If a crocodile eats you for breakfast, here are two consoling thoughts. First, your jewelry may survive -- the hungry reptile could vomit back your rings, necklace and watch. Secondly, if it does cough up your valuables, we now know exactly how it will do so.

How crocodiles vomit, you see, has just been ticked off the 'things to look at in biology' list. (They seem to sneeze, snap their jaws together and then shake their heads from side to side, in case you were wondering).

We generally associate vomiting with ill health, but it is a bona fide part of some animals' digestive process. Sharks throw up indigestible squid beaks, for example, and owls 'cast-up' small balls of fur and bones that would otherwise clog their guts.

And legend had it that crocodiles do the same. A crocodile reportedly vomited hair when it arrived at a Chicago, Illinois zoo in 1960 and electrical stimulation can make the toothy reptiles retch. And there is other evidence that crocodiles vomit out unwanted leftovers -- not least the grapefruit-sized hair-balls found in the Australian bush that are clearly beyond even the biggest owls.

Paul Andrews of St George's Hospital Medical School, London and his colleagues have been studying the crocodile (Crocodylus porosus) vomiting reflex, or emetic reflex. Man-eating reptiles being fairly thin on the ground in England, Andrews flew to Brisbane, Australia. There, the team made captive crocodiles throw up by giving them the plant extract 'veratrine'; they recorded the results on video. The team also took the crocodiles' blood pressure and electrocardiogram measurements -- from a safe distance.

When crocodiles vomit they follow a clear pattern, Andrews' team reports in the Journal of Experimental Biology1. The animals jerk their heads, snap their jaws and contract their throats. Some lunge forward as if to attack, then raised heads and more jaw snapping herald several brief episodes of noisy vomiting. The expulsion of food and liquid is not particularly forceful (crocodiles lack a muscular diaphragm) so they shake their heads to free it.

Nobody has studied vomiting in crocodiles before, so why have they bothered now? The answer is that the gigantic family tree relating species through evolution is full of holes. Common traits and behaviour patterns can help fill in the blanks by tracing animals back to common ancestors. Hair-ball-vomiting crocodiles and owls could be more closely related than rats, say, that are unable to vomit at all.

Common behaviour is a fair indication of shared ancestors, agrees Larry Witmer, an evolutionary biologist at Ohio University College of Osteopathic Medicine, Ohio. "Absolutely. Shared behaviour is valid evidence of relationships. Behaviour is not an incidental attribute, but rather an intrinsic aspect of an animal's biology -- one that is, in most cases, innate and under genetic control," he says.

But behaviour is the most fragile of evolutionary data, Witmer cautions. It is more prone to evolutionary selection than size and shape, for example. In other words crocodiles and owls could have independently evolved to produce hair-balls because it was useful, not because they inherited the behaviour from a shared ancestor.

This is not the first time that Andrews has studied the emetic reflex in animals. Earlier this month he was part of a team that described how thorn back rays (Raja clavata) literally throw their guts up, turning their stomachs inside out to rinse away undigested food2.