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2021 Research Roundup

Welcome to a new blog post series from Squirrel-Net’s digital media specialists! We’re calling it the “research roundup” - a collection of the semester’s best new science stories about, what else? Squirrels! Read on to learn about some of the most interesting, wild, wacky, and/or important squirrel studies from the last year!


The elusive giants of the Himalayas (no, not the Yeti!)


An ancient giant, rediscovered:

The 1994 rediscovery and description of the Himalayan Woolly Squirrel has shaken up our preconceptions of squirrels. This genus (Eupetaurus) contains the largest flying/gliding squirrels, topping the scales at 5 lbs and measuring a whopping 1m from tip to tail! This animal was first described in 1888 when a zoologist obtained 2 skins and a skull from north Pakistan. But, after this discovery, it was assumed to have gone extinct. Even after its rediscovery, little research has been done on these squirrels.



"Eupetaurus cinereus" by Richard Lydekker, 1888 {{PD-US}}


First things first—how many species are there?

Rediscovery of the squirrel has introduced many questions. How did they evade detection for so long? How do they survive such cold climates at high-elevations? What do they eat? How social are they? But, Dr. Stephen M. Jackson and his research team thought one mystery was the most important to tackle first: how many species of Himalayan Woolly Squirrels are there? The Himalayas are one of the longest mountain ranges in the world, spanning a variety of habitats and the world’s highest elevational gradient, so the researchers thought that any animal that lives across the entire mountain range is likely multiple species of the same genus.


What they did and why:

How do you research an animal that has previously evaded detection for over 70 years? Well, if you didn’t think museum specimens and records were important before, think again.


The researchers compiled data from all 24 skin and skull specimens in museums across the world, as well as published records of the squirrel. They measured every little detail of the skulls and jaws of these specimens, and sequenced DNA from preserved muscle tissue. Comparing these data to the close relatives of this elusive giant allowed the researchers to determine how many distinct species were represented and approximately how long it has been since the species diverged from one another.


What they found:

Two new species! The animal formerly simply known as the western woolly flying squirrel (Eupetaurus cinereus) is actually three species. In addition to E. cinereus, the researchers also described the Tibetan woolly flying squirrel (Eupetaurus tibetensis) and the Yunnan woolly flying squirrel (Eupetaurus nivamons). E. cinereus is endemic to northern Pakistan, E. tibetensis is endemic to Tibet, and E. nivamons is endemic to Yunnan, China.


The researchers think that this genus likely diverged from other flying squirrel species in the early to mid-Miocene (~15-20 million years ago), supporting the hypothesis that the genus diverged at about the same time as the Himalayan mountains reached their approximate current positions. Since that time, populations have migrated away from one another and become their own species.


Taxonomy, or describing the evolutionary relationships between species, is one of the first biological sub-disciplines. Presently, it remains an important field of research, as new species are constantly being discovered in our quest to document all of the weird and wonderful organisms that call this planet home!


Citation: Jackson SM, et al. (2021). Across the great divide: revision of the genus Eupetaurus (sciuridae: Pteromyini), the woolly flying squirrels of the Himalayan region, with the description of two new species. Zoological Journal of the Linnean Society. 20: 1-25, DOI: https://doi.org/10.1093/zoolinnean/zlab018


Another dark side of exotic pet trade


The problem at hand:

The exotic pet trade is often criticized for its inhumanity and its potential to transmit diseases. In fact, this industry is one of the leading causes of new and potentially deadly diseases spilling over from animals to humans, which is all too familiar these past years.


One such recently described zoonotic virus, Variegated Squirrel Bornavirus-1 (VSBV-1), has been retroactively traced to the deaths of many humans over the last decade. Sciurus variegatoides, the variegated squirrel, along with other squirrel species that can host the virus, are bred and trafficked through exotic animal trade networks and are also found in many zoos. Bornaviruses are a group of viruses that infect the nervous system of their hosts - typically horses, sheep, cattle, rodents, birds, snakes, and humans. Unfortunately, in the case of VSBV-1, infected squirrels are asymptomatic and therefore serve as ideal reservoirs and carriers for the virus, whereas infected humans develop fatal encephalitis.


Dr. Daniel Cadar, an expert in ecological virology, and his team of specialists conducted a study to determine the origins of VSBV-1 and its past transmission network. The researchers discovered that the virus has only a few variants, and that it did not, in-fact, originate from the variegated squirrel (for which it was named).


What they did and why:

The researchers began by sequencing the genome of viral strains obtained from several species of squirrels. Molecular evidence helped them determine the virulence of the virus and build an evolutionary tree of viral strains and respective hosts.


The researchers also studied the network of transmission through conversations with exotic animal traders and breeders, as well as records from zoos. As one might expect, records of exotic (and sometimes illegal) trade are incomplete, so any animal caretaker who may have been in contact with exotic squirrels was also tested for VSBV-1 antibodies.


What they found:

The virus was surprisingly homogenic, meaning that it had very few mutations and variants. This is good news, because it means that fewer vaccines and/or preventative measures need to be developed to control the virus in the case of a spillover.


The researchers also compiled a comprehensive transmission network between species in different facilities. No animal caretakers tested positive for antibodies or encephalitic symptoms, suggesting that zoonotic transmission of the virus is fairly rare.


Interestingly, the study also showed that Prevost’s squirrel (Callosciurus prevostii) of Southeast Asia was the original host of the virus, not the variegated squirrel of Central America, whose good name has now been exonerated. This information will be helpful in tracing the virus in the wild, and helping prevent the future spread of the virus to humans or other spillover hosts.


Who cares about squirrel diseases?

This kind of study is critical in helping educate policy-makers and citizens about the need for surveillance and regulation of exotic animal trading and breeding facilities, given that the exotic pet trade is unlikely to simply go away. The authors of this study specifically emphasize the need for an official pet trading database, which can be used in the detection and prevention of future outbreaks of animal-borne diseases. Tracking this information may help us stay one step ahead of the next pandemic outbreak!


Citation: Cadar D, et al. (2021). Introduction and spread of variegated squirrel bornavirus 1 (VSBV-1) between exotic squirrels and spill-over infections to humans in Germany. Emerging Microbes & Infections, 10:1, 602-611, DOI: https://doi.org/10.1080/22221751.2021.1902752


American Ninja Squirr-ior


“Go with the flow,” says squirrel:

Many wild animals have the impressive ability to weave their way through tall tree branches and leap from tree to tree at unimaginable heights. But a new study suggests that nine-lived cats and nimble primates have nothing on the acrobatics of squirrels! Researcher Dr. Nathaniel Hunt and his colleagues were interested in exactly how the squirrels are capable of accomplishing such feats of their feet! Through a series of trials, they found that squirrel agility on precarious branches is not planned or deliberate, but depends instead on the squirrel's biomechanical adaptations and past experiences.


Squirrel agility studies like this one have uncovered the secrets of how squirrels always land on their feet: gyrating their tail and torso mid-air to orient themselves. These impressive physical abilities are part of a field called biomechanics, and humans are interested in studying them for use in robotic development. (For a slow motion clip of a ninja squirrel and a description of the physics behind their abilities, watch this video by famed YouTuber and Squirrel Enthusiast, Mark Rober).


What they did and why:

The researchers used a series of different launching and landing platforms, each with a different amount of flexibility. Combined with different gap distances, the researchers were able to determine how squirrels assess these factors to adjust how they jump. The researchers also used fast-action cameras to capture aerial positions of the squirrels. This helped them determine the biomechanics of the squirrels mid-air.


Squirrel parkour:

It turns out that the squirrels make many decisions about how and when to jump, depending on the position and flexibility of the launching surface and gap distance. However, it seems that their preparation ends there! After launching, the squirrel is at the mercy of the landing surface, but thanks to their high adaptability, literal “on-their-toes” thinking, fast decision making, and pliable bodies, the squirrels can quickly adjust to any type of landing surface based on their experiences in the first split seconds of landing.


No wonder the researchers compare the squirrels’ movements to parkour! Adaptability is key, and squirrels are experts at it!


Citation: Hunt NH, Jinn J, Jacobs LF, Full RJ (2021). Acrobatic squirrels learn to leap and land on tree branches without falling. Science. 373(6555): 697-700. DOI: https://doi.org/10.1126/science.abe5753