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How does Fur become Prime?

There are many common misconceptions about how and when a fur becomes “prime”. The most common of these misconceptions is that cold weather is the trigger for an animal to begin building that lush (prime) winter coat that we trappers lust over.


So what exactly is a prime fur?  A prime fur is a fur that has thick, dense “underfur” (similar to down feathers on a bird) which is used to insulate the animal during the winter months. This dense underfur is protected by long “guard hairs”. A fur reaches its peak primeness when the underfur is at its thickest and the guard hairs are at the longest they will be throughout the year. This is when we as fur trappers get the most benefit from harvesting that particular furbearing species.


What makes an animals fur start to transform to into a prime state?  Contrary to popular belief, cold temperatures are not a contributing factor to a fur becoming prime. What triggers the biological change in an animal to start growing their winter coats is a term called photoperiodism. Photoperiodism is defined as the physiological reaction of an organism to the amount of daylight in a 24 hour period. This occurs in both plants and animals and is a driving factor for many biological changes. It is photoperiodism which provokes change in the color and density of fur as well as sexual behavior and hibernation.


Biologically, temperature has no effect on furbearers. For example, even if there was no snow and the temperature stayed at 45° degrees F° throughout the fall into December, a fishers pelt would still become prime by late November. This is driven by photoperiodism or amount of daylight in a given time period. Temperature can affect daily habits and travel patterns due to altering an animals food source and choice of shelter but that’s the extent of the role temperature plays.


In mammals, day length is registered in the suprachiasmatic nucleus (SCN), which is informed by retinal light-sensitive ganglion cells, which are not involved in vision. The information travels through the retinohypothalamic tract (RHT)* to the brain.

In less scientific terms, daylight is absorbed through the eyes retina, transmitting information to the animals brain and triggering the appropriate seasonal response. The thickening of fur on fisher or fox, the changing of color from brown to white on weasel and hare and even the signal for bear to go into their den for the winter (bears actually don’t hibernate) is all based upon daylight. Different animals respond differently to the loss of daylight. While some mammals like raccoon go through biological or physiological changes during early winter, others such as beaver and muskrat respond at different times and their fur will become prime later into the winter.

The US Fish and Wildlife Service along with State Wildlife Agencies have done extensive biological analyses to identify when individual furbearer species pelts become prime. These analyses are major factors in establishing legal trapping seasons. While population dynamics and cultural carrying capacity are certainly factors for biologists, pelt primeness is weighted heavily in the management decision of season timing and duration.

So the next time someone complains that it’s not cold enough to trap, you can correct them and dazzle them with some science.

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