This time of year, many of our park’s mammals are wearing the result of a remarkable biological process: seasonal coat growth. In species like bison and elk, the transition to a thick winter coat actually begins months earlier. The primary driver is photoperiod, the changing length of daylight. As days shorten in late summer and fall, light signals received through the eyes influence the pineal gland’s release of melatonin, which in turn triggers hormonal changes. These hormonal shifts stimulate hair follicles to produce longer, denser guard hairs and a woolly undercoat, creating built in insulation just in time for dropping temperatures.
The structure of that winter coat is a feat of natural engineering. Many Midwestern mammals, from American badger to coyote, grow a dual layer system: coarse outer guard hairs that repel moisture and block wind, and a soft underfur that traps air close to the skin. Air is an excellent insulator, and by holding it in place, the coat reduces heat loss even during subzero snaps. In large bodied animals like bison, the long shaggy hair over the head and shoulders can become frosted with snow while the skin beneath remains warm, evidence of how effectively that insulation prevents body heat from escaping.
Shedding, which often begins before winter has fully loosened its grip, is guided by the same daylight driven system. As days lengthen after the winter solstice, melatonin production decreases, and different hormonal cues signal follicles to shift from growth to rest. In species such as the red fox and white-tailed deer, the dense underfur loosens and is gradually shed, sometimes in dramatic tufts. Temperature can influence the pace of shedding, but it is the predictable change in day length, not a warm week in February, that sets the process in motion. This ensures animals are physiologically prepared for spring even when Midwestern weather remains unpredictable.
Winter coats offer a tangible way to connect guests to wildlife science. When visitors notice shaggy outlines on the bison or thicker neck ruffs on elk, we can explain how changing daylight triggers the growth and eventual shedding of these insulating layers. The bison and elk hides displayed at the Mills Education Center feature full winter coats and can be lowered so guests can feel the dense underfur and coarse guard hairs for themselves. Encouraging visitors to compare texture, thickness, and structure helps turn a simple touch into a lesson about adaptation, energy conservation, and seasonal change in Midwestern mammals.