High levels of testosterone at independence reduce fitness

Reaching independence is a critical moment in the life of an individual (not just for humans). At this moment, both environmental factors –food availability, weather, pathogens- and individual strategies –age of first reproduction, investment in reproduction vs. survival- strongly affect individual lifetime fitness. In a recent paper we explored this period in the Red Grouse (Lagopus lagopus scoticus). In this system, testosterone -a hormone that mediates territorial behavior- and parasites -the intestinal nematode Trichostrongylus tenuis- levels are known to influence survival and reproduction, the two key components of individual fitness. In a large scale experiment, we experimentally and simultaneously manipulated testosterone and parasites in 195 young males at three levels (control, intermediate and high) in 5 different populations across England and Scotland. Birds were equipped with radiotransmiters and we explored the effects of our treatments on fitness by monitoring reproduction and survival throughout their lives. Parasite challenges did not exert significant effects on fitness, possibly because parasite abundance did not increase to harmful levels. In contrast, high, but not intermediate, increases in hormone levels decreased overall fitness. This was caused by reduced offspring production in their first breeding attempt rather than by any effect on bird survival. Our results highlight that investing in high testosterone levels at independence, a strategy that might enhance short-term recruitment probability in territorial species such as Red Grouse, has a fitness cost, and can influence the resolution of the trade-off between reproduction and survival later in life.

The results of this research have been published in the journal Ecology, within an article signed Jesús Martínez-Padilla, Lorenzo Pérez-Rodríguez, Francois Mougeot, Sonja C. Ludwig y Stephen M. Redpath, entitled " Experimentally elevated levels of testosterone at independence reduce fitness in a territorial bird ". The PDF is available on the left-side pannel of this page.