Pineal and gonadal influences on ultradian locomotor rhythms of male Siberian hamsters

► The period of behavioral ultradian rhythms (URs) was longer in short days relative to long days. ► Most features of the short-day ultradian phenotype were unaffected by pinealectomy. ► URs were not affected by gonadectomy or testosterone or estradiol treatment. ► Melatonin-independent seasonal phenotypic transitions are uncommon.