Inside the brains
of this olfactory king of the roost is a powerful cellular mechanism
for detecting carrion from hundreds of feet away
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Until now, no one had
been able to show at a microscopic level that the turkey vulture's
larger olfactory bulbs conferred advantage in the smell department.
(Wikimedia Commons/Tony Hisgett)
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Turkey vultures have evolved to have the most finely-attuned sense
of smell among nearly all birds, which has also allowed them to
be the most ubiquitous of all the 23 vulture species in the worldthat's
according to an important new
study co-authored by Smithsonian Institution researcher and
vulture expert Gary
Graves.
The study is the first to definitively provethrough difficult-to-conduct
comparative anatomical studies and histologywhat has long
been surmised by bird watchers: that the high-flying turkey vulture
has no match when it comes to navigating to a meal via smell alone.
Without these swooping scavengers, the globe could potentially
be looking at a greater incidence of disease or pestilence, and
more certainly, a larger body count alongside highways and byways.
A decade ago, a massive die-off of the endemic vulture species
in India and Pakistan provides evidence of what can go awry, says
Keith
Bildstein, interim president and Sarkis Acopian director of
conservation science at the Hawk
Mountain Sanctuary in Orwigsburg, Pennsylvania. As the birds
became almost nonexistent, dogs stepped into the breach and thrived
on the carrion that previously was mostly ingested by the vultures.
No one is certain why the dogs were not harmed by an arthritis drug
in the dead
cattle that was found to be felling the vultures. But the dog
population exploded, as did the spread of rabies. Some 30,000 Indians
die from rabies each year, with the majority of cases caused by
dog bites. Scientists believe the inability to keep human rabies
deaths in check was correlated with the loss of the vultures, Bildstein
says.
The paper by Graves and his colleagues should give people another
reason to respect vultures, says Bildstein. "This is a very
significant report," he says, adding that "it's a
slam dunk answer" on the question as to whether turkey vultures
(Cathartes aura) indeed have a superior sense of smell.
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Without these swooping
scavengers, the globe could potentially be looking at a greater
incidence of disease or pestilence. (Wikimedia Commons/Kevin
Cole)
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Turkey vultures are raptors, yet unlike the black vulture (Coragyps
atratus), they do not appear to primarily use visual cues to
locate their next meal. But that has not been a settled question,
and bird expertsgoing back to the times of the illustrious
ornithologist John
James Audubon in the mid-1800shave debated how those species
locate carrion.
In the 1960s, Kenneth
Stager, a senior curator of ornithology at the Natural History
Museum of Los Angeles County, made a breakthrough. Anatomical studies
he conducted showed that turkey vultures had an extremely large
olfactory
bulban area of the brain responsible for processing odorssays
Graves, curator of birds at the Smithsonian's National
Museum of Natural History in Washington, D.C.
But the debate over how the birds did it raged on, mainly because
no one had been able to show at a microscopic level that the turkey
vulture's larger olfactory bulbs conferred any kind of advantage
in the smell department.
Like the post-apocalyptic living dead roving the earth in zombie
films, scientists needed fresh brains to determine exactly what
was going on inside the turkey vulture's enlarged olfactory
bulb.
Fresh brains are not easy to get, says Graves. Vultures are protected
by law from hunting, and a special permit is required to collect
the birds for scientific research. Given its mission, the Smithsonian
has been granted such a permit, but even so, vultures are not purposely
executed very often.
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The brain of a turkey
vulture has twice as many mitral cells as black vultures,
despite being a fifth smaller. (EOL/Wikimedia Commons/Shravans
14)
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In 2012, however, Graves heard about a legal culling operation
by the U.S. Department of Agriculture that was to be conducted at
Nashville's airport. He decided it was a chance to make the
best out of an unfortunate situation. He and several colleagues
loaded up a van and set up a mobile necropsy unit at a warehouse
near the airport. "We were there to receive the corpses that
ordinarily would be disposed of," says Graves.
For five days, the researchers processed the birds as they came
in. The brains were removed and the heads preserved in formaldehyde.
They will be added to the Smithsonian's collection, making
a rare commodity available to researchers for generations to come.
Graves and his colleagues also worked quickly to take hundreds
of slices of brain, fixing them for microscopic study.
In their study, published online in Scientific
Reports on December 12, they reported that, on average, the
turkey vulture has an olfactory bulb that is four times larger than
that of the black vulture, and, compared with 143 other species,
the bulb is significantly larger relative to brain volume.
In addition, turkey vultures have twice as many mitral cells as
black vultures, despite having a brain that is a fifth smaller.
Mitral
cells, which are found in all animals, help transmit information
about smell to the brain, and serve as a proxy for the sensitivity
of the sense of smell. This microscopic look was important, because
no published data on the number of mitral cells in avian olfactory
bulbs exists, say Graves and his colleagues in the study.
"In absolute numbers, the turkey vulture has more mitral cells
than any other species measured," the authors say, noting that
they conducted comparison studies against 32 species of 10 different
avian orders.
While it may not be a huge surprisegiven field observationsthat
the turkey vulture has a much bigger olfactory bulb, "it's
comforting," says Bildstein. He was also impressed that Graves'
study was able to pinpoint that black vultures did not have superior
eyesight to the turkey vulture, as has been previously thought.
Graves and his colleagues postulate that eons ago, when Cathartes
and Coragyps diverged, through the "enlargement of its olfactory
system, the turkey vulture was able to occupy a new sensory niche
among vultures that depended on olfaction."
The birds "can smell very diluted plumes of volatile gases
in the air column, hundreds of feet above the ground," says
Graves, adding that they "circle around like bloodhounds to
seek the source of the odor."
That sense of smell and the superior ability to find food in closed-canopy
forests and jungles (unlike the black and many other vultures, which
cannot see carrion in those situations), "has likely contributed
to the turkey vulture having the most widespread distribution of
any vulture species in the world," say Graves. An estimated
18 million turkey vultures roam the globe, according to the
Cornell Lab of Ornithology.
Bildstein agrees with Graves' conclusions. The turkey vulture's
superior sense of smell is what is known as a "key innovation"
in the evolution of its species, he adds.
In the future, Graves hopes to look even deeper into the turkey
vulture's sense of smell. One avenue of exploration is to determine
whether olfactory receptors in the bird's genome might differ
from those of other birds, or humans, or other mammals, he says.
Graves is also interested in looking into what the turkey vultures
are actually detecting in those smells. "The odor of death
is pretty complex," he says, noting that it involves hundreds
of chemicals. It's not clear whether turkey vultures are keying
in on one particular scent or perhaps a cocktail of odors.
It's important to keep studying the massive birdswhich
can have six-foot wingspanssays Graves. They "continuously
clean up all sorts of things that could cause human and livestock
illnesses," he says, and save highway departments millions
by scavenging road kill.
Humans tend to be unaware of the valuable work the birds are doing
while they go about their usual business. "They're just
doing what they've been doing for millions of years,"
says Graves.
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