This week – October 2nd to 8th, 2017 – is Rodent Week, an unofficial online event kick-started by UK palaeontologist, SciComm wonder-woman and small mammal fan Elsa Panciroli (hat-tip also to Order Rodentia). I’ve been doing my bit to tweet some rodent love, and today I’m going to blog on rodents too. I’ll do more this week… if I can.

The jerboa Jaculus showing its different locomotor strategies: (A) quadrupedal walking, (B) bipedal walking, (C) fast bipedal leaping. These drawings are from Hanney (1975), probably my favourite book on rodents and one that you should definitely track down and obtain if you can. Credit: Hanney 1975

Here I’m going to talk briefly about one of my favourite rodent groups: the jerboas. Part of the text here is recycled from material that appeared at Tet Zoo vers 1 and 3 in 2006 and 2009, but even 2009 is such a long time ago now that I’m sure this won’t be anything you remember. Right? Anyway… jerboas – properly, Dipodidae – are Northern Hemisphere members of the mouse lineage (Myodonta), famous for their prodigious leaping ability and also for their remarkably odd proportions. There are about 50 species (assuming the inclusion of jumping mice and birch mice, an issue I’m avoiding here). Indisputable jerboas (that is, excluding jumping mice and birch mice) are fully bipedal but still diverse enough to be grouped into three, four or five subfamilies differing in skull form, toe count, anti-predator behaviour (freezing versus leaping) and much else besides (Lebedev et al. 2012). The name Dipodidae comes from ‘dipodes’ meaning ‘two-footed’, the term apparently used for jerboas by Herodotus (writing sometime around 430 BCE). Given that many animals share the characteristic of having two feet, I presume this was a reference to bipedality. In comparison, this is not so common among mammals.

Portrait of Lesser Egyptian jerboa Jaculus jaculus, again from Hanney (1975). Jaculus is not an especially big jerboa but some of its cousins - like the Great jerboa Allactaga major of central and western Asia and eastern Europe - can reach 45 cm in total length. Credit: Hanney 1975

Jerboas are specialised for saltation, or leaping. Despite a body length of mostly around 10 cm, 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).

The superficially bird-like tarsometarsus -- though with a characteristically mammalian tarsus -- of the tridactyl jerboa Jaculus. Another illustration from Hanney (1975). Credit: Hanney 1975

The remarkably long hindlimbs of jerboas in part owe their length to radically long, weird feet, in particular to the metatarsus (the part formed of the normally separate metatarsals). The metatarsus in most jerboas is a single element where the individual metatarsals have fused together, forming a long ‘cannon bone’ [UPDATE: not ‘canon’!] with three distal condyles. Side toes are absent in these particular jerboas (species of Paradipus, Dipus, Stylodipus, Eremodipus and Jaculus), so they have tridactyl feet. As you can see from the diagram here, tridactyl jerboa feet look remarkably like those 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. Not all jerboas are like this, by the way: in the bizarre Long-eared jerboa Euchoreutes naso (sole member of the group Euchoreutinae), short first and fifth digits are still present.

Portrait of Long-eared jerboa Euchoreutes naso by Joseph Smit. As with all older illustrations of this species, this illustration seemingly doesn’t make the ears large enough! Credit: Josef Smit Wikimedia

Incidentally, most of the cervical vertebrae in jerboas are fused together as well, and in some dwarf jerboas (members of a group – Cardiocraniinae – that seems to be the sister-group to the remaining jerboas; Lebedev et al. 2012) 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.

Experts have disagreed over exactly where jerboas fit in phylogeny. There’s enough to say about that issue and its implications that I’m going to leave it for now. I promise I’ll come back to it. It involves the jumping mice and birch mice, some of which are not that similar to classic jerboas and are associated with forests and grasslands more than deserts.

I may need to do some recalculating, but the figure is at least roughly correct. Buy merchandise here. Credit: Darren Naish

More rodents soon! For previous Tet Zoo coverage of rodents, see...

Refs - -

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

Lebedev, V. S., Bannikova, A. A., Pagès, M., Pisano, J., Michaux, J. R. & Shenbrot, G. I. 2012. Molecular phylogeny and systematics of Dipodoidea: a test of morphology-based hypotheses. Zoologica Scripta 42, 231-249.

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