The following is a description of the background for my dissertation research, “Stress and sociality in a Patrilocal Primate: Do Female spider monkeys tend-and-befriend?” We will be going over my findings in class.
Physiological stress can be measured using glucocorticoids, the “stress hormones.” The secretion of glucocorticoids is an adaptive response to immediate stressors that mobilizes glucose in the bloodstream. However, in situation of chronic stress, a positive feedback system becomes established and further increases glucocorticoid levels, rather than bringing them down to baseline (Sapolsky, 1992). As a consequence, the immune system and other aspects of physiological functioning become compromised. Other hormones can mediate this process by increasing or decreasing glucocorticoid levels (Sapolsky, 1992; Taylor et al., 2000; von der Ohe, CG, Servheen, 2002). For example, oxytocin serves to inhibit the release of glucocorticoids. This hormone is associated with birth and lactation, and aspects of social bonding (Uvnäs-Moberg, 1998). Thus, engaging in social affiliation increases oxytocin levels.
Research in humans demonstrates that loneliness, bereavement, and personal conflict increase stress, whereas social support and physical contact can reduce it (Geary and Flinn, 2002; Heinrichs et al., 2003; Cohen, 2004; Segerstrom and Miller, 2004; Taylor, 2006). Such findings have been mirrored by studies on non-human primates. Baboons that lose close kin to predators are more likely than others in the group to have increased glucocorticoids. Of females that lose a kin member, those that increase their grooming networks return more quickly to baseline levels (Engh et al., 2006 a; b). The potential for infanticide is another of the biggest stressors for female primates, and studies have documented that lactating females exhibit elevated glucocorticoids relative to other females in response to takeovers by potentially infanticidal immigrant males (baboons: (Beehner et al., 2005; Engh et al., 2006 b); howler monkeys: (Cristóbal-Azkarate et al., 2007). Thus, although direct aggression does raise glucocorticoid levels (Wallner et al., 1999), for cognitively sophisticated animals, the threat of aggression alone is enough to elevate physiological stress levels.
The social mechanisms of coping are labeled “tend-and-befriend” by Taylor and colleagues (2000). This phrase refers both to tending offspring as well seeking out affiliative support, particularly from other females. They further hypothesize that this strategy is an adaptive trait that is a widespread, ancestral trait among female primates, and is related to increased activation of oxytocin in female physiology. Support for this strategy has been demonstrated in Western psychological studies on humans, as well as among matrilineal monkeys, particularly macaques and baboons (Taylor et al, 2000). However, these species may not be representative of over-arching primate patterns, especially since matrilineal social organization may be a derived trait that is characteristic only of Old World monkeys (Strier, 1994). In female-philopatric species, related females are characterized by strong bonds and stable dominance hierarchies; conversely, in species characterized by female dispersal, females are not as strongly bonded (Wrangham, 1980; Van Schaik, 1989; Sterck et al., 1997). However, strong “friendship” bonds have been documented among some unrelated females (Silk, 2002).
The tend-and-befriend hypothesis is intended to demonstrate an evolutionary basis for human female bonding and coping strategies. However, in contrast to many other primate species that form cohesive social groups, humans have a highly flexible, dispersed social organization (Geary and Flinn, 2002; Aureli et al., 2008). Furthermore, Manson and Wrangham (1991) argue that humans have a tendency toward patrilocal societies with weak social networks among females, and that this may be indicative of the ancestral hominid condition. Under these conditions, human females would be predominantly unrelated to other females in their adult lives.
Dispersed, patrilocal societies are rare among the primate order, but are found among our closest relatives, the chimpanzees and bonobos, as well as the more distantly related Ateline clade of New World monkeys (Goodall, 1986; Chapman, 1990; Symington, 1990). Although studies on chimpanzees and bonobos are valuable for providing an evolutionary context to examine human patterns, they can only document patterns characteristic of the African apes. In order to evaluate whether the tend-and-befriend pattern is part of a wider ancestral pattern, these questions need to be studied in a species that is phylogenetically distant to humans, yet closely mirrors their social organization. Spider monkeys are a New World species that exhibit fission-fusion social organization in conjunction with male philopatry and female dispersal. By examining this question in spider monkeys, we can focus on the selective pressures that shaped social relationships and coping responses in a convergent social organization.
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