Natural breeding conditions and artificial increases in testosterone have opposite effects on the brains of adult male songbirds: A meta-analysis

A meta-analysis of the literature shows that in adult male songbirds, brain mass, telencephalon volume and n. rotundus (a thalamic visual nucleus) volume increase from the nonbreeding season (low testosterone) to the breeding season (higher testosterone). These effects can at least partially be mimicked by photoperiod manipulations in captivity. In contrast, an artificial testosterone (T) titer increase by chronic implants yields the opposite results: telencephalon, n. rotundus, and n. pretectalis volumes are lower in T-treated animals than in controls. These results suggest that artificial testosterone manipulations do not necessarily mimic the effects of natural variations in hormone levels and that results from experiments using T implants to mimic natural hormonal effects should be interpreted with caution. (C) 2002 Elsevier Science (USA).