Makes sense. But how does the sex that invests more get selected? How did it get that way that female bisons invest more and male seahorses invest more than their opposite sexes? Why were females mammals selected to lactate? Males, since they only contribute the sperm, could have well taken the responsiblity of lactation and the loss of nutrition for the female would stop as soon as the placenta was snipped and she could be in heat quicker. I wonder why this wasn't so. Does anybody have anything on the evolution of lactation?