Sex steroids are gonadal steroids that bind to androgen and estrogen receptors. These circulating hormones are regarded as the prime motivating factors behind the multifaceted social behaviors of animals. This is evidenced by the fact that surgical or chemical removal of the gonads results, in most animal species, in dramatic behavioral changes.
In many cases, the sexual behavior will completely disappear after castration, and behaviors such as roaming, mounting, spraying, and marking, occur less frequently in animals with decreased levels of freely circulating sex steroids. Testosterone is probably the most common, naturally occurring sex steroid found throughout the animal kingdom.
Aggression is the animal behavioral trait most dependent on testosterone activity. When animals of all species experience diminished testosterone levels, male to male aggression ceases almost entirely. The fact that subsequent androgen administration typically restores sexual behaviors and aggressiveness in most species indicates a clear causal link.
It is important to note that in various animal species, testosterone secretion appears to be influenced, to a significant degree, by social and environmental variables as well. However, as a general rule of thumb, dominant and aggressive individuals tend to have, for the most part, higher serum testosterone levels than their submissive and less aggressive counterparts.
HOW DO ANDROGENS CONTROL ANIMAL BEHAVIOR?
Androgenic activity in animals is not too dissimilar from what is observed in humans. In fact, the androgenic control mechanisms are virtually identical. Let us take a closer look:
Behavior is often accomplished through a coordinated system of effectors. The control of this complex system is enacted by the central nervous system and the endocrine system’s hormonal secretions. Deep, feedback loop-mechanisms control the plasma concentrations of the various animal androgens.
These androgenic hormones exert a direct effect on the hypothalamus and pituitary gland, which begin a cascading impact involving the secretion of luteinizing hormone, stimulating follicle hormone, and the adrenal cortical stimulating hormone.
In male animals, this hormonal circuit results in the production of more testosterone and spermatozoa. Just like in humans, there is a negative feedback loop that decreases the production of gonadotropin-releasing hormones when serum levels push past a certain threshold.
One exciting aspect of animal testosterone systems is what is known as the organization and activation model. This model stipulates that only after exposure to testosterone during a critical developmental period can the brain tissue mediate aggressive behavior patterns. In other words, if not exposed to testosterone during early prenatal development, the central nervous system will only mediate female-like behaviors in the brain.
Androgen activity in relation to aggression in nature looks like this:
For the most part, adult male animals are usually more aggressive than adult females. When plasma testosterone concentrations are high, as they are during the breeding season, fights and other aggressive behaviors, surge significantly.
To most people, it comes as a great surprise to learn that animal behavior is ruled by the same hormonal mechanisms that govern human physiology and psychology, yet testosterone plays an essential role in the biological and social life cycles of virtually all vertebrates. Even insects have a similar hormone compound called ecdysone.
For the curious, here are the top 10 animals with the most testosterone in the animal kingdom:
#10 – Bears:
These furry giants are classified as caniforms because they are more doglike than their outward appearance would suggest. There are eight species of bears widespread over various habitats around the planet. Most bears are omnivorous, meaning they eat both animal and plant foods. With the exception of mothers tending their young, bears are mostly solitary. Several types of bear present serious danger to human beings unlucky enough to encounter them. Their typically gentle nature belies a capacity for savage aggression.
#9 – Rhinoceros:
The rhino is one of the largest animals that walk the earth to this day. An adult male white rhino can weight upwards of 5,000 pounds and measure close to 15 feet from end to end. The massive musculature and capacity for violent charges are supported by high levels of testosterone.
#8 – Boars:
Wild swine are capable of running down a strong male human. During rutting season, male boars are known to charge and strike humans with their sharp tusks blindly. The boar will typically continue to attack until the victim has become completely incapacitated. This type of overtly aggressive behavior is typical of high testosterone levels.
#7 – Horses:
Surprising, since there are graceful, brawny creatures are, pound for pound, some of the strongest animals on the planet. Their highly developed musculature allows them to run at high speeds and pull up to 3 times their body weight for extreme distances.
#6 – Bulls:
Most bulls weight up to 2,000 pounds and are capable of highly aggressive behavior. In fact, careful handling of these animals is required for safety around humans. Bulls are capable of sudden rage filled charges. This aggression has been selectively bred into certain species used for prizefighting.
#5 – Hippopotamus:
The hippopotamus is without a doubt one of the most aggressive species in the animal kingdom. These large-mouth, barrel-shaped beasts are frequently reported to charge and attack humans and even boating vessels, almost always without provocation.
#4 – Buffalo:
The African buffalo, also known as the Cape buffalo, is one of the five big game species found in the African continent. These strong, muscular, beasts gore hundreds of people every year and have won the nickname Widow-Maker. Buffalo are known to be more aggressive than lions and leopards.
When bull elephants enter the periodic musth state, they display highly aggressive behavior. Even normally placid individuals revert to overt violence and will attack sight.
#2 – Wolverine:
This small, bear-like creature is known throughout nature to be one of the most aggressive animals. Think of a weasel on steroids and going through “roid rage” and you will have a good idea of what a wolverine is like. Pound for pound, few animals match the Wolverines strength and aggressive savagery. In a testosterone-fueled rage, adult wolverines have been known to attack and kill moose.
#1 – Bull Shark:
The mighty bull shark has virtually zero tolerance for provocation, which makes them extremely dangerous to humans and all other animal species of the sea. Bull sharks are extremely territorial and overtly aggressive in nature.
Most zoologists believe this highly aggressive nature is due to excessive testosterone levels. It is thought that the bull shark has more serum testosterone than any other living creature on the planet. There is evidence to suggest that even a female bull shark has more circulating testosterone than an adult male elephant in musth.
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- Young, F. G. “The evolution of ideas about animal hormones.” The chemistry of life (1970): 125-155.
- Adkins-Regan, Elizabeth. Hormones and animal social behavior. Princeton University Press, 2005.
- Björkqvist, Kaj. “Sex differences in physical, verbal, and indirect aggression: A review of recent research.” Sex roles 30.3-4 (1994): 177-188.
- BullShark WIKI