A closer look at monogamy as a mating strategy in human and non-human primates
Humans, compared to mammals, are relatively unique in undergoing menopause (Austad 1994) and living very long post-reproductive lives ( (Hawkes et al. 1998) It could be a non-adaptive byproduct of a drastic increase in longevity (lions and baboons; Packer 1988), or a selective advantage because females avoid the complications of childbirth and focus attention on childrearing (Mace 200), but overall this phenomena is not very well understood from an evolutionary standpoint. Researchers look to non-human primates for answers because they share a close ancestry and many derived traits, which makes them prime candidates to understand reproductive senescence and menopause and create future models. Some monkeys and apes undergo abrupt declines in reproductive capabilities or menopause (gorillas; Atsalis et al. 2008,chimpanzees; Videan et. al 2008, colobines; Borries et al. 2008, and common marmosets; Tardif et al. 2008).
The common patterns of reproductive success (RS) with any fish and birds have continually increasing RS as they age (Ryder 1980), some mammals increase in RS until they reach maturity then remain constant until death (Rockwell et al.1993), and many mammals have a peak reproductive success near maturity, increasing as they reach maturity, and then declining again as they age further (Robbins et al. 2006). One hypothesis attempts to explain having a ‘peak reproductive success’ by suggesting that there is a trade-off between allocating resources to infants and growth and maintenance in older individuals especially (Stearns 1992). This makes sense because costs of gestation, lactation, and care for the offspring are very high for female mammals, and increase as they age past maturity (Clutton-Block 1978).
Titi monkeys are unique from other NWM in their social structure, they display monogamy and paternal care (Mendoza & Mason 2006 & Kleiman 1977). Titi monkeys have comparatively smaller inter-birth intervals than other small bodied New World monkeys, possibly because they have paternal investment (Garber & Leigh 1977). Titi monkey reproductive success in relation to age has not been studied, and it will provide data for monogamous primates and New World monkeys.
I am in the process of getting permission to upload data from the California National Primate Research Center.
In this study, female reproductive success in terms of number of offspring could be predicted by age, but survivorship of offspring could not. Females more consistently gave birth to one offspring per year as they grew older suggesting either fewer lost pregnancies or shorter interbirth intervals. Secondly, we found that females who had more mates had twice as many offspring and significantly better reproductive success. However, this appeared to be because females who have lived longer, were likely to have more than one monogamous partner in their lifetime due to death of an old partner. There are a few hypotheses that explain this pattern of reproductive success; 1) experience, 2) selection, and 3) effort (Muack et al. 2004). The experience hypothesis suggests that reproductive experience can increase parallel to age (Curio 1983). This may explain why RS increased with age, and why individuals with more mates (who had been alive longer) had better RS.
Reproductive experience is likely to be a confounding variable, but it is difficult and sometimes impossible to entangle and test. Pairing experience and cooperation can also contribute to ‘experience’ (Curio 1983) and is easier to test. One way that pair experience can be measured is pair bond duration, and this is measurable. In future studies investigating this species, looking at pair bond duration could prove to be very useful, and is necessary to draw strong conclusions. The effort hypothesis suggests that the experience that comes with age will increase the reward per effort ultimately leading to a higher RS (Stearns 1992). The output from effort at different ages is another measurable factor, but experience could yet again be a confound. The selection hypothesis suggests that individuals who have low reproductive success will eventually disappear from the cohort (Curio 1983). Another related explanation proposed by Packer 1998, is that several species simply have not had a drastic change in longevity to result in the same deleterious effects of menopause (selection has not had enough time to act on older ages because species recently started living so long) After longitudinal studies on lions and baboons, he found that many females had serious reproductive and physiological declines right at the end of their lives, but most would die quickly after or never even make it this far. His theory suggests that these detrimental reproductive genes have not yet been selected out of the cohort in humans,since increase in longevity was drastic. If this is the case, it is possible that titi monkeys that lived significantly longer than what was found at the CNPRC may exhibit similar declines in reproductive success.