[Ibby]

Originally Posted by ZenGum

Ibram, interesting thoughts. Arguing that a scenario is impossible is a reasonable response.

Nevertheless, I disagree.
I imagine some combination of hormones and pheromones and physiological response to suckling could well provide a possible mechanism to stimulate lactation, and behaviour guided by imprinting could limit this to direct offspring.

Would it have an evolutionary advantage? Hard to say, in lots of situations it probably wouldn't, but we only need a few situations in which it did, for it to be preserved.

I get your point about relative investments, but that raises the issue of sexual selection, and females would have reason to prefer partners who showed more promise as providers for the young.

I can imagine it being useful in mostly monogamous species by reducing the burden on a single parent and allowing bigger broods, leading to faster population growth. Mutual suckling could be pleasurable and reinforce pair bonds.

I can imagine it being beneficial for tribe/herd species in both sharing nursing burden and reinforcing group cohesion.

I think it is possible and could well be selected for. I mean, it is less useless than the peacock's tail.

Ah, but that was sexually selected for. that's a whole 'nother ball game.
edit: oh! i missed the part where you brought sexual selection up yourself. I guess I still don't buy it. it's already physiologically possible... if it was evolutionarily advantageous, I still feel like it would already exist. Females in species where males help raise young already select for males that have an advantage there, and still in almost every species, the male investment in offspring is still lower. The idea, though, that we "only need a few situations in which it did for it to be preserved", I think, is wrong. Just the same way that the investment in eyes quickly drops to none in cave animals shows how reward for investment needs to be constant, and evolution doesn't save things just because they might be useful. Since the majority of mammals do not raise their young in pairs, it wouldn't reasonably be a trait all or most mammals share, only a few, and while any mammal could evolve to have lactating males as things stand now, they don't. Don't doesn't always mean could never, I guess - I can imagine a potential scenario in which, like seahorses, some sort of convoluted set of circumstances leads to a substantial increase in male investment, in which case i think the males would probably develop the ability to nurse, but mammals, to me, don't look like they're in much of a position to buck the trend there.