Testosterone makes birds more colourful
Male birds benefit in several ways from an increased testosterone level in the blood: In addition to increased assertiveness during courtship or a positive influence on their vocal tract, the hormone ensures more intensive coloring of the plumage through carotenoids, as a Canadian research team has now found in red partridges (Alectoris rufa) discovered. The testosterone increases the availability of the color pigment.
The scientists led by Julio Blas from the University of Saskatchewan experimentally increased the testosterone level in some red chickens that initially had a low concentration of the hormone in their blood. At the same time, they measured the level of carotenoids in the blood and the strength of the immune system of the birds, since the color pigments not only determine the yellow and red colors of feathers, feet or beaks, but also stimulate the animals' body defenses. Testosterone, on the other hand, is said to have a suppressing effect on the immune system. Accordingly, only he althy animals can afford to invest a particularly large number of carotenoids in their plumage.
The additional testosterone administration increased the carotenoid content in the blood plasma and in the liver of the birds by around a quarter compared to untreated animals. At the same time, males with high testosterone levels in both groups also showed a more intense red coloration of the skin around the eyes - an important signal during courtship. However, since not all of the additionally available carotenoid went into the feather headdress, a larger proportion also remained in the blood, where it mitigated the oxidative and thus potentially harmful effects of the androgen.
Hormones such as testosterone play an important role in the development of male sex organs such as secondary characteristics and make a major contribution to shaping male behavior. Blas and his colleagues therefore evaluate the positive influence on the carotenoid level as the original mechanism for protecting the immune system from androgens. In order to convey this improved he alth to the outside world, intensively red, yellow or orange colored parts of the body developed as a secondary quality feature in the course of evolution. It is possible that testosterone also accounts for the sex-specific differences in plumage colours, which are based on carotenoids, while this is not the case with structural colors - they arise from a special structure of the feathers - or pigments based on melanin.
It is still unclear to the scientists how testosterone raises the carotenoid level: This may be related to increased enzyme activity in the intestine or an increase in lipoproteins in the blood, both of which are also controlled by the hormone, according to Blas.
