Factors Behind Fawn Mortality

There have been lots of studies on fawn survival and what kills fawns during their first six months of life. One obvious answer is predators — particularly coyotes.

Using DNA, researchers can confirm that coyotes ate a fawn, but DNA does not prove that they killed that fawn. It could have died of starvation, abandonment, disease, or other things, and then was found by a coyote.

In fact, the study of fawn survival is complicated by many variables and how those variables interact. A fawn could be malnourished because the mother didn’t do her job. If she selected poor habitat, wasn’t in the best condition, or was young when she gave birth, her fawns might suffer. It’s complicated and makes the study of fawn survival difficult.

There is research to prove coyotes do kill fawns. It involves capturing does and then inserting a vaginal-implant transmitter that comes out when the doe gives birth to a fawn. From there, researchers can then go to that spot and place a radio collar on the newborn fawn. The collar gives a signal when the fawn dies, and researchers can then rush in and determine via the sign left, or DNA, what predator killed the fawn, if any.

Without those transmitters, things can get tricky. If there is coyote poop at the site and deer hair in that poop, that doesn’t always mean the fawn was killed by a coyote. That fawn may have been sick and died, or it may have been malnourished (and a surprising number are).

One interesting Delaware study changed our perspective on the importance of predators killing fawns. They compared fawn mortality in a large area that had almost no predators (no bobcats or black bears, and only a few coyotes) to another area in Delaware that had all those predators. Guess what? Fawn survival was about the same in both areas. That means that if predators don’t kill fawns, they die of other things.

No one has done more research on the fawn-coyote relationship than Dr. Duane Diefenbach and his graduate students at the Pennsylvania State University Cooperative Fish and Wildlife Unit. I’ve mentioned this before, but they post summaries of their research projects on a blog. Google “The Deer-Forest Blog,” and sign on for free weekly e-mail updates.

One of their latest research ventures is looking at fawns before they die, to see why they may be susceptible to dying. Graduate student, Tess Gingery, is studying live fawns to see if there is a way to determine if some fawns might have a higher rate of mortality than others. They are doing this by looking at cortisol: A hormone that is released when a mammal is under stress. Humans also release cortisol when under stress. When the human body perceives stress, our adrenal glands release cortisol, and this causes an increase in our heart rate and blood pressure. That’s why cortisol is called the “stress hormone.”

The question then becomes: What causes a fawn to have stress? No studies have ever been done to measure cortisol levels in an attempt to figure out if fawn stress has anything to do with their survival. Dr. Diefenbach notes that continued stress causes continued release of cortisol, and that can negatively affect the fawn’s immune system, their food digestion, and their brain function. Not good.

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To look at this, when the Penn State researchers captured fawns, they collected saliva and quickly measured the amount of cortisol it contained. They believed that fawns with high cortisol levels would have a greater rate of dying, and indeed, that is what their research showed. Fawns with high cortisol levels had a lower survival rate than fawns that didn’t. The cortisol didn’t cause fawns to die, but whatever caused the stress that led to high cortisol did. And as cortisol levels increased in fawns they captured, survival later on decreased.

What conditions might cause a fawn to be stressed? Maybe poor fawn habitat. Maybe a fawn lives in an area with poor cover, and this increases stress. The presence of many predators might lead to fawn stress and higher cortisol. Previously, Penn State researchers found that there are higher densities of predators where you have more edge. Thus, you’d expect fawn survival to decrease in such areas…but it didn’t. This is one example of how factors that affect fawn survival interact.

Dr. Diefenbach considered the overall density of fawn predators — black bears, coyotes, and bobcats. All three have increased in Pennsylvania over the past 20 years, but in the forested areas of northern Pennsylvania, fawn survival in 2000-2001 was the same as fawn survival in 2015-2017. In the ridge and valley regions of Pennsylvania, fawn survival was the same in 2000-2001 as it was in 2015-2017.

Maybe the mothers are the cause of fawn stress? In fact, researchers found that the areas that had the highest cortisol levels in fawns weren’t the areas with the most predators. They were the areas with more young mothers. This could lead to fawns being born in poorer habitat, or fawns being malnourished because the mothers weren’t in the best of health. In fact, one could speculate that the age of mothers might play a bigger role in fawn survival than predator density.

However, as Dr. Diefenbach points out, maybe fawn survival isn’t a big deal relative to healthy deer herds. Other than in one study from South Carolina, most coyote-deer studies show that fawn survival is 50 percent or higher. Even if you wanted to, there really isn’t a practical way to control coyote numbers. The key to population growth is adult female survival. That’s what ultimately controls population growth — even when fawn survival is low. But that’s a story to be told in another column.

We hunters want to blame fawn deaths on predators such as coyotes. But the real world just isn’t that simple. With researchers like Dr. Diefenbach and some of his exceptional graduate students, the answers will keep coming. And when they complete one study, you can bet that opens up new questions for further research.