The correct Fig. 5 is now given below. Fig. 5 Activity levels (number of crosses in test tank) in the cold-water treatment was positively related to the relative size of the nuchal hump (hump area/body area)The original article has been corrected.Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

trait was demonstrated in wild-caught X. multilineatus females, suggesting the nuchal hump could be a target of sexual selection if it developed in males in the wild. We consider the results from this study in relation to the diversity in development of the trait across species and introduce a nuchal hump metabolic growth hypothesis to help explain its function in swordtails.