Sunday, May 22, 2011

New Deer Survive Due To Concealment, Timing

Sunday, May 22, 2011
There are thousands of them out there right now -- six to eight pounds each, warm brown dappled with white, looking like they're all legs and ears. The sight warms the hearts of animal lover and seasoned hunters alike.

They are whitetail fawns, nearing the peak of their springtime arrivals.
Doug Furin of Jumonville, Fayette County, saw a fawn last week, while digging "ramps" or wild leeks.

"I didn't even know it was there," Furin said. "I laid my bag of ramps on the ground and started to dig when I caught movement out of the side of my eye. There was this fawn, curled up in the leaves beside my bag. I'd almost stepped on it."

Furin did the right thing by the fawn and its mother. He left in a hurry.

"I know enough not to disturb them," he said. "I took a quick picture, heard the doe snort and got out of there. I think [the doe] had another one nearby because she kept looking back and forth."
Furin's encounter gave him a glimpse of the white-tailed deer's simple, but effective, strategy for ushering enough fawns into adulthood to sustain the species -- camouflaged young, concealed in the doe's absence, emitting little or no scent.

When a fawn lies motionless, the spots, numbering about a hundred on each side, give the appearance of dappled sunlight, an effective camouflage unless the young deer flinches, as did Furin's. Although Furin found the newborn lying down, it is likely that it had already taken some steps. Biologists' accounts document that fawns typically try to stand within 10 minutes of birth, and that most can stagger a few steps soon after. Young of large ruminants like deer, caribou and antelope must be able to leave the birth site quickly to avoid predators attracted to the scent of fluids lost by the doe. By six days of age, fawns can easily follow their mother anywhere.

Despite those built-in precautions, predators still exploit the protein windfall of whitetail fawns. In a two-year (2000-2001) study of fawn mortality on two sites in central Pennsylvania, Game Commission and Penn State biologists identified predation as the greatest source of fawn mortality. Researchers monitored marked fawns until the fall hunting season and found 46 percent of all spring and summer fawn mortality attributable to predators. Fawns were especially vulnerable during their first nine weeks. Bears and coyotes killed roughly equal numbers. Bobcat predation was minor.

Still, biologists concluded that enough fawns survived to at least sustain the herd.
"We have no evidence to suggest that the fawn survival rates we observed were preventing population growth," the authors wrote.

Another, more complex, ploy has helped whitetails thrive for hundreds of thousands of years, despite predators that find and eat as many fawns as they can.

"It's called a 'birth pulse' or 'birth synchrony.' The idea is that most fawns are born in a short window of time, which overwhelms predation. Even bears and coyotes can only eat so many fawns at the same time," said Marrett Grund, farmland deer project leader for the Minnesota Department of Natural Resources and a former deer biologist with the Pennsylvania Game Commission.

Grund explained that when the ratio of adult does to bucks in a population is balanced, bucks are able to breed all available does during a tight span in the fall rut. After a seven-month gestation, then, does drop their fawns at roughly the same time.

Among Pennsylvania's deer herd, 70 percent of fawns are born within 14 days of the first day of June.

But in herds with higher proportions of females, some does do not conceive until later 28-day estrus cycles, spreading out the birth pulse throughout the summer.

"If the pulse is extended, predators can feed on fawns for a longer period of time," Grund said. "Over the season, predators form a 'search image,' sort of like humans do when they are searching for mushrooms or something else that's hard to see. Given enough time, predators get good at finding [fawns]."

Grund explained that roughly equal numbers of male and female fawns are born each spring.
"But when deer densities become unnaturally high, the birth proportions shift to about 52 percent males. We believe that is a natural way for the population to self-regulate," Grund said.
Grund agreed that Furin did the right thing when he found his fawn. "It's a rare opportunity. Enjoy it," Grund said. "But do not touch. The proximity of a large predator, represented by your presence, is stressful to the fawn. If you handle or touch it, you only add to the stress."

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