Of the 100 species on this year's list, 49 are new additions since the last EDGE update, in 2007—and some may already be extinct. In addition to the Eastern long-beaked echidna, the Western long-beaked and Attenborough's long-beaked echidna topped EDGE's list this year. All three—and most of the other EDGE species—are deemed critically endangered by the International Union for Conservation of Nature (IUCN).
The EDGE project calculates a score for a species' uniqueness by looking at a so-called supertree, a "huge family tree showing the evolutionary relationships between all mammals," said Carly Waterman, program manager for EDGE. That number—combined with the animal's scarcity according to the IUCN—determines a species' rank on the EDGE list. ZSL created a separate EDGE list for weird and rare amphibians.
Though a 2010 expedition to Indonesia's Papua Province did not reveal any live echidnas, the team did find telltale holes that the creatures poke in the earth while searching for worms.
While experts suspect that hunting is the biggest danger to echidnas, "we don't really know what the relative impacts of the threats are," Waterman added.
Photograph by Tim Laman, National Geographic
4. Greater Short-Tailed Bat
The greater short-tailed bat (shown in an illustration) is likely totally gone from its native islands off New Zealand. The ground-dwelling bat is defenseless against voracious, introduced rats and has not been seen since 1967. (See "Extinct Walking Bat Found.")
Hopes were raised a few years ago when a scientist recorded a call that could belong to the greater short-tailed bat.
But a local Maori tribe owns the area where the call was heard, making it difficult for scientists to obtain permission to search for the bat, biologist Stuart Parsons said. So the greater short-tailed bat's future is still in the dark—at least for now.
Illustration by Peter Schouten
China's baiji, or Yangtze River dolphin, may already be extinct—just 13 were found during a population survey between 1997 and 1999.
Baiji are often caught in nets, struck by boats, or killed by pollution in the industry-choked Yangtze. Authorities have placed restrictions on fishing and dumping in the river, according to ZSL, but those restrictions are widely ignored.
ZSL's Waterman said that a colleague's extensive 2006 survey turned up no dolphins, which "left him absolutely convinced that there are no animals left." However, at least one baiji sighting was confirmed in 2007. (Related: "'Extinct' River Dolphin Spotted in China.")
Overall, the IUCN "tends to err on the side of caution," Waterman added, so the baiji remains listed as critically endangered.
Photograph from Oxford Scientific/Photolibrary
6. Mountain Pygmy Possum
Ski resorts in southeastern Australia have caused the decline of the mountain pygmy possum, according to EDGE experts.
That's because grooming snow on ski slopes can disturb the snow-hibernating creatures or destroy their habitat. Thought to be extinct until one was found inside a ski chalet in 1966, the possum's range is now confined to just a few square kilometers.
This may look like a rodent of unusual size, but the rare Hispaniolan solenodon (pictured in 2009) isn't a rodent at all.
More closely related to shrews and moles, the solenodons are the only mammals that inject prey with venom, through special grooves in their teeth. There are only two species: the Hispaniolan solenodon—native to the island of Hispaniola, shared by Haiti and the Dominican Republic—and the Cuban solenodon, which was rediscovered in 2003.
Until the introduction of predators such as dogs, cats, and mongooses to their island habitats, the "slow and clumsy" solenodons had no natural enemies, according to EDGE.
Photograph by Gregory Guida, Press Association/AP
9. Riverine Rabbit
The riverine rabbit (pictured in a file photo) is unlike most of its kin in one crucial way: It doesn't breed like a rabbit.
While many rabbit species can raise up to 12 babies per litter and bear multiple litters in a season, the riverine rabbit usually raises one kit at a time—and it lives for just three or four years in the wild.
Commercial farming has caused trouble for the species for many years, she added. "Either farmers plow the area—because the only arable land in the Karoo is along the river—or there is overgrazing."
Photograph by Tony Camacho, Images of Africa
10-11: Sumatran and Black Rhinoceros
The fast decline of Southeast Asia's Sumatran rhinoceros (pictured, a mother and her calf at the Cincinnati Zoo) and the black rhinoceroses of central and southern Africa have landed the species near the top of the EGDE list.
All rhinoceros species are considered to be threatened or vulnerable by IUCN due mostly to poaching, which has doubled in the past year, according to Rick Schwartz, an information "ambassador" and keeper for the San Diego Zoo.
Compounding the issue, rhinoceroses prefer different breeding environments than many mammals.
"You can't just put a boy and a girl together and make more," Schwartz said. "The girls need girlfriends. They need a cohesive female herd, and then your top female in the herd will breed and raise her young, surrounded by her friends."
For instance, unlike koalas or kangaroos, the burrowing marsupials carry their young in pouches that face backward, which keeps dirt out. (See a picture of a common wombat.)
Predators introduced to Australia, such as the dingo, have nearly wiped out Earth's biggest wombat species. Today, only one colony of fewer than a hundred individuals remains—protected by a dingo-proof fence.
Photograph by Jason Edwards, National Geographic
13. Bactrian Camel
The Bactrian camel (pictured, a herd in the Gobi) is distinguished from the more familiar Arabian (dromedary) camel by its two humps. The species has lost out in a competition for food and water with livestock, including its own domesticated brethren. Likewise, interbreeding with the domesticated species may wipe out the wild camel for good, experts say.
"We have a lot of camels on the planet, but if you look at the wild ones, they're critically endangered," said the San Diego Zoo's Schwartz. "It's hard for people to understand, because they say, I see camels everywhere."
The wild Bactrian also has unique adaptations to its extreme climate: Its coat goes from incredibly thick in winter to almost hairless in summer, and the animal has squishy foot pads—like "bags filled with slime"—that act like snowshoes, to keep it from sinking into the desert sand, Schwartz said.
Photograph by David Edwards, National Geographic
14. Javan Rhinoceros
Considered "more primitive" than Africa's white or black rhinoceroses, the Javan rhino (shown in a file photo) is the rarest of all living rhinoceros species, according to the San Diego Zoo's Schwartz.
Though the approximately 50 remaining Javan rhinoceroses live inside protected national parks in Vietnam and Java, Indonesia, the risk of poaching for their horns remains."What's really unfortunate is this horn is keratin, the same as our fingernails and hair"—and thus not very valuable, Schwartz said.
The two Javan rhino colonies are also at risk of disease. Earlier this year, the International Rhino Foundation discovered three dead Javan rhinoceroses with their horns intact, suggesting that the animals had died of different causes. In the past, domestic livestock have spread diseases to the rhinos, according to the foundation.
Photograph by Mary Plage, Photolibrary/Getty Images