Saturday, March 11, 2006

It’s not a rhinogradentian: it’s the most fantastic jerboa, Euchoreutes

It’s funny how things work out. Today I am obsessed with rodents. Why? Most of my day was spent clearing out an old loft, and while rummaging through decades of accumulated rubbish I came across multiple copies of old Brooke Bond picture card albums, and among them one of my favourites: Tunnicliffe’s Asian Wild Life. Brooke Bond pictures cards were given away free inside boxes of tea (the tea-producing branch of the company later became known as PG Tips) and, for a small fee, collectors could send off for an album. Hugely influential to young people that grew up in tea-drinking households during the 1960s and 70s, many of the series were devoted to natural history, and they are fondly remembered by many people who work today in the biological sciences. They explain my fascination with the artwork of Peter Scott, Charles Tunnicliffe and Maurice Wilson.

While, mostly, I looked after the albums that I inherited from my mother – who collected the cards herself as a girl – there were a few that I unfortunately defaced and mutilated, Asian Wild Life among them. So today I’m happy to have back in my hands not one, but two, pristine, completed albums. Like several of the Brooke Bond picture cards series, Asian Wild Life was both written and illustrated by the fantastic Charles F. Tunnicliffe (1901-1979). And there is always one picture in particular that fascinated me, and today still does: it’s Tunnicliffe’s painting (reproduced above) of two Yarkand jerboas Euchoreutes naso, bounding together across the steppes of north-west China.

Euchoreutes has to be one of the oddest-looking rodents and, years later, when I learnt about the rhinogradentians (on which I will post soon), I wondered if Euchoreutes wasn’t really a jerboa at all, but in fact a wayward rhinogradentian, perhaps related to the Earwing Otopteryx volitans. Even the binomial – Euchoreutes naso – is suggestive of some link with rhinogradentians given that the latter group includes the nasobemes (genus Nasobema). Like earwings, Euchoreutes has ridiculously enormous ears, and its alternative name is the Long-eared jerboa. If anything, Tunnicliffe’s painting actually doesn’t make the ears appear large enough: in photos, the ears look to be about as long as the entire body.

And in body length, Euchoreutes is just 70-90 mm long. That’s small, but not as small as the Baluchistan pygmy jerboa Salpingotus michaelis: with a body length of 36-47 mm it’s the smallest living rodent (it’s also relatively new to science, having only been discovered in 1966 and named in 1973). At the other end of the scale, some species of Allactaga (the four- and five-toed jerboas) exceed 260 mm in body length, and can then have a tail over 300 mm long. Getting back to Euchoreutes, it’s odd not just for its large ears, but also for its unusually long snout. It does however resemble most other jerboas in having proportionally small forelimbs and tremendously elongate hindlimbs.

Like all jerboas (well.. nearly all: read on), Euchoreutes has fused metatarsals. Is metatarsal fusion a synapomorphy for the group? There’s a problem with that: the Five-toed dwarf jerboa Cardiocranius paradoxus lacks metatarsal fusion. Is this because it’s the most basal jerboa, because it exhibits a character reversal, or because it’s not a jerboa at all? While few phylogenetic studies incorporate it (it is a very obscure and little-studied species), it is usually implied in classifications that it’s down at the base of the jerboa clade (properly called Dipodidae).

Though pedal digits I and V are reduced in Euchoreutes, they are still present. This contrasts with the dipodine jerboas Paradipus, Dipus, Stylodipus, Eremodipus and Jaculus, all of which lack lateral digits and are tridactyl. Their elongate, fused metatarsi thus bears three distinct distal condyles and look, at least superficially, remarkably like the tarsometatarsi of birds. This similarity has not been lost on ornithologists (Rich 1973) and is a remarkable case of convergent evolution. If the proximal end of the metatarsus were broken off (and this bit is the giveaway, as it of course shows the presence of tarsals charactestically mammalian in form and number), I suspect that even some experienced zoologists would be fooled into misidentifying a jerboa metatarsus as an avian one. Sadly I don’t have many jerboa leg skeletons lying around so cannot test this idea. Incidentally, most of the cervical vertebrae in jerboas are fused together as well, and in some dwarf jerboas the first three dorsal vertebrae are also fused together, and to the fused cervicals. I don’t know why this is, but it might be to prevent dislocation or jarring during the violent acceleration and deceleration incurred during leaping and bounding.

And on the subject of leaping and bounding, jerboa feet are clearly specialised for saltation (jumping). With body lengths of mostly around 100 mm, jerboas can cover about 3 m in a single leap. This is a neat and useful trick if you want to cross large distances on hot sand, but of course jerboas are mostly nocturnal, and the predominant function of saltation in jerboas is to move quickly away from predators. One species – the Rough-legged jerboa Dipus sagitta – exhibits particularly interesting predator-avoidance behaviour: it not only leaps from predators, but, as it leaps, grabs at over-hanging foliage with its teeth and forelimbs, and then clambers into the vegetation to hide (Hanney 1975).

Specialised as they are for impoverished steppes, sub-deserts and deserts, jerboas have apparently benefited from desertification in some regions (Duplaix & Simon 1977). This probably only applies to tolerant generalists among the group, however, and certainly doesn’t work for Euchoreutes. It reportedly declined by about 50% during the 1990s (Nowak 1999) and is regarded as endangered.

Phylogenetic studies demonstrate that Euchoreutes really is a jerboa, and not a rhinogradentian, and it’s traditionally been allocated its own ‘subfamily’ called Euchoreutinae Lyon, 1901 within the jerboa family Dipodidae Fischer de Waldheim, 1817. Whether Euchoreutes is actually a member of either of the two dipodid clades that have been recognised in phylogenetic studies of this group - Dipodinae and Allactaginae (Shenbrot 1992) - remains uncertain. One study of dipodid phylogeny based on cranial characters (Dempsey 1991) didn’t include Euchoreutes as no skulls were available for examination, which isn’t surprising given that only a handful of specimens are present in museums worldwide (Nowak 1999). Classifications have generally listed Euchoreutinae as separate from Dipodinae and Allactaginae, but only because the ‘subfamily’ rankings demand that each be treated as equivalent in rank. So exactly how does Euchoreutes fit into dipodid phylogeny? That’s a good question, and I’d be interested to know if it’s yet been answered.

Dipodidae appears to have evolved in the Miocene from ‘a taxon at the sicistine/zapodine [viz, birch mouse/jumping mouse] level of evolutionary dental development’ (Martin 1994, p. 99). Incidentally, Dipodidae is sometimes used for the clade that includes birch mice and jumping mice, as well as jerboas. However most rodent workers seem to favour the use of the family-level name Zapodidae for birch mice and jumping mice, with Dipodidae restricted to jerboas proper. Dipodids + Zapodids = Dipodoidea. The name Dipodidae obviously comes from ‘dipodes’ meaning ‘two-footed’, the term apparently used for jerboas by Herodotus (writing some time around 430 B.C.).

On the subject of dipodid phylogeny I can’t resist mentioning Krasnov & Shenbrot’s (2002) study of co-evolution between fleas and jerboas. It’s an interesting study in that they found no good correlation between flea phylogeny and jerboa phylogeny: the distribution of fleas on their jerboa hosts depends instead on ecological and geographical factors. You might argue that this is predictable, given that parasites like ticks, lice and fleas mostly switch between hosts that inhabit similar environments, rather than those that are closely related. Bed bugs Cimex, for example, are well known for parasitizing humans, but before this they were bird and bat parasites which ‘transferred their attentions to man when he began to live in caves and stayed with their new host when he moved away from the forests into other living quarters’ (Andrews 1976, p. 162). Unfortunately for my research on Euchoreutes, it wasn’t included in Krasnov & Shenbrot’s (2002) study as it generally lacks fleas entirely!

So there you have it. I should have said at the beginning why I have a thing about rodents right now. Firstly, I’m quite excited about the new paper in Science showing that the recently described Laotian rodent Laonastes isn’t actually an entirely new taxon, but in fact belongs to a group otherwise known only as fossils, the Diatomyidae. I have a blog post planned on that, and about other, similar cases from the rodent world (go here to see it). Then there are the tie-ins with what I was saying in previous posts about Amazonian fragmentation and Pleistocene refugia, as rodents are involved in these stories too. Right, I need to get back to those Crato turtles. For the latest news on Tetrapod Zoology do go here.

Tunnicliffe’s painting of Euchoreutes shown above is from…

…. where you can see the complete run of Asian Wild Life picture cards.

Refs - -

Andrews, M. L. A. 1976. The Life That Lives on Man. Faber & Faber, London.

Dempsey, M. A. 1991. Cranial foramina and relationships of dipodoid rodents. Unpubished B. A. thesis, Baruch College of The City University of New York.

Duplaix, N. & Simon, N. 1977. World Guide to Mammals. Octopus Books, London.

Hanney, P. W. 1975. Rodents: Their Lives and Habits. David & Charles, Newton Abbot.

Krasnov, B. R. & Shenbrot, G. I. 2002. Coevolutionary events in the history of association between jerboas (Rodentia: Dipodidae) and their flea parasites. Israel Journal of Zoology 48, 331-350.

Martin, R. A. 1994. A preliminary review of dental evolution and paleogeography in the zapodid rodents, with emphasis on Pliocene and Pleistocene taxa. In Tomida, Y., Li, C. K. & Setoguchi, T. (eds) Rodent and Lagomorph Families of Asian Origins and Diversification. National Science Museum Monographs 8, 99-113.

Nowak, R. M. 1999. Walker’s Mammals of the World, Sixth Edition, Volume II. The Johns Hopkins Univesity Press (Baltimore and London).

Shenbrot G.I., 1992. A cladistic approach to the analysis of phylogenetic relationships among dipodid rodents (Rodentia; Dipodoidea). Archives of the Zoological Museum, Moscow State University 29, 176‑200.

Rich, P. V. 1973. A mammalian convergence on the avian tarsometatarsus. The Auk 90, 676-677.


Blogger Dr. Vector said...

Hi Darren,

Speaking of Pleistocene refugia and rodents--and possibly anticipating what you were planning to say anyway--has the isolation of mammals on the mountains of the American Southwest played much of a role in inspiring or testing Pleistocene refugia hypotheses? This is something I know very little about, but I'm intrigued.

6:15 AM  
Blogger Darren Naish said...

Thanks to both of you for your comments. Aha, the answers to your queries will follow in due course... watch this space.

Must finish Crato turtles and Galve vertebrates manuscripts...

12:10 PM  

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