One of a number of posts that I’ll be sharing based on things discovered during research into my book on cat physics, coming next year! The previous post on the Chandler wobble is another post in this series.

The ability of cats to land on their feet when they fall from a height, no matter how they fall, is almost legendary. It has been explored by scientists and engineers for a variety of reasons for at least 150 years (though I have found that scientific interest actually dates back 300 years).

Cats are not the only animals to have a so-called “righting reflex,” however. Étienne-Jules Marey, the French physiologist who produced the first high-speed photographs of a falling cat righting itself, also demonstrated that rabbits have the same ability, as seen below.

The initial interest in falling cats and rabbits focused on the physics of the problem: what sort of motions does an animal need to make in order to properly flip itself over? Later research, however, starting in the early 1900s, asked a different question: how does the animal know which way it needs to turn in order to land on its feet, i.e. how does it know which way is down?

The question is a surprisingly profound one. In freefall in a gravitational field, an object or animal is truly weightless — it experiences no forces on its body at all. This realization was, in fact, the key element that inspired Albert Einstein’s general theory of relativity: that we only feel our weight when we are resisting the free acceleration that gravity tries to impart on us. A key foundation of general relativity, in fact, is that a uniform acceleration is indistinguishable from the force of gravity. If one is in an elevator that is accelerating upwards in the absence of gravity, for instance, it would feel the same as being in a stationary elevator that is in the presence of gravity.

So, once a cat or rabbit is dropped, it experiences no gravitational field, and therefore has no physical sensation of which way is “down.” Naturally, one would expect that it figures out which way it is going by simply using its eyes; however, experiments with blindfolded cats showed that they are still able to land with their feet facing the ground. So how do they know?

In the 1960s, the British physiologist Giles Brindley (b. 1926) hypothesized that the only remaining possibility is that the animal maintains a memory of which direction is down, and uses that memory to instinctively fix the landing direction. He then instituted a series of very peculiar tests on rabbits to see whether his hypothesis was correct.

“Peculiar” could really be said to be Brindley’s scientific specialty. In addition to his cat studies, in the 1960s he also invented his own electronic instrument, the “logical bassoon,” as he is pictured playing above. He is also infamous for a presentation at a urology conference in Las Vegas in 1983 which is so not-safe-for-work that I will only provide a link to an eyewitness account here.

Brindley’s rabbit tests were also rather unusual. Described in a pair of conference proceedings¹, Brindley set out to expose a rabbit to an acceleration that would, as far as the rabbit could tell, change the direction of gravity. The rabbit would be subjected to the false direction of gravity and then released, to see whether it fell true to the ground or true to the apparent direction.

The conference proceedings do not provide a lot of detail, or any photographs, of the apparatuses used for the experiments, so I have to make some deductions based on the very brief descriptions, as follows.

The first experiment involved a rabbit in a box, suspended from a pair of rails that inclined upwards at 13° 6′. The box was catapulted up and down the slope, and when it returned to the bottom, a trap door opened to release the rabbit, where its righting reflex in falling was photographed. My illustration of the system is shown below.

The catapult acceleration of the rabbit lasted 0.3 seconds, and a full trip up and down the slope would be 8 seconds. During that time, the box is tilted, and the rabbit feels gravity at a 13° angle. I assume that the platform where the rabbit begins and ends is level, as this is the point: the rabbit experiences a tilted gravitational field for an extended period of time, but then it is suddenly changed just before release — what happens to the rabbit?

To quote Brindley:

On the ninth second the floor of the box was automatically opened, and the rabbit was photographed while it was falling 150 cm (0.553 sec) to a cushion. It retained throughout the fall an inclination of about 13° to the upright. In contrast, if it was held stationary at 13° 6′ and then dropped, it turned in the air and became upright within 30 cm.

In other words: the rabbit orients itself based not on what the gravitational field is the very instant of falling, but on what it was for some seconds before that. In hindsight, this makes sense: just as an animal falls, it may experience any number of weird forces as it flails around and pushes/bounces off of its perch. Memory of the direction of gravity in the recent past is more reliable than that memory at the last instant.

But Brindley wasn’t done: he also took the rabbit in a “motor-car.” I use his description here.

The same box, without its wheels, was mounted in a car. A rabbit was put in as before. The car was driven at 32 km/hr in a straight line for 30 sec, and then suddenly turned without change of speed into a circular orbit of 50 m diameter, so that the rabbit suddenly experienced an apparent gravitational field inclined at 17° 51′ to the vertical. After 10 sec the floor of the box was opened and the rabbit photographed while it was falling 80 cm (0.404 sec) to a cushion. It retained throughout the fall an inclination of about 18° to the upright.

My illustration of this is below. During the circular path, the rabbit experienced a centrifugal “force” which made the force of gravity feel like it had tilted toward the outside of the circle. As before, the rabbit fell according to the direction of gravity it had historically experienced, not towards the vertical.

Note my question mark about the “release point”: I assume that the car returned to a straight path the moment before release, so that the cat’s memory would be tested, and not the sensation it fell right at the instant of falling.

A curious note about this latter experiment, as later recounted², this experiment was done on a disused runway at the Duxford Aerodrome, and Brindley’s wife Hilary was doing the driving while Giles did the photography.

Both the railway and motor-car experiments were recounted in Brindley’s first conference proceeding; in the second, he added an additional test: spinning rabbits by a centrifuge!

… rabbits were placed in a box with a spring trapdoor floor, mounted 105 cm from the axis of a centrifuge (the ‘merry-go-round’ of the Engineering Laboratories, Cambridge University). The centrifuge was brought to a speed giving here a gravitational-accelerational field inclined at about 30° to the vertical. After 1/2 min or more the box was moved quickly to the axis of the centrifuge, and at a time between 1/4 and 15 sec later the trapdoors were opened… Those dropped 1 sec or less after the change usually fell in a very oblique posture roughly corresponding to the field at the periphery of the centrifuge. Intermediate times gave intermediate postures.

Confirming my suspicion about the railway and motor-car, we see that the rabbits were quickly brought to the axis of the centrifuge, where they would experience no centrifugal force, before dropping.

In Brindley’s experiments, the change in the apparent direction of gravity is increasingly extreme — from 13° 6′ to 17° 51′ to 30°. In the second proceeding, Brindley gets even more ambitious: he suggests performing the experiments with a rabbit in a diving airplane to give an apparent change in the direction of gravity of 40°! These experiments were apparently never carried out, possibly due to the fact that nobody wanted to perform risky airplane dives to test rabbit reflexes.

The takeaway from all of Brindley’s experiments: rabbits, and presumably cats, keep a “memory bank” of roughly the last 6-8 seconds of the direction of gravity, and fall based on what that memory bank tells them. To put it another way, a dramatic change in the direction of gravity usually takes about 6-8 second for the animal to completely acclimate to.

It’s hard to say how the rabbits felt about all this experimentation. I imagine that they were not terribly traumatized by the drops — they were small drops, onto a cushion, and it is a reflexive action — but they were probably not terribly amused by them, either.

Though clearly not the most revolutionary science related to the animal righting-reflex ever performed, Brindley’s experiments gave a fascinating insight into how animals solve the problem of finding “down” when in a weightless environment. In rabbits vs. relativity, one might say that the rabbits won a limited victory.

***********************************

¹ G.S. Brindley, “How does an animal that is dropped in a non-upright posture know the angle through which it must turn in the air so that its feet point to the ground?” J. Physiol. 180 (1965), 20-21P and G.S. Brindley, “Ideal and real experiments to test the memory hypothesis of righting in free fall,” J. Physiol. 184 (1966), 72-73P.

² J. Kan, T.Z. Aziz, A.L. Green and E.A.C. Pereira, “Biographical sketch, Giles Brindley, FRS,” British J. Neurosurgery 28 (2014), 704-706.