Chernobyl birds have smaller brains

Abstract

Background: Animals living in areas contaminated by radioactive material from Chernobyl suffer from increased oxidative stress and low levels of antioxidants. Therefore, normal development of the nervous system is jeopardized as reflected by high frequencies of developmental errors, reduced brain size and impaired cognitive abilities in humans. Alternatively, associations between psychological effects and radiation have been attributed to post-traumatic stress in humans.

Methodology/principal finding: Here we used an extensive sample of 550 birds belonging to 48 species to test the prediction that even in the absence of post-traumatic stress, there is a negative association between relative brain size and level of background radiation. We found a negative association between brain size as reflected by external head volume and level of background radiation, independent of structural body size and body mass. The observed reduction in brain size in relation to background radiation amounted to 5% across the range of almost a factor 5,000 in radiation level. Species differed significantly in reduction in brain size with increasing background radiation, and brain size was the only morphological character that showed a negative relationship with radiation. Brain size was significantly smaller in yearlings than in older individuals.

Conclusions/significance: Low dose radiation can have significant effects on normal brain development as reflected by brain size and therefore potentially cognitive ability. The fact that brain size was smaller in yearlings than in older individuals implies that there was significant directional selection on brain size with individuals with larger brains experiencing a viability advantage.

Figure 1. Background contamination levels (Ci/km²) in the Chernobyl region and location of study sites.

Adapted from Shestopalov .

Figure 2. Head volume of birds (mm³) in relation to level of background radiation (µSv/h), after controlling for species and body mass.

The line is the linear regression line with the equation log₁₀(Head volume)  = 3.3918−0.0045 log₁₀(Background radiation). Residuals from a model that included species as a predictor were added mean log₁₀-transformed head volume 3.3934 to facilitate interpretation.

Figure 3. Body mass of birds (g) in relation to level of background radiation (µSv/h), after controlling for species.

The line is the linear regression line with the equation log₁₀(Head volume)  = 2.3922−0.0004 log₁₀(Background radiation). Residuals from a model that included species as a predictor were added mean log₁₀-transformed body mass 2.3922 to facilitate interpretation.

Figure 4. Beak length of birds (mm) in relation to level of background radiation (µSv/h), after controlling for species.

The line is the linear regression line with the equation log₁₀(Head volume)  = 3.0742−0.0004 log₁₀(Background radiation). Residuals from a model that included species as a predictor were added mean log₁₀-transformed beak length 3.0742 to facilitate interpretation.