The image of a hooded cobra with its neck flared is one of the most iconic from the animal kingdom. But how do they do it? Bruce Young from the University of Massachusetts Lowell and Kenneth Kardong from Washington State University explain that P. Russell and E. Home suggested in 1804 that muscles were entirely responsible for the hood's erection. Since then most people had suggested that the reptiles use their ribs and some had suggested that the reptile's skin might be involved in pulling the ribs into place, but no one had tried to find the mechanism. Unfazed by the snake's reputation, Young and Kardong decided to find out exactly how cobras hood (p. 1521).
The duo decided to measure the electrical activity of all of the muscles in the neck region as the animals erected their hoods, to find out exactly which ones were involved. ‘Doing the surgery was the riskiest part of the study because you have to work around the head but the snakes are prone to waking, which can be disconcerting,’ says Young. Having successfully inserted as many as 20 minute EMG electrodes into the tiny neck muscles, Kardong and Young waited for the snake to recover before filming and recording the muscle activity as the animals flared their necks. But it soon became clear that no two hooding events were the same. Sometimes the animals would hold a 70% hood for minutes, while on other occasions they could flare the hood completely and retract within 45 s, making it almost impossible to map the muscle activity to specific phases of the hood's erection.
However, after months of patience, Kardong and Young could see that eight muscles were involved in erecting the hood with two groups playing major roles: the levator costae and the intercostal muscles. Young explains that the large levator costae and the supracostalis lateralis superior muscles raise the first few ribs nearest to the head, intercostal muscles hold the ribs together while the hood is erect, and muscles connecting the ribs to the skin hold the skin taut.
Collecting X-ray images of a relaxed and hooded cobra, Kardong and Young could also see how far the ribs rotate away from the vertebral column and wondered if the joint between the rib and vertebra was different from that of ribs further down the body that are not involved in hooding. Scrutinising the joint section of neck and torso ribs with scanning electron microscopy, the team could see a protrusion on the neck rib's articular surface, which was missing on the torso ribs, that could displace the rib along the vertebra as it rotated, extending the range of the rib's movement. And when they looked at the joint structure, the team could see that the joint was encased by an elastic ligament that holds the joint in place. The duo realised that this ligament could also recoil and pull the ribs back when the levator costae and intercostal muscles relax, to tuck the hood away with the aid of additional intercostal muscles.
Young explains that cobras are not the only snakes that hood. ‘Several groups of unrelated snakes show almost identical defensive behaviour,’ says Young, and he is keen to find out how these other snakes raise their hoods.