Challenge hypothesis
It proposes that testosterone promotes aggression when it would be beneficial for reproduction, such as mate guarding, or strategies designed to prevent the encroachment of intrasexual rivals.
This additional boost in testosterone appears to facilitate male-male aggression, particularly during territory formation and mate guarding, and is also characterized by a lack of paternal care.
Level A is presumed to maintain feedback regulation of both GnRH and gonadotropin release, which are key factors in testosterone production.
[6] Generally, mating behavior is demonstrated in the spring and accordingly, male birds show a sharp increase in LH as well as testosterone during this time.
[8] Estradiol (E2), a type of non gonadal estrogen, seems to play a key role in regulating aggressive behavior during the non-mating season in several species of birds.
As previously noted, many bird species during the non-mating season have low testosterone levels yet still manage to display aggression.
The challenge hypothesis predicts the testosterone influence on aggressive male-male interactions between male northern fence lizards.
Similar to the original hypothesis, they predicted that there would be increased male-male aggressive interaction when a receptive and fertile female chimpanzee was present.
[11] Therefore, chimpanzees significantly increased both testosterone levels and aggressive male-male interactions when receptive and fertile females presented sexual swellings.
[15] This evidence suggests that the increase in testosterone is related to only aggression – not sexual activity – as male chimpanzees mate equally with both parous and nulliparous females.
When aggressive interactions among males arise in species that exhibit paternal care, testosterone levels are expected to be elevated.
Males are predicted to exhibit an increase in testosterone to Level C (physiological maximum), but only during periods of territory establishment, male-male challenges, or when females are fertile so that paternal care is not compromised.
In order to gain the benefits of reproductive effort, individuals have to suffer the costs of testosterone, which can hinder their physiological maintenance.
This is a form of life history tradeoff, due to the fact that natural selection favors reproductive success rather than maintenance.
[3] Prolonged high levels of testosterone have also been seen to suppress the immune system, with evidence ranging from human natural experiments to male-ring-tailed lemurs.
[23][3] Maintaining high levels of testosterone is energetically expensive, which can hinder reproductive success when a male frequently finds himself in aggressive and physically demanding situations.
[22] Due to increased aggression as a result of high testosterone levels, individuals expose themselves to higher injury risk than usual.
An increase in male-male aggression in the reproductive context as related to testosterone is strongest in situations of social instability, or challenges from another male for a territory or access to mates.
In continuous breeders, females are sexually receptive during estrus, at which time ovarian follicles are maturing and ovulation can occur.
Evidence of ovulation, the phase during which conception is most probable, is advertised to males among many non-human primates via swelling and redness of the genitalia.
[28] The predictions of the challenge hypothesis as applied to continuous breeders partially rests upon males' ability to detect when females are sexually receptive.
For example, testosterone is lower in fathers as compared to non-fathers,[32] and preliminary evidence suggests that men may be able to discern cues of fertility in women.
[33] The support for the challenge hypothesis in non-human animals provides a foundation for which to explore the relationship between testosterone and aggression in humans.
[35] One such context is ‘victory-defeat’, where testosterone and cortisol will interact to increase desire to compete again more after losing than winning, as a method of regaining social status.
