Sex, drugs and mating role : testosterone-induced phenotype-switching in Galapagos marine iguanas

Abstract

Males of many vertebrate species have flexible reproductive phenotypes and must decide before each mating season whether to adopt sneaker, satellite, or territorial mating tactics. How do males gauge their abilities against others in the population? We tested experimentally whether hormone–behavior feedback loops allow Galapagos marine iguana males to activate their three behavioral phenotypes as predicted by the relative plasticity hypothesis. Territorial males defended small mating areas and had significantly higher plasma testosterone (T) levels (75 ± 11 ng/ml) than did satellite males that roamed around territories (64 ± 8 ng/ml) or sneaker males that behaved like females within territories (43 ± 11ng/ml). In territorial males, temporary pharmacological blockade of T slowed head-bob patrolling, decreased territory size threefold, and reduced the number of females on territories 20-fold. This supports previous data that females may gauge male attractiveness by using head-bob patrolling, here shown to be a T-dependent trait. Control-treated neighbors reacted to the weakening of T-blocked males by increasing head-bob rate fivefold and territory size 1.6-fold, and female numbers increased 2.5-fold. Unmanipulated or control-injected males remained unchanged. Behavioral effects were partly reversed after 7 days. T injections induced satellite males to establish temporary territories, even at unconventional locations. Some T-boosted satellite males suffered serious fighting injuries. T-injected sneakers left female clusters and behaved like larger satellite males that roam around territories. Thus, territorial and mating tactics are activated by T, but experimental (de-) activation at the wrong ontogenetic stage is costly: manipulated males switched phenotype but thereby lowered their access to females. We hypothesize that T levels of males that are based on early-season behavioral interactions influence a males' subsequent phenotypic role.