Dominance Hierarchy - Hormones and Dominance

Hormones and Dominance

It has been shown that modulation of hormone levels after hibernation may be associated with the establishment of dominance hierarchies within the social order of the Paper wasp (Polistes dominulus). Former studies have shown that dominance hierarchies within the Paper wasp species is dependent on the queen (also known as a foundress, though the mechanism behind the establishment is currently under investigation. Recent research suggests that this manifestation may be dependent on specific hormones contained within the foundress. Laboratory experiments have shown that when foundresses are injected with juvenile hormone (JH), a hormone responsible for regulating growth and development in many insect species including wasps, the foundresses exhibit an increase in dominant behavior compared to those who had not been injected. Further evidence shows that foundresses that have a larger corpora allata, a region of the female wasp brain responsible for the synthesis and secretion of JH, are naturally more dominant. In an effort to determine if JH is indeed responsible for manifesting dominance, a follow up experiment was done utilizing 20-hydroxyecdysone, an ecdysone known to enhance maturation and size of oocytes. The size of the oocytes plays a significant role in establishing dominance within Polistes dominulus and thus researchers used this hormone to compare which condition (either JH or 20-hydroxyecdysone treated) induced higher levels of dominance intensities within foundresses. The results showed that the 20-hydroxyecdysone treated foundresses showed increased dominance compared to those foundresses treated with JH and suggests that 20-hydroxyecdysone, not JH may play a larger role in establishing dominance (Roseler et al., 1984). Subsequent research however, suggests that JH is implicated in the manifestation of dominance, though it only exerts dominance-type effects on certain individuals. By recording the number of mounting attempts between rival foundresses as a measure of dominance, researchers found that when injected with the same amount of JH, larger foundresses showed more mounting behaviors compared to foundresses that were smaller. Additionally, the more dominant foundress tended to show an increased number of oocytes contained within her ovaries. It was concluded that larger foundresses are more reproductively fit and thus JH, which is responsible for the growth and maturation of the ovaries, be more active within these individuals compared to the smaller, less fertile foundresses though the mechanism of action and/or any synergistic effects between JH and other hormones remains unknown.

The hormone model of dominance and reproductive capacity has also been demonstrated in the naked mole-rat (Heterocephalus glaber). It has previously been established that the dominance hierarchy within the species is dependent on the highest ranking female (queen) and her ability to suppress critically important reproductive hormones in male and female sub-dominants. In sub-dominant males, appears that lutenizing hormone and testosterone are suppressed while in females it appears that the suppression involves the entire suppression of the ovarian cycle. This suppression reduces sexual virility and behavior and thus redirects the sub-dominant's behavior into helping the queen with her offspring. though the mechanisms of how this is accomplished are debated. Former research suggests that primer pheromones secreted by the queen cause direct suppression of these vital reproductive hormones and functions however current evidence suggests that it is not the secretion of pheromones which act to suppress reproductive function but rather the queen’s extremely high levels of circulating testosterone, which cause her to exert intense dominance and aggressiveness on the colony and thus “scare” the other mole-rats into submission. Research has shown that removal of the queen from the colony allows the reestablishment of reproductive function in sub-dominant individuals. To see if a priming pheromone secreted by the queen was indeed causing reproductive suppression, researchers removed the queen from the colony but did not remove her bedding. They reasoned that if a primer pheromones were on the bedding then the sub-dominant's reproductive function should continue to be suppressed. Instead however, they found that the sub-dominants quickly regained reproductive function even in the presence of the queen’s bedding and thus it was concluded that primer pheromones do not seem to play a role in suppressing reproductive function.

Evidence also suggests that glucocorticoids, signaling molecules (commonly known as stress hormones) produced by the adrenal glands which stimulate the fight or flight response may also be implicated in the establishment of dominance hierarchies. Field research has shown that higher ranking individuals tend to have much higher levels of circulating glucocorticoids compared to subdominant individuals within the hierarchy. This is in stark opposition to the original suggestion stating that being subdominant was more stressful than being dominant within a hierarchy. Two core hypotheses attempt to explain these unusual findings. The first suggests that higher ranking individuals exert more energy and thus need higher levels of glucocorticoids to more readily mobilize glycogen stores for energy use. This hypothesis is supported by research showing that when food availability is low, cortisol levels tended to increase within the dominant male thus allowing increased glycogen metabolism and subsequent energy production. The second hypothesis suggests that elevated stress hormones are a result of social factors within the hierarchy particularly when the hierarchy is in transition. Proponents of this theory assert that when a hierarchy is unstable, aggressive interaction and confrontations increase within the hierarchy. As a result, the dominant individual ends up fighting much more than when the hierarchy is stable and as consequence of the increased fighting, glucocorticoids are elevated during this period. Field studies of olive baboons in Kenyaseem to support this hypothesis as it has been observed that dominant individuals tended to have lower cortisol levels in a stable hierarchy than did subdominant individuals. When the hierarchy shifted to being unstable however, the dominant individual showed much higher levels of cortisol compared to subdominant individuals. Many are quick to point out however, that this is extremely unusual as dominant individuals rarely have low levels of glucocorticoids regardless of hierarchy state. Nonetheless, it appears that being dominant may not be as beneficial as previously thought since chronic levels of glucocorticoids can have a variety of adverse health effects including immunosuppression, increased appetite, osteoporosis etc. see cortisol and perhaps is the reason why subdominant individuals are so reluctant to challenge the dominant individual for their position during hierarchy transitory periods.