Do Deer Inbreed?

Astronomical Odds

Zack Vucurevich, wildlife biologist and owner of Whetstone Habitat, says whitetails have a few factors working to their advantage that essentially make inbreeding a non-issue.

“Dispersal is probably the biggest one. When does are getting ready to drop their fawns this spring, they’re going to kick out the bucks from last year’s fawns,” he said. “The doe’s going to send him off, and he’ll reestablish his home range somewhere else. It’s been documented for yearlings to travel up to 100 miles away, but typically it’s only a couple miles. With does, it’s not nearly as common. They do disperse, but the likelihood is far greater for a buck to go out and disperse and set up a new home range.”

Vucurevich says radio collar data also tells us bucks take excursion expeditions that further minimize the risk of them breeding with a mother, sister, or another close relative.

“It looks like they’re going on vacation, where they’re just leaving their home range, and they’ll check out another area. It might be a couple miles from their home range,” he said. “So if that coincides with the rut, that’s another thing working against the potential for inbreeding to occur.”

Further, playing the percentages, it’s just not likely to happen.

“If it ever occurs, it is extremely rare with whitetails. If you look at their breeding strategy, it’s pretty much equal opportunity for any male deer on the landscape to breed any female deer on the landscape.”

Foremost expert on whitetail genetics, Dr. Randy DeYoung of the Caesar Kleberg Wildlife Research Institute, told the National Deer Association that the Institute has never documented an individual buck successfully breeding the same doe in multiple years or a buck ever mating with a known relative.

But Vucurevich says biologists have found no evidence of a “hard stop” such as a pheromone or scent that would prevent them from breeding with a relative.

“There’s nothing currently to suggest that it would occur—not saying it can’t happen,” he said. “But the likelihood of them ending up in the same area? It’s one of those things that unless it’s forced upon them, like a breeding facility, it’s not something that you’re really going to run into in the wild.”

Artificial Circumstances

While the odds of inbreeding in the wild are slim, Vucurevich says it’s possible in scenarios where the natural order is manipulated or displaced.

“When it’s artificial circumstances, it can and will occur.”

In a situation where they’re confined to an island (similar to the small wolf population secluded on Isle Royale) or escape from a zoo in an area where whitetails aren’t native, Vucurevich says inbreeding is possible because animals are hardwired to reproduce.

But a recent study found the isolated population of whitetails on St. John showed “little evidence” of inbreeding. So even under these conditions, it seldom happens.

Vucurevich says disease and the subsequent management of a whitetail population could also potentially lead to inbreeding.

“If CWD or EHD just absolutely hammers a population, that might be another circumstance where it could occur. Or say there’s a new CWD zone, and the wildlife agency offers unlimited tags to get control of the situation. You’re artificially decreasing the numbers to where it could potentially happen,” he said. “But if I were a betting man, I wouldn't put my money on it happening on the natural landscape.”

So while inbreeding is highly unlikely, it’s sometimes intentionally forced in the captive deer industry in the form of line breeding to promote desirable traits such as trophy antlers. But these artificial circumstances are far from natural.

Actual Effects

In some species, inbreeding can cause poor development, immunity, and fertility as well as innocuous physical changes. The wolves of Isle Royale suffered from spinal deformities, while inbred Florida panthers grew fur with telltale cowlicks, among other more serious issues. Avian species, on the other hand, are generally affected less than mammals.

“These genetic bottlenecks are not always as detrimental to certain species,” Vucurevich said.

It’s possible inbreeding could have some effect on whitetail antlers or hair. But the more likely result would be stillborn fawns.

“Even if inbreeding does occur, it might not produce a viable offspring,” he said. “Something might happen during gestation that would interrupt it due to the lack of genetic variability.”

Without widespread inbreeding, any one-off cases would primarily affect individual deer rather than the overall population in a given area.

Because inbreeding hasn’t historically been an issue among wild whitetails, not much is done at the state or property level to prevent it from happening. But for landowners faced with diminished deer populations, Vucurevich says they can play it safe.

“If the deer population where you hunt is low enough for you to be concerned with inbreeding, you probably shouldn’t be harvesting any antlerless deer until after the rut,” he said. “But those circumstances are extremely rare, and it is essentially a non-issue for hunters dealing with a sustainable population of deer.”

Vucurevich says some areas have gone from news of a deer track in town making the front page of the local paper to thriving populations. Whitetails are resilient, and inbreeding has yet to become a threat.

“The natural history of this animal, how they utilize the landscape, their breeding habits—they’re a very successful animal,” he said. “They’ve been here for a long time. They’re occupying spaces that would’ve amazed people when we had lower deer numbers back in the day.”

Feature image via Matt Hansen Photography.