The Gymnasts of the Animal Kingdom

There is no doubt about it: doing a handstand is not easy. The act requires the coordination and strength of dozens of muscles. But for a peculiar breed of rabbits that do not hop, the handstand is their go-to. This rare breed, known as sauteur d'Alfort, was discovered in 1935 by French veterinarians who noticed the rabbits’ ability to move around on their front paws effortlessly.

The typical movement for a rabbit is referred to as saltatorial locomotion, or hopping. Saltatorial locomotion is a type of terrestrial locomotion that is mainly bipedal, requiring two muscular hind limbs. Though the motion appears to be simple, it requires a precise sequence of movements, from the angle of extension at launch to the protraction of limbs upon completion. However, sauteur d'Alfort rabbits lack this coordination and cannot hop as a result.

For nearly a century, scientists were unable to pin down an explanation for this breed’s abnormal locomotive behavior. However, in a recent study published in PLOS Genetics, researchers reported that the bizarre bipedal gait of sauteur d'Alforts is accredited to a loss-of-function mutation in the RAR Related Orphan Receptor B (RORB) gene. RORB codes for proteins and is expressed in regions of the nervous system that process circadian and sensory information. The gene is also responsible for the presence of RORB-positive neurons and interneurons in the spinal cord, passing signals that control limb coordination to allow for hopping. Due to the mutation, however, many introns, or non-coding segments, are incorporated into the expressed proteins. This reduces the number of RORB-positive neurons and causes deformities in the interneurons in the spinal cord. As a result, while these rabbits walk normally when moving slowly, they intrinsically lift their hindlimbs and depend solely on their forelimbs when moving quickly instead of hopping.

This strange scamper is not confined to rabbits. Stephanie Koch, a neuroscientist at University College London, found that mice that possessed this genetic mutation also did handstands when they started to run. Similarly, horses with this mutation exhibited an altered gait where they ambled instead of trotting and galloping.

In addition to explaining the unusual gait, studying the genetic abnormality of RORB contributes immensely to our understanding of animal movement. Nevertheless, scientists still lack the clues to pin down the specific function of the interneurons that cause a handstand as opposed to a different motion. This limits their ability to quantify the extent of RORB’s functions since the gene may be affecting not only locomotion through the spinal cord, but also every process in the rabbit. Regardless, breakthroughs, such as the identification of the RORB gene, have helped piece together the various components of the nervous system that work together to stimulate muscular contractions and, ultimately, movement.

The RORB gene’s function is not constrained to locomotion. It also affects vision, several biological rhythms, and sensory transduction, the process of converting a sensory stimulus into electrical signals processed by the nervous system. Defects in the gene have also been linked to epilepsy, a neurological disorder that causes seizures. While medication can treat epilepsy by controlling it and preventing seizures, there is no cure for the disease. However, studying these hand-standing rabbits can expand scientists’ knowledge of the gene, allowing them to develop measures to combat RORB-related diseases and prevent harm in those possessing a mutated RORB gene.

It is a far cry from the traditional hop, but the acrobatic habits of the sauteur d'Alfort breed have unlocked a paramount breakthrough that can help scientists decipher the elaborate process of locomotion once and for all. Moreover, it has exposed RORB, a gene worth exploring due to its extensive functions in the body’s processes, its morbific connection to epilepsy, and its peculiar ability to turn bunny hops into handstands.