The case for monogamy

Marmosets are generally described of as monogamous. Since this was a prevailing notion for some time, one only discovers instances in journals where researchers have found an alternative mating system in some marmoset groups. However, most scientists that study marmosets begin by conceding that these animals are, for the most part, monogamous. In some groups of marmosets, there appears to be a distinct pair bond. This pair bond has been "described quantitatively in terms of their socio-sexual interactions" (Evans, 1983). When bonded, males show soliciting behavior towards the females. Also, when bonded, females show aggressive behavior to other, "stranger" males not in the group (Evans, 1983). Other studies also found that females that leave their group are allowed to return at a later time without high levels of aggressive behavior from the other group individuals (Ziegler 2002). Interestingly, one study found that daughters are not necessarily kept from breeding as was previously thought, and that she may form bonds with other males in the group (Ziegler 2002). It has also been found that postpubertal males do not engage in sexual activity in a group dominated by other males and that these postpubertal males do indeed give up reproduction to become helpers (Baker, 1999). If members of both species give up some breeding rights it may indicate there is some fluctuation at the top, and there could be some monogamous pair-bonds being formed. Marmosets appear to have a high level of pair-bonding in captivity. However, in the wild, an enormous amount of data has been collected on the polyandrous nature of this animal.

The case for polyandry

The prevailing data indicates that marmosets tend to be in a polyandrous-type situation for much of their time in groups. There are many findings that are grouped as correllaries to this initial mating system distinction. First, there is a unique system of female suppression of reproduction in the marmoset (Abbott, 1987). When a female becomes the "breeding female" of a group, she forces the suppression of her subordinates (Abbott, 1987). Apparently, there is a physiological mechanisms that is related to this suppression that which looks at a determination of social status. Being conncected with a subordinate status causes a range of reproductive possibilities, from low offspring success to complete infertility (Abbott, 1987). Clearly, something more than just monogamy is going on here. The neuroendocrine and behavioral responses by females to their subordinates limit the reproductive ability of these subordinates (Abbott, 1998). They can repress the subordinate females reproductive neurocrinology, and in particular their hypothalamus-pituitary- adrenal function as well as their glucose levels (Abbott, 1998). Female physiological activity in this species is more important that male physiological activity regarding deciding who mates, because females can completely cause a subordinate to become infertile (Abbott, 1987). Psychological and physiological stress imparted by the breeding female to her subordinate "helper" females ensures that they aren't able to breed (Abbott, 1987). Some authors go even so far as to talk about cooperative breeding and how marmosets may indicate convergent adaptation to insects (Abbott 1998).

Some studies have shown that non-breeding females employ different strategies to attempt to become breeding females. These are 1) remain in natal family and wait, 2) emigrate to another group and breed along side dominant female, 3) form a new group, or 4) try to initiate intergroup matings in natal group (Ziegler, 2002). This would initially point to a type of polyandry, where being in the prestigious breeding position is essential for gene succession. Also, in one study, after the loss of a dominant, breeding female, the males immediately exhibited polyandry (Lazaro-Perea, 2000). This study also found that the marmosets in the study lived in cooperatively breeding groups with a pair of reproductive animals and one to eight non-reproductive helpers, that male and female offspring remained in group as helpers, and that there were very few instances of dispersal by marmosets (Lazaro-Perea, 2000).

The father marmoset puts in a lower investment when adult helpers are present (Santos 2000). Again, this supports the argument that marmosets behave in a polyandry-like way, as father marmosets would be less likely to assume that alloparenting would be practiced in a monogamous pair-bond. Helpers cooperate in territorial defense and their help decreases the cost of defense for the breeding pair (Lazaro-Perea, 2001). Non-breeding females are non-territorial and do not exhibit agonistic behaviors at group members from other territories (Lazaro-Perea, 2001). This may facilitate forays into other groups if food is low. These intergroup interactions are important for marmosets, and they are facilitated by the non-breeding females. This action is thought to be an attempt by these females to assess their ability to become the breeding female at an adjacent group, again supporting the idea that she must find her own group where a system such as polyandry can aid her in offspring production (Lazaro-Perea, 2001). In addition, the female bears twins 20% of her weight, which in turn contributes to a polyandry-like situation, where the group must care for her and her non-identical twin offspring (Santos 2000).

Please note that this page was created for an undergraduate course at Davidson College