PSU Deer Study

While we have put the topic of fawn predation to bed on this blog, it’s still shows up. A recent publication in Functional Ecology took a look at reproductive synchronization in white-tailed deer as it relates to predator defense strategies. 

Reproductive synchrony is just a fancy way of describing how spread out or clumped births are.

Before we get into how this might affect predation, let’s talk about the weather or more specifically seasonality. It’s widely acknowledged that climate factors (think food and temperature) are the muscle behind whether how much births are synchronized.  Indeed, deer exhibit a wider birthing season in the mild and relatively seasonless south than in the north where there are distinct windows of plant growth. 

But can predators reinforce or increase birth synchrony too? 

There are two strategies used by prey species that may have arisen as defense strategies against predation. Predator swamping and predator avoidance hypotheses.

Predator swamping hypothesis predicts a high synchronization of births. Strength in numbers or, in this case, it’s a they-can’t-get-all-of-us mentality. If all young are born at the same time (birth pulse), it’s like an all you can eat buffet. Predator bellies are too full. They can’t eat all of them so overall young have a better chance of survival. 

If predators really influence birth synchronization, then you might expect fawns born on either side of the birth pulse to be at greater risk of predation and experience lower survival. 

Predator avoidance hypothesis predicts asynchronous births. If you are a hider like the white-tailed deer fawn, then fewer of you playing hide and seek will make it more difficult for a coyote or a bear to find you than if everyone is playing at the same time. 

These seem like plausible theories. 

But to which strategy does the white-tailed deer subscribe? And are there data to support it?

This research looked at birth timing and survival data for fawns from 9 study areas including Pennsylvania. If the predator avoidance hypothesis was at play, population level survival probability would increase with asynchrony (more spread out births) and individual fawn survival would not be affected by its birth date relative to peak fawning.

I’ll cut to the chase: Patterns of fawn survival better support the predator swamping hypothesis, not predator avoidance; and predators may present a selective force great enough to shift reproductive synchrony. 

That means that along with temperature and food availability, predation may also influence when does give birth.

I’m a fan of the predator swamping hypothesis because it makes complete intuitive sense. I like things that make sense!

However, one of the findings didn’t quite jive. Remember our pyramid of births showing predator swamping hypothesis? When looking across data from 9 different areas, fawns born AFTER, but not before, peak parturition were at increased risk of predation. 

If predators really had an influence on synchrony then fawns born on either side of the peak should be at greater risk, not just those on the tail end.

Why did only fawns born AFTER see higher mortality?  

Well I seem to remember that young females (first breeding cycle) and those in poor physical condition are bred later than older does and those in good physical condition. That would mean their fawns are born later. Fawns of inexperienced first-time moms and those in poor condition are already at a disadvantage. Even this study noted that heavier fawns had a decreased predation risk for the first 30-days of their life. 

The upside of this paper is the large scale and giant sample size. The fates of over 860 fawns were analyzed. This allows us to see patterns that are invisible with smaller studies or on individual study areas. 

The downside of this paper is that it ignores other aspects of deer biology. Fawns born on the back side of the peak already have a strike against them and it’s not just their birth date. Their fate may already be sealed, and it has nothing to do with coyotes, bears, or bobcats lobbying for synchronicity. Proximate vs ultimate mortality – an underweight fawn born at a subpar birth site to a yearling doe is eaten by a bear. The bear is blamed but really that fawn was going to die no matter what. 

I love the predator swamping concept. But it’s just a happy byproduct of climate seasonality. Serendipity as they say. Like when I realized I could use Nature’s Miracle to clean up cat pee AND get the stink out of my running clothes! 

-Jeannine Fleegle
(Graphics by Tess Gingery)

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