The effects of autumn testosterone on survival and productivity in red grouse, Lagopus lagopus scoticus

Abstract

There are costs and benefits to individuals of having high levels of circulating testosterone, and the costs can be particularly apparent outside the breeding season. Unlike many birds, red grouse have high testosterone levels in autumn and in this study we explored the consequences of variation in autumn testosterone levels, in terms of survival and breeding success. We caught 123 male grouse in September, from six sites in Scotland and England, U.K. Grouse were given either testosterone or control implants, then fitted with radiotransmitters and released. Control males tended to survive better, but the difference was not statistically significant. During the first 3 months, cause of mortality varied with treatment, with birds of prey killing only testosterone-implanted birds. In spring, females paired with testosterone-implanted males had larger combs (sexual ornaments) than those with controls, suggesting that they might have been better-quality individuals. Testosterone-implanted birds were more likely to be paired and to be bigamous, and each bird alive in spring produced on average 2.5 more offspring than control birds. The benefits of investing in elevated levels of testosterone are clearly high for those males that survive the winter. However, these benefits are reduced by overwinter mortality and the reproductive success of all males alive at the start of the experiment did not vary statistically with treatment. We discuss the longer-term consequences of high testosterone levels, which are likely to be very different to short-term ones.