It’s not lonely at the top, after all: dominant chimps have more parasites

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It wasn’t more than a few decades ago that stress was seen merely as an unpleasant mental state or a mild irritation. Stanford neurologist Robert Sapolsky recognized early on, though, that it had real, significant impact on one’s health. In a Wired piece from last summer,  “Under Pressure,” Jonah Lehrer relates how Sapolsky first connected the dots between stress and health while studying baboons: “He noticed, for instance, that the males at the bottom of the hierarchy were thinner and more skittish. ‘They just didn’t look very healthy,’ Sapolsky says. ‘That’s when I began thinking about how damn stressful it must be to have no status. You never know when you’re going to get beat up. You never get laid. You have to work a lot harder for food.’”

The sustained social stress that social animals with low status put up with can grind down their bodies and numerous studies have shown the health gap between high and low ranking individuals several species. High ranking greenfinches clear viral infections more quickly than lower ranking ones, high ranking dairy goats have fewer gastrointestinal parasites less dominant ones and high ranking pigs have more white blood cells available to fight off Aujeszky disease than lowlier swine.

This isn’t to say its all fun and games for the animals at the top of the social hierarchy, though. Sure, dominance has its perks. Everyone wants to groom you, no one’s trying to beat you up and you can always get laid, and, therefore, have the best chance of reproductive success, but acquiring and maintaining that dominance is no easy task and often involves frequent aggression. Testosterone is considered the driving physiological factor of aggression – the hormone has a permissive effect, altering bodily functions that heighten pre-existing patterns of aggression – and is frequently associated with social dominance rank (though no study has shown solid, significant correlations among the three). Testosterone might facilitate a climb up the dominance ladder, but it is appropriately costly, and elevated testosterone levels are associated with increased metabolism, production of oxygen radicals and – mirroring the effects that stress has on low ranking animals –suppression of the immune system.

Michael Muehlenbein and David Watts from Indiana University wondered, since testosterone is immunosuppressive, and high testosterone levels are associated with a high dominance rank, if a high rank would also be associated with a higher parasite burden, a common effect of a suppressed immune system. To get a better grip on the links between dominance, testosterone, health and parasitic infection in nonhuman primates, Muehlenbein and Watts collected fecal samples and behavioral data from a community of chimpanzees in Ngogo in Kibale National Park in western Uganda. With 150 members, the Ngogo community is the largest described in the wild and contained 24 adult males and 14 adolescent males when Muehlenbein and Watts did their research.

The pair logged 1,700 hours of observational data to determine the dominance rank of 22 adult males and collected 67 fecal samples from those individuals. They hypothesized that dominance rank would be directly associated with fecal testosterone and cortisol levels (cortisol is a steroid hormone that’s produced by the adrenal gland in response to stress) and intestinal parasite burden. In order analyze the fecal samples for hormone and parasite levels, the researchers had to dehydrate the chimp droppings for two hours in a small portable oven on top of their camp stove before packing them up and shipping them stateside, an activity that likely caused the occasional cortisol spike at camp.

After analysis, Muehlenbein and Watts found that 1) fecal testosterone, but not cortisol, levels were directly associated with dominance rank, 2) both testosterone and cortisol were directly associated with intestinal parasite richness (number of unique species recovered) and 3) dominance rank was directly associated with worm-like helminth, but not single-celled protozoan, parasite richness (possibly because helminth parasites impose greater immunological costs than protozoans). As suspected, higher ranking chimps had higher testosterone levels and greater parasite burden than the lower ranking ones. Heavy is the head that wears the crown, and the gut below is isn’t doing so well, either.

Testosterone’s effect on the immune system is mainly suppressive. It inhibits antibody production and impairs natural killer cell activity. Muehlenbein and Watts think that a testosterone-suppressed system can’t muster the allergic responses needed to clear gastrointestinal infections. They also suggest that it could be the behavioral, and not the physiological, aspects of alpha male status that leads to an increased risk of parasites. A chimp’s daily travels, social network, nutritional status and certain personality factors – like sociability – all impact its susceptibility to disease to varying degrees, and all those factors vary greatly depending on where a chimp sits in the social hierarchy. High-ranking males usually have wider social circles, get around more and have more mating opportunities and, hence, more risk of acquiring directly-transmitted infections.

Reference: Muehlenbein MP, & Watts DP (2010). The costs of dominance: testosterone, cortisol and intestinal parasites in wild male chimpanzees. BioPsychoSocial medicine, 4 (1) PMID: 21143892

Image: “Alpha male chimp and older chimp jump on fig tree” by Wikimedia Commons user Caelio, used under a Creative Commons license.

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