Cooperative Polyandry


"Groups in which two or more males mate with a single female during a single breeding season and collaborate to raise her offspring"

Expectation of Polygyny and Selfish Behavior

Cooperative polyandry seems to fly in the face of expectations in two ways- First of all, males are expected to raise their reproductive success by gaining access to females. Females don't maximize reproductive success by getting more males! Secondly, it looks like altruism- one of the males is taking care of offspring not related to him! Why is he being a sucker?? Cooperative polyandry is pretty rare for these reasons.

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.

Cooperative Polyandry in Callitrichids

Characteristics of callitrichids
They're the smallest of anthropoid primates; all under 1kg. Generally territorial. Diet is mixed- fruit and small prey items. Some eat gums, especially the marmosets who are specialized gummivores. Another important feature is that they produce twins- about 80% of births are twins. They're only primates who twin routinely.

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.

Saddleback tamarins: 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
3%All males

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.

Helpers and Helping

Do helpers actually help?
In some cases, the results have been mixed. In some studies of birds, the nests with helpers really do better, but in others they don't seem to. Some behaviors look to us like they would be helpful but you have to look closely to see if they actually increase reproductive success. In tamarins, having help does actually help increase reproductive success. This help is mainly gotten from polyandrous males. You also get helping from other non-reproductive helpers in the group- usually older siblings. Some data on carrying from a monogamous callitrichid group:

Family MemberPercent of Carrying Time
Female sibling #125%
Female sibling #213%

Why do helpers help? What do they get out of it?

"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.
Thus, monogamous pairs do not attempt to breed, and helping appears to be necessary for successful reproduction."

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

The average weight of newborns in all primates is 8% of the mother's body weight, but in callitrichids it's 18% of the mother's body weight.

Costs of lactation
When they're pregnant, females spend 25% of their time feeding, but when they're lactating, they spend 35% of their time feeding. Since the babies are growing so quickly, they're expending a lot of energy and mom is still providing it all.

Costs of carrying
Look at how time budget is affected when moms are carrying infants around- they need to spend more time resting when they're carrying children around.

ActivityPercent of time budget when carrying kidsPercent of time budget when not carrying kids

So you can see that carrying children around really affects how much they can get done.

The Evolution of Cooperative Polyandry

"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.

Monogamous and Extended Family Groups of Marmosets

There is some more recent evidence noting the differences between tamarins and marmosets. It seems that tamarins are more polyandrous and the marmosets are more into monogamy with non-reproductive helpers.

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.