Comment on ''How the Horned Lizard Got Its Horns'' Recently, Young et al. (1) purported to identify the mechanism behind the origin and maintenance of horns in horned lizards (Phrynosomatidae: Phrynosoma) by demonstrating directional selection for increasing horn length in one population of Phrynosoma mcalli from the southwestern United States. In drawing their conclusions, Young et al. asserted rather than demonstrated the current function of the trait of interest (that is, they assumed that horns are an antipredatory defense mechanism). In addition, Young et al. assumed that current function reflects past selection. In doing so, they have failed to recognize the crucial distinction between adaptation and exaptation (2, 3). These criticisms could have been avoided had the authors given due consideration to the phylogenetic definition of adaptation (3-5), which uses robust criteria for identifying adaptation.

The early postnatal period is a critical neurodevelopmental stage characterized by rapid neural maturation and is adversely affected by early-life stressors. This study explored the behavioural, physiological, and epigenetic consequences of early-life stress in a population of homeless rescue kittens. This longitudinal study included 50 kittens rescued and placed into foster care by the Prince Edward Island Humane Society. They underwent behavioural testing at 8, 10, and 12 weeks of age. Hair cortisol concentration was measured at 8 weeks and served as a physiological marker of the previous 3 months’ cumulative stress response, which, for these kittens, included the late gestation period. A blood sample for relative telomere length measurement was taken at 10–12 weeks to estimate epigenetic changes as young kittens. Data were analyzed with respect to age and performance in all repeated measures tests, status as a stray or a surrender, and the presence of the dam in their foster homes. As expected, the performance of kittens in all tests changed over the 5 weeks of testing. Kittens separated from their mothers exhibited significantly higher hair cortisol concentrations (p = 0.02) and elongated relative telomere lengths (p = 0.04). No correlation was found between hair cortisol concentration and relative telomere lengths (p = 0.99). These results support the need for further study on the effects of epigenetics and early-life stress, both in kittens and across species.