When geoecologist Steffen Zuther
and his colleagues arrived in central Kazakhstan to monitor the calving of one
herd of saigas, a critically endangered, steppe-dwelling antelope,
veterinarians in the area had already reported dead animals on the ground. "But
since there happened to be die-offs of limited extent during the last years, at
first we were not really alarmed," Zuther, the international coordinator
of the Altyn Dala Conservation Initiative, told Live Science.
But within four days, the entire
herd — 60,000 saiga — had died. As veterinarians and conservationists tried to
stem the die-off, they also got word of similar population crashes in other
herds across Kazakhstan. By early June, the mass dying was over. [See Images of
the Saiga Mass Die-Off]
Now, the researchers have found
clues as to how more than half of the country's herd, counted at 257,000 as of
2014, died so rapidly. Bacteria clearly played a role in the saigas' demise.
But exactly how these normally harmless microbes could take such a toll is
still a mystery, Zuther said.
"The extent of this die-off,
and the speed it had, by spreading throughout the whole calving herd and
killing all the animals, this has not been observed for any other
species," Zuther said. "It's really unheard of."
Crucial steppe players
Saigas play a critical role in
the ecosystem of the arid grassland steppe, where the cold winters prevent
fallen plant material from decomposing; the grazing of the dog-size,
Gonzo-nosed antelopes helps to break down that organic matter, recycling
nutrients in the ecosystem and preventing wildfires fueled by too much leaf
litter on the ground. The animals also provide tasty meals for the predators of
the steppe, Zuther said. [Images: Ancient Beasts of the Arctic]
"Where you find saiga, we
recognize also that the other species are much more abundant," Zuther told
Live Science.
Saigas, which are listed as
critically endangered by the International Union for the Conservation of
Nature, live in a few herds in Kazakhstan, one small herd in Russia and a herd
in Mongolia. The herds congregate with other herds during the cold winters, as
well as when they migrate to other parts of Kazakhstan, during the fall and
spring. The herds split up to calve their young during the late spring and early
summer. The die-off started during the calving period.
Die-offs of saigas, including one
that felled 12,000 of the stately creatures last year, have occurred frequently
in recent years. But the large expanse of the country affected by last year's
die-off meant veterinarians couldn't get to the animals until long after their
deaths. The delay hindered any determination of a cause of death, and
researchers eventually speculated that an abundance of greenery caused
digestion problems, which led to bacterial overgrowth in the animals' guts.
Detailed analysis
This time, field workers were
already on the ground, so they were able to take detailed samples of the
saigas' environment — the rocks the animals walked on and the soil they crossed
— as well as the water the animals drank and the vegetation they ate in the
months and weeks leading up to the die-off. The scientists also took samples of
the ticks and other insects that feed on saiga, hoping to find some triggering
cause.
The researchers additionally conducted
high-quality necropsies of the animals, and even observed the behavior of some
of the animals as they died. The females, which cluster together to calve their
young, were hit the hardest. They died first, followed by their calves, which
were still too young to eat any vegetation. That sequence suggested that
whatever was killing off the animals was being transmitted through the mothers'
milk, Zuther said.
Tissue samples revealed that
toxins, produced by Pasteurella and possiblyClostridia bacteria, caused
extensive bleeding in most of the animals' organs. ButPasteurella is found
normally in the bodies of ruminants like the saigas, and it usually doesn't
cause harm unless the animals have weakened immune systems.
Genetic analysis so far has only
deepened the mystery, as the bacteria found were the garden-variety,
disease-causing type.
"There is nothing so special
about it. The question is why it developed so rapidly and spread to all the
animals," Zuther said.
Mystery endures
A similar mass die-off of 400,000
saigas occurred in 1988, and veterinarians reported similar symptoms. But
because that die-off occurred during Soviet times, researchers simply listed
Pasteurellosis, the disease caused byPasteurella, as the cause and performed no
other investigation, Zuther added.
So far, the only possible
environmental cause was that there was a cold, hard winter followed by a wet
spring, with lots of lush vegetation and standing water on the ground that
could enable bacteria to spread more easily, Zuther said. That by itself
doesn't seem so unusual, though, he said.
Another possibility is that such
flash crashes are inevitable responses to some natural variations in the
environment, he said. Zuther said he and his colleagues plan to continue their
search for a cause of the die-off.
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