Seasonal Cycles
Most animals experience dramatic seasonal cycles: they migrate, hibernate, mate and molt at specific times of the year. In spring, for example, some 500,000 sandhill cranes make a migratory stopover at the Platte River in Nebraska. As mating season approaches, the testicles of hamsters quadruple in size. These cycles are hard-wired: captive ground squirrels continue to hibernate seasonally even when kept in constant temperatures with unvarying periods of light and dark. Likewise, birds in stable lab conditions get restless at migration time and keep molting and fattening in yearly cycles.