Magazine article Natural History

Brainy Sharks?

Magazine article Natural History

Brainy Sharks?

Article excerpt

Some people think of sharks as small-brained feeding machines. This is far from true. In cartilaginous fishes (sharks, rays, and skates), in fact, brain weight relative to body weight is about ten times that of bony fishes in the same size range. The relative brain weights of many cartilaginous fishes are close to those of mammals. The brain of a 40-pound Atlantic stingray weighs nearly 3 ounces-twice the weight of a 40-pound beaver's brain. A 440pound tiger shark has a brain of nearly 4 ounces, which is in the same size range as the brains of such mammals as the mountain lion or wolf and almost half the size of the brain of a 350pound tiger.

Shark-brain expert Glenn Northcutt has pointed out that not only are the brains of cartilaginous fishes comparatively large, but the relative development of their major divisions closely parallels that of birds and mammals. Among the factors that could account for the presence of such large brains may be a more complex behavioral and sensory repertoire than has hitherto been recognized. A large brain is probably especially useful in a complex environment such as a coral reef or a river in Amazonia, where some of the brainiest cartilaginous fishes live. Moreover, a few sharks are known to maintain a higher temperature than that of the surrounding water; the result is a higher metabolic rate and thus a comparatively lower cost for a big brain.

Other factors may also favor large brains in cartilaginous fishes: many species produce living young rather than eggs and have long gestation periods. It is intriguing that the groups of cartilaginous fishes with the biggest and most complex brains (whaler sharks, hammerhead sharks, stingrays, and eagle rays) have evolved yolk sac placentas or placental analogues that greatly increase the energy supply to embryos. It has been suggested that in mammals, an above-average brain size demands an increased energy flow from mother to offspring. …

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