Cover legend: An arctic ground squirrel (Urocitellus parryii). Torpor onset in this seasonal hibernator is regulated within the central nervous system by adenosine A₁ receptor signaling that depends on a seasonal change in sensitivity: an agonist delivered into the lateral ventricle induces torpor in winter, but not in summer. Photo by Lesa Hollen and Leone Thierman. For more information, see the article by Jinka et al. in this issue (pages 10752–10758).