BYU-led study points to potential reason for mule deer population decline in the West
by Logan Stefanich ksl · KSL.comKEY TAKEAWAYS
- A Brigham Young University-led study reveals increasing the ratio of male mule deer decreases female nutritional condition.
- This imbalance leads to lower survival and growth rates in mule deer fawns.
- Research suggests reducing male population could boost overall mule deer population growth.
PROVO — Over the last few decades, mule deer populations have declined across western North America.
As such, strategies to manage their populations have evolved, with one being to increase the proportion of mature male mule deer.
On the surface, it makes sense. More males could mean a higher likelihood of reproduction. But it might actually have the opposite effect.
New research from Brigham Young University, the Utah Division of Wildlife Resources and nongovernmental organizations like Sportsmen for Fish and Wildlife, Mule Deer Foundation and Rocky Mountain Elk Foundation, to name a few, found that increasing the ratio of male mule deer is associated with decreasing female mule deer nutritional condition, thus lowering population growth rates.
In what BYU wildlife ecology professor Brock McMillan described as "large-scale collaboration," researchers used 15 years of data on mule deer productivity and seven years of data on deer body reserve dynamics to quantify the influence of decreasing male harvest on the population dynamics of free-ranging mule deer.
The study began with the idea of looking at "everything that might be affecting population dynamics," McMillan, who is also a co-author of the study, said.
Over the course of several years, the research team annually captured female mule deer at the beginning of each winter using helicopter net-gunning, then recorded body mass, size, rump fat and other measurables before releasing the deer.
Their measurements revealed decreasing body fat accumulation in females — critical for their survival and the survival of their fawns — and lower female population growth the following year in areas managed for high male-to-female ratios.
The takeaway, McMillan said, is that land and wildlife agencies typically manage herds for a higher proportion of male to female ratio than they have in the past, leading to a decline in population growth.
McMillan called the revelation a "game changer."
"I'm not sure that the public's going to accept it, and I bet that a lot of the managers are hesitant to believe it. But what it suggests is that as management agencies have continued to increase the proportion of the populations that are comprised of males, they have been driving population growth down," McMillan said. "And it could partially explain the population decline of mule deer over the last 30 or 40 years throughout the West."
But where does the sense of looming skepticism over the notion come from?
"It's a little counterintuitive that if you remove more animals from the population, that there will be more animals in the population," McMillan said. "There's been a big push from the public to close the hunt in some units so that the populations can recover, and this study says that's exactly opposite of what you should do. You should remove more of the males."
"It's really hard ... to get buy-in, but if you spend time discussing the biology, we get a lot more buy-in," he added.
So, what is the biology behind what the study is saying?
McMillan said there are a couple of ways male mule deer are driving female nutritional condition down.
The first, he said, is competition for food.
"All the animals on the landscape are probably going to the same grocery store. Males are 50% bigger than females, so really a male counts for one and a half females. So when we increase the number of males on the landscape, we're increasing direct competition with those females," McMillan said.
Another reason is associated with the mule deer rut, or their annual breeding season.
"When you have more males on the landscape, the females are pursued more heavily during that two or three week period of the rut, and they have less time to forage. They have to move more to evade all the males ... which doesn't allow them to put on as much fat," McMillan said. "If they're always trying to evade a male, they don't have time to forage. They don't put on as much fat, or they lose more because they're active more."
With that in mind, McMillan said researchers don't know which of those reasons, and it could be all of them, are the primary driver of population decline.
"All we know is if you increase the proportion of males on the landscape, the average nutritional condition or fat content in the female declines, and that decline in fat content leads to lower survival, lower birth weights of their fawns, lower growth rates of their fawns and lower survival of their fawns," McMillan said.
A trickle-down effect, essentially.
Going forward, McMillan said future research could involve a study where the proportion of the population comprised of males is reduced to see if that leads to increased population growth.
The study was published Monday in the journal Ecological Solutions and Evidence.
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