In species where females are promiscuous, there is a big conflict of
interest between males and females over the amount of resources each
child should get. The father of one child probably won’t be the father
of the next (or even of others in the same litter), so his genes —
which he passes on to the child — should evolve to try to get more
resources for his offspring. Genes that come from the mother,
meanwhile, evolve to suppress this effect, so that all her offspring
aren’t fighting each other for resources. In species where females are
monogamous, in contrast, male and female interests are more or less the
same, since the same male is likely to sire many litters with the same
In humans, mice and other mammals of our sort, the
activities of the placenta are — ready for this? — largely controlled
by the father’s genes. Now consider what happens when certain close
species try to mate. Female deer mice are much more promiscuous than
female oldfield mice, so a male deer mouse’s genes are predicted to
fight for resources much more than a male oldfield mouse’s genes would.
Consistent with this, when a male deer mouse mates with a female
oldfield mouse, both placenta and fetus become huge, and the mother
often dies. Any fetus that manages to be born is one third bigger than
babies from either species usually are. When the oldfield mouse is the
father, on the other hand, the pregnancy is much less risky for the
mother — but the baby is a runt.
…female chimpanzees are much, much more promiscuous than human females.
So, assuming you could get fertilization, here’s my prediction: if the
chimpanzee were the father, the pregnancy would be extremely dangerous
for the mother. Probably, few pregnancies could be carried to term. Any
children that did result would be huge. In contrast, if the human were
the father, the children would be small, and both mother and child
would be more likely to survive.