Rigor Vitae: Life Unyielding

Friday, January 05, 2007


Darren Naish has let the...er...squirrel out of the bag, so it's time for me to continue my armchair speculation about the Bornean Mystery Mammal (BMM). When the news was released at the end of 2005, some suggested the creature was a primate—in fact, I heard several authorities remark on its similarity to a lemur. The only qualifications a primate needs for lemurship, though, is a Madagascan pedigree, and the BMM falls short there. Neither of the Southeast Asian prosimians, tarsiers (Tarsius spp.) nor slow lorises (Nycticebus spp.), even vaguely approach the form of our friend. Other, even goofier, identities were suggested: a Bay Cat (Felis=Catopuma badia), and even a tree kangaroo (Dendrolagus sp.). Most of us assumed the animal was some kind of civet (family Viverridae). The commonly circulated photograph showed a crouching, chestnut mammal with a long, arching tail. What little of its face wasn't obscured by a leaf was washed out by its eyeshines. The most striking feature to me was the long hind limb, with its particularly meaty back of the knee, and plantigrade-looking posture. In the less-seen photo of the departing BMM, the hind limbs had a similar look that convinced me the animal was a civet, either one of the 7 species (not including linsangs and binturongs) known to occur on Borneo or something previously unknown on the island.

Most people I heard from agreed, in fact, a number of them seemed to think it was a Hose's Civet (Diplogale hosei), a Bornean endemic known only from 15 specimens. According to the description in Walker's Mammals of the World, D. hosei is endowed with a tail barely ½ of the snout-vent length, much shorter than the tail in the photos, and I rejected that notion outright (although an IUCN camera trap subject identified as that species sports a long tail--above).

At the time, I considered the Small-toothed Civet (Arctogalidia trivirgata) (top), and the Sulawesi Civet (Macrogalidia musschenbroekii) (lower) to be more likely candidates. Arctogalidia has the problem of a dark, grayish tail, and Macrogalidia is known only from Sulawesi, not to mention that its tail has light-colored bands (although the departing BMM photo could suggest similar markings).

Enter Erik Meijaard, an expert on Bornean mammals, and his colleagues Andrew C. Kitchener and Chris Smeenk, whose paper in Mammal Review (Vol. 36, No. 4, 318-324) proposes that the BMM was a flying squirrel, specifically Aeromys thomasi, a large and little known Bornean endemic. In their paper, the authors selected 17 known species that could possibly be mistaken for the BMM, and scored them on 14 morphological characteristics and one behavioral one, then compared the results against the BMM's score. A. thomasi came out on top, with 12 matches, compared to 4 for D. hosei. According to their table, the WWF photos were more likely to have depicted one of several squirrel species, a Maroon Leaf Monkey (Presbytis rubicunda), or a Housecat (Felis catta) than any known species of viverrid. Extra credit goes to the authors for including a pair of nice drawings by Ivan Noortwijk, that fill in the photographic shortcomings, to show how squirrelish the BMM might actually look (below).
After reading the paper, the squirrel hypothesis has a lot of appeal. Those meaty knees begin to look more like the little patagia that stretch from a flying squirrel's ankles to the base of his tail, and a piece of crumpled main patagium just might be peeking from behind the left elbow. I'm still bothered, though, by those hind legs, and a pelvis that just looks more viverrine to me than sciurine. I have never even seen a photograph of an Aeromys species (there are two), but as a boy I caught and kept two American flying squirrels (Glaucomys sabrinus), which are much smaller, and not particularly close relatives of the much more diverse Asian flying squirrels. That said, though, I've spent hours watching the American rodents negotiate on the ground, both walking and running, and the high-rumped posture in both BMM photos is extremely different.

As clever and interesting as the squirrel hypothesis is, I have a couple of minor disagreements with some of the supporting evidence the paper presents for it. The authors point out that the eyeshine of a flying squirrel is whiter than that of a civet, but photographs of eyeshines are usually quite washed out, and I regard them as poor evidence of true color. The tapeta of goatsuckers and crocodilians shine brilliant scarlet, but often appear white or yellow in photographs. The 15 characteristics that candidates were graded on did not seem particularly fair to me. Two of them, terrestrial habit and presence or absence of patagia, seem irrelevant. Much was made of the arched tail in the first photograph. The evidence that this posture is typical for the BMM is poor; a civet turning to its left could very well be captured in such a position, and the second photo shows no arching. The fact that a monkey and housecat scored far higher than a Small-toothed Civet illuminates the shortcomings of the process.

The authors mention a citation that describes another Southeast Asian flying squirrel, Iomys horsfieldi, on the ground with its tail “stretched out behind and arched sharply.” The arch of the BMM's tail is hardly sharp (note the exaggerated arch of the Noortwijk illustration and the far sharper arch of shown in the I. horsfieldi photo above).

I'm ultimately left feeling less sure of the creature's identity. Meijaard et al presented a good but inconclusive case, and though I'm unconvinced, I'm sure not going to say the creature isn't a squirrel. Rather than conclusively identifying the BMM, the authors confirmed the unreliability of photography, illustration, of our very eyes. The best lesson they provide is outlined in their conclusion: “We...recommend that wildlife photographers become more circumspect in announcing 'new' species, especially with media that are only too willing to widely publicize such news. The WWF has taken the right steps towards formal description of the 'new' mammal by attempting, so far unsuccessfully, to collect a specimen. This case highlights the importance of formal description based on type specimens and a review process.”
upper: Aeromys thomasi illustration by Karen Phillips (Field Guide to the Mammals of Borneo; WWF Malaysia)
second: Diplogale hosei photo from the IUCN
third: Arctogalidia trivirgata photo by Lim Boo Liat; Macrogalidia musschenbroeki photo by Christen Wemmer (Walker's Mammals of the World; Johns Hopkins)
fourth: BBM photos from the WWF; Aeromys thomasi illustrations by Ivan Noortwijk (Mammal Review)
lower: Iomys horsfieldi photo by El Rey Ardilla


Blogger Darren Naish said...

Good post, and I appreciate your scepticism. I agree that eyeshine is a poor feature to cite, though note that the authors did not rely on this as an important feature. However, some photos of Petaurista show eyeshine identical to that of the Bornean animal (e.g., see Kitchener's BBC Wildlife article of March 2006).

On tail shape and posture, a few photos of Petaurista also show that the Bornean animal more resembles giant flying squirrels that you might think: the photo accompanying the Kitchener article shows the tail hanging straight down from an animal perched on a branch, quite different from the S-shape that's more typical for squirrels. However, your point about the problems with this observation is a good one.

The high-rumped posture evident in the head-on photo results, I think, from the fact that the animal is bending down to sniff the ground. But this can't apply to the walking-away photo so... is Aeromys really this different from other squirrels? We just don't know.

3:34 PM  
Blogger Neil said...

In my experience photographic eyeshine can vary widely in a single individual, depending on conditions. I agree that it a rather spurious trait to use for species level ID. Likewise, drawing any broad conclusions about general body or tail posture from two photos, one of which certainly captures the creature in motion, seems difficult.

Still, I suppose I'm inclined to buy the flying squirrel hypothesis just on the basis of the strong visual hints of a patagium. I'm still rather partial to the idea that it might be an extant volaticothere.

12:54 AM  
Blogger Darren Naish said...

Of course - an extant volaticothere! Having blogged about both volaticotheres and the Bornean animal recently, it never occurred to me to connect the two.. :)

4:23 AM  
Anonymous Anonymous said...

I do not want to toot my own horn, but if I don't who will. When if first looked at the picture I said, "It looks like a squirrel." Of course, I have a limited number of species to draw from. Squirrel or cat, that about sums it up...

2:18 PM  
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11:11 PM  

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