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Animal Antics: Inbreeding koalas compromised their genetic diversity

By MELANIE HSU | October 25, 2012

Sleeping, eating and having sex all day — who wouldn’t want to have that kind of life? Australian koalas (Phascolarctos cinereus), widely known as the biggest sleepers in the animal kingdom, are doing just that. In order to digest the high-tannin eucalyptus foliage that comprise the majority of their diets, these cuddly marsupials spend most of their day (a stunning 16 to 18 hours) motionless.

But don’t underestimate koalas because of their low metabolism and extreme need for sleep. Young koalas (also called joeys) spend the six months of their lives chilling in their mothers’ pouches. After they emerge, they spend another six months riding on their mothers’ backs. Considering that these joeys can reach up to one-fourth of their mother’s weight, it is quite amazing that these koalas can still swing from tree to tree with such grace!

Sadly, life as an koala can be very tough; due to the scarcity of options in the dating arena, many individuals have resorted to mating with their own kin. Inbreeding is a common occurrence in animals with declining populations, and the koala is no exception.

Thanks to popular demand for their pelts, koalas were nearly hunted to extinction in the 20th century. The brutality of these killings, which often involved the use of guns, poisons and nooses, instigated a public outcry and one of the first wide-scale environmental rallies in Australia.

Unfortunately, the growing environmental activism did not do much to protect these poor koalas: in 1926-1928, a severe drought led people to send another 600,000 of them to the grave. Although the Australian government has since placed bans on hunting and owning these critters, koala populations have yet to recover from these horrific events.

While researchers expected to find low genetic diversity in koalas, they also found that extensive inbreeding in these marsupials dated back a lot earlier than expected. Through comparisons of modern specimens and 14 museum samples at the Leibniz Institute for Zoo and Wildlife Research in Germany, Alex Greenwood and his colleagues found evidence that the koalas may have lost much of their genetic diversity as far back as 50,000 years ago, during the late Pleistocene Epoch. This epoch marked the extinction of the giant koala (Phascolarctos stirtoni), which were about a third larger than their modern-day relatives.

To assess the extent of inbreeding in koalas, Greenwood and colleagues compared DNA in mitochondria, an organelle that generates chemical energy for cell in the form of adenosine triphosphate (ATP). Despite the fact that the museum samples originated from different regions and different points in time, their genetic profiles did not differ from that of today’s specimens. This finding was used to support the hypothesis that genetic diversity in koalas was lost a long time ago.

Contrast the koala situation to that of the gray wolf (Canis lupus). Due to concerns about loss of livestock, this top predator was hunted to extermination in northeastern United States in the late 1800s, and from Montana, Idaho and Wyoming by the 1930s.

Fortunately, because of their ecological importance as a keystone species, these wolves, along with other large carnivores such as grizzly bears and cougars, eventually became the focus of conservation efforts. Because their populations declined so recently, modern gray wolves have noticeably less genetic diversity than museum samples from an earlier era.

Initially, koalas were thought to have lost most of their genetic diversity at the hands of humans. When Europeans set foot in Australia in the late 1700s, they noticed that koalas were a rare occurrence, most likely because of overhunting by Aborigines.

By the mid-1800s, the species started to recover due to a decline in hunting, but the fashion fever that soon followed pushed the koalas to the brink of extinction. In addition to extensive inbreeding, which increases the chances of offspring inheriting harmful recessive alleles, today’s koalas are also threatened by habitat loss and disease, particularly Chlamydia.

Koalas are listed by the Australian government as “vulnerable,” and by the United States government as “threatened.” Worryingly, the International Union for Conservation of Nature (IUCN) lists the koala as “least concern.”

The Australian Koala Foundation, an international not-for-profit organisation that aims to diminish the threats to koala survival and raise global awareness to help save endangered flora and fauna, disagrees with this listing, stating that the koala’s status as “common” in a particular area does not necessarily mean that they are indeed plentiful in that area.

Beyond these classification disputes, the problems facing koalas are undeniably real. The low genetic diversity of koalas indicates that they may have trouble adapting to changing environmental conditions or new diseases.

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