The hammer of the hammerhead shark – just one of many strange outgrowths that evolution has produced. Like almost everything in …
The hammer of the hammerhead shark – just one of many strange outgrowths that evolution has produced. Like almost everything in nature, even this structure has its meaning, even if science is not entirely in agreement.
Just think of a huge elk antler, which can be up to 2.5 meters long and weigh over 40 kilograms, the ultra-long giraffe neck or the enchanting but incredibly impractical train of the peacock. A whole series of disadvantages for the owners of such “excesses” suddenly springs to mind. But on closer inspection, it quickly becomes clear: Each of these anatomical features has its right to exist. The elk needs his headdress to impress and fight, and maybe even to shovel snow, and the rule here still applies: the bigger the better. The giraffe comes with its neck to the leaves of trees that other cloven-hoofed animals can only dream of, and the peacock pulls the shorter when a tiger is around, but otherwise – thanks to the train – secures the prettiest female and most of them Progeny. The supposed whims of nature are in most cases the result of a tough cost-benefit analysis. What doesn’t work disappears.
The hammerhead shark also stands out from its colleagues in the shark tank because of its unusual structure, its head shape. The hammer-shaped head, called cephalofoil – a combination of the Greek word for “head” (cephalos) and the Latin for “foil” (folium) – is the trademark of the Sphyrnidae family, which has a total of nine species. Among them, the great hammerhead shark is the largest at almost six meters and the bow-headed hammerhead shark the most common. Amazingly, however, the origin and biological function of the cephalofoil, in contrast to other funny or graceful structures in the animal kingdom, is still rather unknown – despite 50 years of research . But there are many theories, because one thing is clear: the hammer wasn’t suddenly there for no reason and just like that.
The “hammer”, known in technical terms as the cephalophoils, has proven itself over 55 million years in evolution – that is how old are the oldest fossil finds.
THE HYDRODYNAMIC HYPOTHESES
According to the “hydrodynamic lift hypothesis,” the hammer supposedly functions like the wing of an airplane, especially that of a canard or duck plane, which gives lift to the front end of the body. Incidentally, the pectoral fins also fulfill a similar function, but they are smaller in hammerheads with a large cephalofoil than in other shark species. One fact that supports this theory is that the total area of the cephalofoil and pectoral fins is always the same in all hammerhead species; in the end all of them have the same ‘lift’, be it through hammer or fin size. Shark species that do not have a cephalofoil have larger pectoral fins to compensate for the lack of the “hammer surface”.
The second hydrodynamic hypothesis is that the maneuverability of the shark is increased by the hammer. Experiments have found that hammerhead sharks are indeed more agile and flexible than sandbar sharks.
THE PREY THEORY
A third hypothesis is that the sharks use the hammer to manipulate their prey.
In the Bahamas, at least with the great hammerhead shark, it has been observed sporadically how it downright ‘knocked out’ eagle rays and American stingrays with a hammer, threw them to the ground and pinned them.
While the poor ray was pressed to the ground, the shark turned around its own body to take a “bite-favorable” position.
The wider the head, the more sensory cells – so one of the theories.
HYPOTHESES FOUR AND FIVE: SENSE PERCEPTION
The fourth and fifth hypotheses assume an improvement in the sense of smell and sight through the hammer. The “olfactory gradient hypothesis” postulates that, among other things, better alignment along a scent trail (“olfactory clinotaxis”) takes place. Odor gradients, i.e. a stronger or weaker odor, could be better perceived if the nostrils are further apart. The arrangement of the nostrils on the hammer and their lateral enlargement, which is only made possible by the hammer, also allows more seawater to be tested for odor molecules. And finally, through the larger nostrils, the shark could have more olfactory epithelium at its disposal, which further improves the sense of smell.
While previous findings support the first two assumptions of this hypothesis, it could not be proven experimentally that the olfactory epithelium, i.e. that layer of tissue equipped with olfactory cells, is larger in hammerhead sharks than in sharks without a cephalofoil. The »improved binocular vision hypothesis«, in turn, states that binocular vision is improved and the field of vision as a whole is enlarged by placing the eyes on the hammer. There are already studies on this hypothesis, the results of which support both assumptions.
With the help of the Lorenzini ampoules, sharks can also locate prey hidden in the sand.
THE MAIN THESIS: ELECTRO SENSORS
The best known and most frequently put forward hypothesis is the “improved electrosensory hypothesis”. It says that the larger area of the cephalofoil must be covered with more electroreceptors in order to achieve a pore density similar to that of other sharks. As a result, the shark would have more pores overall and thus an improved electrical sense. Sharks use this sensory system, among other things, to perceive bioelectrical impulses that are involuntarily given off by prey. Using these impulses, they can easily track down animals hidden in the sand, for example.
A higher sensitivity of the electrical sense can also be achieved by lengthening the channels within the electrical pores. The cephalofoil enables such an extension in the hammerhead shark. There is also a whole series of studies on this hypothesis that at least partially confirm it.
As with the elk, which uses its antlers to fight, impress and shovel, there are probably several reasons why the hammerhead shark could develop such a remarkable head shape – a feature that has not changed significantly over the past 55 million years has changed.
Subclass: Euselachii (sharks and rays)
Neoselachii (recent sharks and rays)
Parent order: Galeomorphii
(includes four orders of modern sharks)
Order: Ground Sharks (Carcharhiniformes)
Family: Hammerhead Sharks (Sphyrnidae)
Species: wingtip hammerhead shark (Eusphyra blochii)
Species: Corona hammerhead shark (Sphyrna corona)
Bow-fronted hammerhead shark (S. lewini)
Spoon-headed hammerhead shark (S. media)
Great hammerhead shark (S. mokarran)
Shovel-nosed hammerhead shark (S. tiburo)
Small-eyed hammerhead shark (S. tudes)
Smooth hammerhead shark (S. zygaena)
Sphyrna gilberti (described for the first time in Aug 2013)
by: https://www.tauchen.de/author/redaktion/ / https://www.tauchen.de/
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