By Blaise Jones
A large part of the fearsome reputation of sharks comes from the idea that they are all mindless eating machines. Many people believe sharks to be ravenous killers, stalking the ocean in a constant search for prey. While sharks are indeed opportunistic hunters, they are by no means mindless eating machines.
Tough Pill to Swallow
While sharks may not greedily swallow down everything they come across, they aren’t exactly neat and tidy eaters. Like bony fish, sharks tend to swallow their prey whole. However, unlike bony fish sharks are able to eat things bigger than their mouths.
Bony fish tend to not have the same kinds of sharp, cutting teeth that many species of shark have. These specialized teeth allow many species of shark to tear off bite-sized chunks of larger animals. These sharks do not chew these chunks, however. They swallow them whole, just like bony fish.
There are, however, a few species of shark that do “chew” their food. Bottom-dwelling sharks that feed on animals with hard shells have specialized teeth for crushing their prey’s shells. They chomp down repeatedly upon their prey until the hard and spiked shells are destroyed, letting the shark suck up all the tasty meat like an aquatic Roomba.
Despite their ravenous reputations and opportunistic nature, sharks don’t actually need to eat to all that much. Even the most active and energetic of sharks only needs to eat anywhere between 0.5 to 3 percent of its body weight a day. This means that a shark weighing 100 pounds (45 kg) would only have to eat three pounds of fish (1.3 kg) per day. That doesn’t mean that the 100-pound shark eats a 3-pound fish (or an equivalent amount of smaller fish) every single day. Most sharks only eat once or twice a week. A good sized shark can prey upon pretty much any animal smaller than itself and the bigger the meal the longer a shark can go without eating.
Sharks can do long periods between meals because they have very slow metabolisms. A metabolism is, “all chemical reactions involved in maintaining the living state of the cells and the organism.” In other words, the process by which an organism breaks down the things it consumes and transforms them into energy it can use, as well as how efficiently that energy is used.
Sharks are metabolic miracles. It can take anywhere from several hours to several days for a shark to fully digest a meal, depending on the size of the meal, and once that meal is digested much of that energy is conserved and store in the shark’s liver.
Even the most energetic and metabolically intense sharks have a surprisingly slow rate of energy consumption. The great white shark is a huge animal and, because of unique aspects of its physiology, has an equally huge need for energy. However, a great white still only needs to eat 3 percent of its body weight per day because whenever the shark is just cruising around, it only uses 0.2 calories of energy per kilogram of body weight every hour. Humans use eight times as much energy per unit of body weight.
Expert Ergonomic Energy Expenditure
It takes very little energy for a shark to move through its environments, which means a little food goes a long way. On top of this, sharks have extremely efficient digestive systems.
While the thought of the digestive process taking up to a week may sound inefficient, it is quite the opposite. Every extra second that food stays in a shark’s system gives it that much more energy. Shark intestines are uniquely designed to maximize the amount of time they have to digest their food.
Sharks have a special adaptation, found nowhere else in the animal kingdom, called a spiral valve. The spiral valve is, like its name suggests, a corkscrew-shaped structure located in the intestines of a shark. This valve sends the food rotating around inside the intestine as much as 40 times before the food passes. Not only does this maximize the amount of time the food is in the intestine, it also greatly increases the surface area which as shark can use to digest its food.
“Sharks: The Animal Answer Guide” by George H. Burgess and Gene Helfman
“The Encyclopedia of Sharks” by Steve Parker