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Stripes and Spots Forever

Posted: April 9, 2018

Tigers cannot change their stripes. Fawns cannot change their spots…maybe.
Click for larger image

Click for larger image

I’ve said it before.  There are few things in the world sweeter than a fawn.  And while they come in different flavors, all of them have sprinkles!  Their cutest is firmly grounded in those spots.  

Spots can tell you a lot about a fawn.  Like sex.  You can determine sex by spot pattern – straight for males, zig zag for females.  If you use this method, you are right about 50% of the time [flipping a coin works well too!].  However, you can greatly improve your odds of sexing a fawn correctly by examining between their back legs.

Ok, spot pattern does not indicate sex.  But that pattern is important.  Whitetail neonates use what is known as a hider strategy as an anti-predator defense.  Cryptic coloration is key to success of this strategy.  This is what allows fawns to hide in plain sight.  

Deer are incredibly adaptable and successfully occupy a wide range of habitats.  The forests of PA, the beaches of the Keys, the suburbs of New York City, or the agricultural fields of the Mississippi delta – no matter, deer call all these places home.  

Those landscapes look very different and have different predators and, therefore, different selective pressures.  Do a fawn’s spots look different in different places too?  

That’s what a group of researchers wondered.  A paper recently published looked at the heritability and predictability of spot patterns in 3 white-tailed deer populations.  

Heritability is the amount of phenotypic variation in a population that is an attributed to individual genetic differences.  So, is spot pattern genetically determined?  

Researchers hypothesized that camouflage patterns of white-tailed fawns were highly heritable and that spotting patterns would accurately predict their region of origin because those patterns vary spatially and temporally.  

Translation:  if spot pattern is an inherited trait and that pattern helps increase survival, then a population should respond to selective pressures in the environment creating different spot patterns for different environments.  The awesomeness that is evolution!

So did they decode the pattern?  Yes, kind of.  Their data support that spot pattern is moderately to highly heritable.  To blend in, spot pattern needs to match the environment.  If it doesn’t, then those fawns get eaten.  This is how a selective pressure, like predation, can affect camouflage patterns of a population.  

Two of the 3 populations studied had predictable spot patterns.  Meaning you could determine with some level of certainty where a fawn was born based on its spot pattern.  COOL!

But what about the third population?  Why didn’t spot pattern of fawns from that area match?  Milk chocolate fawn

Remember camouflage pattern needs to match the environment but what if the environment keeps changing?  The Delta region where fawn spot pattern was unpredictable has experienced the largest shift in land use of the 3 regions studied.  Over 75% of historic forest cover has been cleared for agriculture.  This drastic shift disrupted the selective pressures on this trait.  Spot pattern disaster! Chaos, dogs and cats living together, MASS HYSTERIA!  

Maybe I’m overstating.  But it does demonstrate how shifting land use patterns can have unintended consequences beyond just the change in habitat.  

So tigers maybe stuck with their stripes.  But not fawns, changing those spots is a necessity.  But don’t rush them.  Evolution takes time.   

-Jeannine Fleegle
Wildlife Biologist
PGC Deer and Elk Section

 

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