Fluctuating asymmetry as an early warning system in the critically endangered Taita thrush

Environmental and genetic factors increasingly, stress natural populations, and conservationists need simple tools to measure the effect of these stresses before populations become irreversibly, affected. The inability of an organism to withstand random perturbations during its development-so-called developmental instability-has been proposed as a potential early, indicator of stress. To act as a true earl), warning system, however, stress-mediated changes in developmental instability, should be manifest in the phenotype before major changes occur in more direct components of fitness. We tested this hypothesis by, estimating the survival Probabilities of 260 Taita Thrushes (Turdus helleri) inhabiting three differentially, disturbed forest fragments (little, moderately,, and highly, deteriorated) and relating these to individual estimates of tarsus asymmetry, (a measure of developmental instability,), two other putative indices of expected fitness (size and body, condition), and a coefficient of inbreeding. Tarsus symmetry, but not survival, was lower in the moderately disturbed population than in the least disturbed one, whereas both symmetry, and survival were lower in the most disturbed population. Thus, higher levels of habitat disturbance were reflected by, increased population levels of asymmetry before a decrease in survival became apparent. In contrast, none of the other variables showed a significant response to habitat disturbance. At the individual level, asymmetric thrushes showed lower probabilities of survival than more symmetric ones, and the magnitude of the relationship increased with habitat disturbance. Birds in better condition (i.e., higher residual body, mass) survived better, irrespective of the quality, of the habitat. The key,finding that stress-mediated increase influctuating asymmetry may, precede decrease in survival suggests that single-trait asymmetry can serve as an early warning system in conservation.