Life history theory is a branch of evolutionary biology that deals with the trade-offs in the allocation of time and resources over an organism's lifespan, as Del Giudice discusses in the target article. Concepts and data from animal research played a central role in the development of life history theory. Animal research can also make an important contribution to the development and testing of life history models of human reproductive strategies. In particular, given the similarities in parenting, attachment, lifespan development, and reproduction between humans and other primates (Kappeler & Pereira Reference Kappeler and Pereira2003; Maestripieri Reference Maestripieri and Maestripieri2003; Reference Maestripieri2005b), studies of nonhuman primates can make a significant contribution to our understanding of human attachment and reproductive strategies.
In rhesus monkeys, infants possess an attachment system whose design features, ontogeny, and adaptive functions are very similar to those of the infant attachment system in humans (Maestripieri Reference Maestripieri and Maestripieri2003; Maestripieri & Roney Reference Maestripieri and Roney2006). These similarities suggest that the attachment system is not a product of the modern human environment, but rather, an adaptation with a phylogenetic history that can be traced back to the common ancestor of humans and Old World monkeys. The infant attachment system in rhesus monkeys is best viewed as an ontogenetic adaptation with the specific function of increasing infant survival during a period of high vulnerability and dependence on a caregiver (Maestripieri & Roney Reference Maestripieri and Roney2006). Attachment theorists have hypothesized that the attachment relationship with a caregiver becomes a template for other relationships later in life, and especially for sexual and romantic relationships. In this view, attachment would be an adaptation not only for early survival, but also for reproduction.
The hypotheses that human adult romantic attachment is an adaptation and that different attachment styles represent different reproductive strategies are currently not supported by strong empirical evidence. These hypotheses, however, could be supported by comparative and phylogenetic evidence showing that attachment serves reproductive functions in closely related primate species, and that humans and these primates are likely to share this reproductive adaptation by virtue of common descent (Maestripieri Reference Maestripieri2005b; Roney & Maestripieri Reference Roney and Maestripieri2002). Unfortunately, this comparative and phylogenetic evidence is currently lacking. Because the sexual and mating behavior of nonhuman primates appears to be fully accounted for by sexual selection theory (Kappeler & van Schaik 2004), the relationship between attachment and mating has not been investigated in primates. It is, of course, possible that attachment has acquired new reproductive functions in humans, which are not shared with nonhuman primates. As is generally the case with all negative evidence, the failure to find an association between adult attachment and reproduction in nonhuman primates would be difficult to interpret. However, evidence that variation in attachment is associated with variation in reproduction among closely related primates would provide important support for one of the crucial assumptions of the model presented in the target article.
Another assumption of the model is that the timing of menarche is an important expression of female life history strategies in humans, and that variation in the timing of menarche can differentiate between individuals pursuing different reproductive strategies.
Although this assumption is probably correct, research with rhesus monkeys and other primates has suggested that the onset of menstrual activity is only one of a suite of reproduction-related traits associated with different life history strategies. In mammals and birds, variation in offspring survival accounts for the largest fraction of variation in female reproductive success (Clutton-Brock Reference Clutton-Brock1988). Accordingly, in rhesus monkeys, the success of a female's first reproductive attempt depends to a large extent on the amount of parenting experience the monkey acquired as a juvenile. Therefore, early menarche and sexual activity are accompanied, and most often also preceded, by early and intense interest in other females' infants (Maestripieri & Roney Reference Maestripieri and Roney2006). Consistent with the predictions of Del Giudice's life history model and its predecessors, research with rhesus monkeys has shown that females exposed to harsh and unpredictable parenting in infancy are more interested in infants early in life than females without this stressful experience (Maestripieri Reference Maestripieri2005a).
By using cross-fostering experiments, we were able to disentangle the effects of early stress from genetic similarities between mothers and daughters, and by collecting physiological data we were able to show that the effects of early stress on the development of parental responsiveness are mediated by long-term changes in the activity of the hypothalamic-pituitary-adrenal axis (Maestripieri Reference Maestripieri2005a). Early and intense interest in infants was also observed among adolescent girls who grew up without their fathers at home and had early menarche (Maestripieri et al. Reference Maestripieri, Roney, DeBias, Durante and Spaepen2004). Therefore, parental responsiveness is an important variable that should be included in life history models of human attachment and reproductive strategies. Studies of nonhuman primates can enhance our understanding of different components of human reproductive strategies, both conceptually and empirically.
Lack of control for genetic effects on variation in attachment and reproduction, and lack of knowledge of the physiological mechanisms through which early stress affects reproductive maturation, are some of the limitations of life history models of human attachment and reproductive strategies. Experimental studies of nonhuman primates in which genetic and physiological variables can be manipulated provide opportunities to test some of the assumptions and predictions of models of reproductive strategies in ways that would not be possible in humans. They also provide the opportunity to conduct longitudinal studies of lifespan development and reproduction in a relatively short period of time. Therefore, proponents of human life history models should explicitly encourage the testing of these models with comparative data, and acknowledge that these data can provide important evidence concerning the adaptive function, physiological regulation, ontogeny, and phylogeny of human social and reproductive behavior.
