the expression of reproductive behaviors, and is not expected to drastically interfere with parenting behavior. In species where males exhibit minimal to no paternal care, testosterone levels are hypothesized to be at Level C throughout the breeding season because of intense and continued interactions between males and the availability of receptive females. In polygynous species, where a single male tends to breed with more than one female, males generally do not exhibit a heightened endocrine response to challenges, because their testosterone levels are already close to physiological maximum throughout the breeding season. Experimental support for the relationship between heightened testosterone and polygyny was found, such that if testosterone was implanted into normally monogamous male birds (i.e., testosterone levels were manipulated to reach Level C) then these males became polygynous. Mating Effort versus Maintenance There is a broader trade-off to consider when it comes to the challenge hypothesis: maintenance vs reproductive effort. Reproductive effort includes both mating and parental effort. 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. Therefore, the ability to find the correct balance between reproductive effort and maintenance would have been positively selected for by natural selection, leading to the physiological and social behaviour we now know as the challenge hypothesis. One such cost is that increased aggressive activity due to high levels of testosterone is hypothesised to expose males to increased predation, which not only endangers them, but also their offspring. A study on the lizard Sceloporus jarrovi, supported this prediction, as those with induced high levels of testosterone for extended periods of time had a higher mortality rate than those with lower levels of testosterone. 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. 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. Due to increased aggression as a result of high testosterone levels, individuals expose themselves to higher injury risk than usual. Therefore, the costs of maintaining a high tesosterone level may outweigh increased reproductive success. A study on male ring-tailed lemurs (Lemur catta) supports the idea of