See subject. I know it’s an oldie, and I have a sense of why species (like us, I believe) are not evolutionarily monogamous (although I look forward to corrections) but not why those lovely birds whose name I forget, for example, stick together. And I can’t think offhand of any other species. Some penguins?
I should have thought the question should be why are any species monogamous.
The advantage of not being monogamous is that it gives males, and to a lesser degree females too, more chances to reproduce.
The advantage of monogamy is that it theoretically causes the males to stick around and help raise the offspring. What’s interesting is that it’s recently been found through DNA testing that a lot of birds previously thought to be monogamous are really so in name only. Both sexes sneak off for a quickie from time to time.
Not much point in reproducing if your heirs don’t live to adulthood, and in species where there aren’t many babies produced per cycle and the mother is at least briefly incapacitated by pregnancy and childbirth, and by lactation or otherwise feeding the young, then it makes sense that the males that stick around to help out would have more living offspring, who then also stuck around to help out.
That’s monogamy till your children reach independence, though, not monogamy for life.
But we find that real world monogamy–that is, a male and a female work together to care for juveniles–is extremely rare in mammals, and much much more common in birds.
And this is because in mammals the females have a much larger investment in the offspring than the males do, due to the biology of mammalian reproduction. Females gestate the offspring inside their bodies, and then feed the babies via lactation. Males do none of this. A female mammal can’t afford to abandon her babies, they are obligatorily dependent on female parental care. A male mammal can behave like a character in a 70s rock song, and decide it’s time to ramble on down the road, but don’t worry baby because we had some fun times and maybe someday I’ll be back in town to fuck you again.
But female birds lay eggs, and then the eggs must be cared for. A female bird’s investment in a newly laid egg is a lot greater than the male bird’s but it’s a heck of a lot less than a female mammal’s investment in a newly born baby. Not to mention that when the egg hatches, the baby bird is not obligatorily dependent on food that only the female can produce. And so a female bird can threaten to abandon a nest of eggs a lot more credibly than a female bird could abandon a nest of babies.
So in egg-laying creatures that don’t just lay the eggs and walk (or swim) away, egg-laying species that have parental care tend to have a lot more instances of males sharing in caring for the eggs and subsequent hatchlings.
Human beings have a very un-mammal-like system of parental care, in that male humans often do some amount of parental care, unlike most mammals where the males do absolutely no parental care.
(Bold and italic mine.)
The bold is key to my understanding. Now, in the italic sentence, “the threatening to abandon the eggs” is a threat to the male’s reproductive success, so he’ll stick around for the babies.
But for life? Those damn love-birds again. Or maybe I have their lives wrong.
Eagles, geese, swans, sometimes pigeons and doves tend to mate for life. Some other ones tend to go back to the same mates at the begaining of the mating season but once the babies are raised they appear to go solo again. I have seen this with some local hawks.
(Italics mine. I mean, the new italics in this post, not the note about italics in that I inserted when citing a different post. It was my post, and is my post now, and I can italicise whatever the hell I want when I cite it. Just want to clear that up.)
Actually, you made my question clearer, which is to add “How come?” after the italic words.
How much longer do mammals live beyond mating age? Once the female stops breeding, do monogamous animals spend the rest of their life together?
My meager level of knowledge tells me that most animals attempt to breed almost right up to the end, whereas humans typically go thirty-thirty five years after the birth of their last child. My point is that I don’t consider humans monogamous. The divorce rate is 50% in the US and more than that are cheating. Once the kids are out of the house, things can get pretty tense.