A research group studied the timing of Arctic ground squirrel emergence from hibernation. They collected data over a 25-year period and found that female and male ground squirrels differed in how variable their emergence times were. Across years, no matter what the above-ground temperatures were, male ground squirrels emerged in the same week in April. Females usually emerged during the same week as the males if it coincided with snowmelt, but in colder years, they re-entered hibernation and emerged later after snowmelt. Which one or more of the following statements is/are possible proximate cause(s) for this phenological pattern?