"Groups in which two or more males mate with a single female during a single breeding season and collaborate to raise her offspring"
It occurs in scattered animals- some raptors like hawks and eagles have polyandrous mating systems. Also african hunting dogs. In primates, humans in Asia with traditional cultures have polyandrous mating systems, usually for much the same reasons as callitrichids do, who are the only non-human polyandrous primates.
Only a few have been studied; common marmoset, cottontop tamarin, golden tamarin, saddleback tamarin.
Results of early studies
When people first started looking at social systems and mating systems, they thought callitrichids were monogamous. This was based on a few things- there's only one breeding female per group in the wild and also polyandry was pretty unknown at that point. Furthermore, in captivity the monogamous groups seemed to breed more successfully and when you added adults of either sex you ended up with a lot of aggression. However, when people got out into the wild, looked at things in more detail, and paid better attention, they figured out how things really work.
mating and communal care systems
Saddleback tamarins group composition: of all groups observed, how many were which composition type?
|22%||One male, one female|
|61%||Multiple males, one female|
|14%||Multiple males and multiple females|
Just because their group composition is mostly polyandrous doesn't necessarily mean that they're mating polyandrously. In many animals, there are males and females in a group together but there's only one breeding male- so the social system is not the same as the mating system. For instance, in red-tailed monkeys there is more than one male in the group but only one of them is mating with the females.
Are tamarins really polyandrous? Let us look at data from a group with two males in it. They each got about half of the mating (60-40% split for male1 and male2). Not only were the matings shared, but so was the parental care. Looking at statistics for infant carrying, male1 carried infants 45% of total carried time, male2 about 37%, and the mom carried less than 20%. We know that one of the males is being altruistic because only one of them could have been the father. He is being a non reproductive helper. These are seen a lot in birds- there are often helpers at the nest, usually young from previous seasons, such as in bee-eaters. Also jackals do this. Some primates have non-reproductive helpers too, like the barbary macaques.
|Family Member||Percent of Carrying Time|
|Female sibling #1||25%|
|Female sibling #2||13%|
"The key to explaining helping and polyandry in tamarins is the high need for parental care.Only one of 33 sets of offspring in the wild was born to a single pair. This one exception was conceived in a trio of 2 males and one female.
Ok so we know that the NEED is high, but does this really provide the motivation for the helpers? No, of course not! We need to know how it benefits the helper. Remember our equation rB>C. We know that helping is absolutely necessary. This means that the benefits to the recipient are really high- thus, if there is even a little bit of relatedness, then it will be worth it to help.
Costs of parental care
High neonate:mother weight ratio
|Activity||Percent of time budget when carrying kids||Percent of time budget when not carrying kids|
So you can see that carrying children around really affects how much they can get done.
"Helping is essential for successful reproduction, and the callitrichid mating system is determined by the number of nonreproductive helpers.
1 monogamy occurs in groups where a sufficient number of older offspring remain to help.
2 polyandry occurs in groups lacking old offspring. In these situations, pairs accept/recruit the help of an additional male."
Basically, males who are mated benefit under some conditions by accepting another male into the group and allowing him sexual access to the female. If the first male is trying to raise kids alone with the wife then it's just too hard. So he can have sole access but no surviving kids, or else he can share with another male and get half the kids sprung. Then if there get to be older siblings, the first male doesn't have to accept the secondary male any more.
So, monogamous pairs are the ones who have several older offspring in the group. Monogamous pairs who don't have older offspring don't attempt to mate. This is the evidence that backs up the previous theory and explains why males would tolerate another male in their group.
What's in it for the older offspring? They seem to be making the best of a bad situation. The habitat is often saturated, and when an offspring reaches the age when it could disperse, it can't because all the available spaces are occupied and defended by older, more experienced animals. They probably could leave the group, but they may as well stay since they're safer from predators. So they do the best they can of the situation; if they can't have their own kids they may as well help raise siblings who have some of their genes. They may also benefit by learning how to parent, gaining experience on parenting skills.
Why? It seems that the need
for polyandry is less in marmosets. Their home range size is smaller on
average than tamarins', as well as their day rage length. This means that
they don't have to travel so much, so carrying infants around isn't as
much of a chore.