Social hierarchies and health in primates

Wow, it's almost like Science is reading my mind! Just after this post on social status and this post on allostasis, the April 29, 2005 Science has printed this review article on social hierarchies and health by Robert Sapolsky (Stanford University).

The idea that the overall level of stress should affect health status is now well-established, due in large part to Sapolsky's prior work. Likewise, the idea that stress level is influenced by social position, especially rank, is clear. But which social ranks should be under the most stress?

Research in the 1950s, since discredited, argued that high rank was more physiologically stressful (that is, the "executive stress syndrome," which was purportedly valid for both humans and other primates). By the 1960s, the prevailing view had become that lower dominance rank carries the greatest risk of stress-related disease. It has now become clear that this is an incorrect generalization. The contemporary view reflects the heterogeneity that is the core of ethology: Rank means different things in different species and populations (Sapolsky 2005:648).

Ultimately, stress resulting from social interactions depends on the interactions themselves, and these are indefinitely varied. Sapolsky tries to break down the possible components of variation, arriving at ten separate dimensions along which social interactions may direct stress to some individuals more than others. These range from the strongly ecological, such as the whether high-rank individuals can dominate resources, to the ideosyncratic, such as culture and personality. Many include different aspects of the breeding system or competition related to mating. Sapolsky does not mention demographic factors that plausibly affect stress levels, such as group size and population density; his attention seems to be mostly directed to "species-typical" solutions. I would term these as "static" solutions, since they do not allow variation or envisage mechanisms of transition; compared to "dynamic" solutions that show any particular species as a flux of different social groupings that change over time. In any event, there appears to be little point in reductionism of social interactions with relation to stress -- a clear answer about whether dominant or subordinate individuals are more stressed in any particular species seems like a concrete step forward.

Sapolsky presents a table of binary tests of his ten factors, performed upon captive animals, to show the way that they influence the stress level in dominant and subordinate individuals. The assay of stress keys in on the level of glucocorticoids.

Consistently, animals who are more socially stressed by the dominance hierarchy show indices of hyperactivity of the GC system. This includes elevated basal levels of GCs, the enlarged adrenal glands that accompany such increased secretion, a sluggish GC stress response in the face of a major homeostatic challenge, and impaired sensitivity of the system to negative feedback regulation (Sapolsky 2005:651).

Unstable hierarchies, hierarchies in which dominant individuals must repulse many challenges, and cooperative breeding systems tend to stress high-ranking individuals. Low-ranking individuals are relatively stressed (compared to high-ranking individuals) in societies with low social support for subordinates and with nonviolent intimidation by dominants.

According to the review, chronic stress has many negative health effects, including:

  • hypertension
  • increased cholestorol and associated plaques
  • inhibition of fertility and reproduction in males via suppression of testosterone
  • reduction in fertility and reproduction in females via delayed maturation, anovulatory cycles, accelerated senescence, and reductions in estrogen and progesterone
  • suppression of immunity
  • neurobiological changes

Finally, the review discusses the analogy between the status-related stress in primates and SES as a factor in human health.

Are these findings relevant to humans? Initially, they seem to be of minimal relevance. Humans are not hierarchical in the linear, unidimensional manner of many species. For example, humans belong to multiple hierarchies and tend to value most the one in which they rank highest (for example, a low-prestige employee who most values his role as a deacon in his church). Furthermore, the existence of internal standards makes humans less subject to the psychological consequences of rank. Finally, health-rank relations that are easy to study can be highly artificial (e.g., examining the physiological consequences of winning versus losing an athletic competition) (Sapolsky 2005:652).

But Sapolsky concludes by noting that SES does correlate with several health indicators in Western societies.

There is no necessity to assume that humans must be hierarchical in the same style as many primate species to still have status-related chronic stress. The elements of the endocrine system that comprise the stress response are highly conserved in vertebrates; they can be expected to respond social interactions in any primate species, whether it has a rigid hierarchy or not. The cumulative effect of social interactions may be patterned in many different ways, some probably leading to an expression of chronic stress even if coping mechanisms (such as hobbies) are present. The endocrine system is about motivation for actions; and ancient humans needed motivating no less than any other kind of primate. Does it mean something for our evolution that low SES in today's humans leads to greater health impacts? I guess we'll have to wait and see.

References:

Sapolsky RM. 2005. The influence of social hierarchy on primate health. Science 308:648-652. Science online