opportunities.”
The obvious question is: If reproductive success really hinged on males cruising for temporary mates (using navigation skills), why would one hav to go so far afield to find an example? Why would that sort of culture not be more common in places with high birth rates? Actually, places with high birth rates usually feature stable two-parent families (cf the Mormons).
Which brings up another question: It’s unclear why children of affairs with distant women would be more likely to survive than children who live with their father. Most data favour the latter conclusion.
Or, as a friend put it, Darwinian assumptions can perform the unique feat of turning gold into lead. 😉
Note: Incidentally, even today, in Namibia, child mortality is 47.5 per 1000 live births and life expectancy is 67 years. In Japan, child mortality is 2.21 per m and life expectancy is 84 years. Most places are somewhere in between. (Different factbooks give slightly different rankings but don’t change the picture much.)
See also: Human origins: The war of trivial explanations
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