Light-headed leporids that run like hell

  

"My, what large ears you have" 

Why are jackrabbits so awesomely charismatic?

What?  You think I'm pulling your leg?

Then you haven't seen a jackrabbit skipping across the prairie like a tumbleweed in a storm.

I'm telling you Jack is a superb runner, and a running Jack is a sublime sight.

He can't bluff his predators, can't blast them with vile chemicals and can't impale them with quills.

Jack's survival depends on speed. It's his main defense against ambushers and fleet-footed coursers.

His tribe -- the leporids -- are natural runners, and those species that have been clocked are about 4 times faster than rodents of the same size.

But among the leporids jackrabbits are the ultimate sprinters.

A black-tailed jackrabbit can crank it up to about 70 km/hr  (or 44 mph), while a coyote, several magnitudes larger, does about 65 km/h (or 40 mph).

Watching a jackrabbit in overdrive is a thrill, and scaring one up from its hideout is also a thrill, though you may have to change your pants when your blood pressure returns to normal.

They can afford to bolt at the last minute because they're faster than you.

Plus, an explosive exit may momentarily startle a predator and give Jack a head start.

If there's a high speed chase rabbits are capable of stunning gymnastic maneuvers.

I once saw some amazing footage of leporid acrobatics in an archival film from the days of the British Raj. An Indian  hare (Lepus nigricollis) dodged and turned, and leapt gamely over a slobbering and snapping German shepherd.

There was no contest. The hare's locomotory skills so excelled those of that particular dog that it would surely have escaped if it hadn't been penned.

If you view running rabbits in slow motion you gain a heightened appreciation of things you just don't see in real time -- namely, the pounding interaction of gravity and acceleration on flesh and joints.

Functional morphologist Dennis Bramble analyzed film of running black-tailed jackrabbits and found that the G-loads or decelerating forces acting on the forelegs of a 5-pound Jack averaged 3.5 G during a fast gallop.

The loads were as great as 12 G when they touched down after high observational leaps.

The jackrabbit's jarring run led Bramble to examine the structural adaptations for dealing with the head-rattling effects of high speed locomotion.

To that end he examined jackrabbit skulls and cadavers and concluded that the curious areas of latticed skull bone (fenestra) are more than a diagnostic curiosity of leporids.

Fenestra confer strength without the weight of solid bone, and a light weight head is more easily stabilized when the body is moving at high speed.

High speed film of running Jacks shows that the head floats quite steadily ("undergoes minimal angular exursion") in relation to the bounding motion of the body.

In fact, the swift jackrabbit's head weighs about 55% of the slower European rabbit's head of the same size. (And by the way, the European rabbit, Oryctolagus cuniculus,  runs only 56 km/hr.)

The jackrabbit's skull has an even stranger feature. The braincase is hinged near the base of the skull with something that approximates a woodworker's box-joint. This is the intracranial joint  or ICJ.

Bramble manipulated the heads of fresh cadavers and found that the ICJ is moveable over a small range not exceeding 1 mm.

A galloping jackrabbit subjects the ICJ to an alternating rhythm of impact loading, and the jointed skull seems to serve as a shock absorber.

It works like this.

When the jackrabbit lands on its forelegs a decelerating force opens the hinge and the anterior part of the skull moves forward. The ears swing slightly forward too.

When the jackrabbit pushes off with its hind legs the accelerating thrust closes the hinge and the ears swing backward a bit.

A few words on Jack's ears: their mass is almost one third that of the head, and they bridge the ICJ via large frontalis muscles -- the only muscles to span the skull hinge directly.

If you are wondering why a galloping jackrabbit would increase aerodynamic drag by holding those big ears erect, Bramble hypothesized that they play a role in restoring the cranium to its normal preloaded condition when the cranial hinge is closed.

If you are still doubtful you are probably wondering if the cooling function of the ears explains their upright position while running.

Well it doesn't, because the ears' radiator function normally kicks in when the rabbit stops running.

So the question remains: is the ICJ and cranial kinesis a biproduct of having a light head?  Or is something else going on?

Bramble believes the most likely target of shock absorption is the jackrabbit's large eyes.

With a visual field of +360 degrees, stereoscopic vision in front of and behind the head, and extraordinary movement detection, rabbits are supremely sighted, and not just to detect danger.

While pursued by predators they are able to navigate the rapidly changing obstacle course ahead while visually tracking their pursuers behind -- and they do this usually at night.

The venous sinuses that surround the eyes lend support to this idea. The sinus vessels lack valves and respond to pressure changes due to movement of the ICJ.

One possibility is that the sinuses maintain constant pressure on the eyes because blood shunts back and forth between them when the intracranial joint expands and contracts.

There's more to the hypothesized mechanism than that.

The ICJ may clamp off the temporal vein and divert blood to sinus hydraulics that assist shock absorption -- all of which has implications  for the evolution of leporids, skull fenestration, and high speed chases.

I believe Professor Bramble has made it quite clear that Jack has a lot going for himself.

Admire him not only for having the fastest feet in the West (at least for his size), but also for his lightweight and hinged head. 

References

Bramble, D.M. 1989. Cranial specialization and locomotor habit in the Lagomorpha. American Zoologist, 29:303-317.

Best, T.L. 1996. Lepus californicus. Mammalian Species, No. 530:1-10.

Garland, Jr., T. 1983. The relation between maximal running speed and body mass in terrestrial mammals. Journal of Zoology, 199:157-170.

http://darrennaish.blogspot.com/2006/05/most-freaky-of-all-mammals-rabbits.html

Wood, A.E.. 1957. What, if anything, is a rabbit?  Evolution 11:417-425,