Biology Index
By Richard Bruce BA, MA, and PhC in Economics

Blood, Frenzy, Poison, Shark Feeding Behavior

The sharks leave the prey alone, then one bites and they all go into a feeding frenzy. Or they leave you alone until you bleed, but you get a scratch and they attack. Their feeding appears mysterious but reasonable explanations are possible when we remember that sharks do not know that we are not poisonous.

Sharks and most other predators live in a world of venomous and poisonous animals. The seas have some twelve hundred poisonous species of fish according to this National Geographic article. This is somewhat less than one in ten species of fish in the sea. In addition to this, many invertebrates are poisonous. It is my impression that the invertebrates are even more likely to be poisonous than the fish. Many sponges, all cnidarians: jellyfish, corals, and sea anemones, echinoderms: starfish, sea urchins, and sea cucumbers, and many cephalopods, octopi, squid, etc are poisonous or venomous. If sharks or other predators are to survive long enough to reproduce they must avoid eating the poisonous animals that can kill them. Just because an animal is smaller than the shark does not mean it is safe to eat.

If an animal is slow moving it frequently is poisonous. For example, pufferfish, boxfish, lionfish, jellyfish, and sea snakes all move slowly and all are poisonous or venomous. It seems likely that slow moving animals are usually poisonous. How could a slow moving animal survive long enough to reproduce if it did not have some means of defense? So the shark looks at you and sees a clumsy, slow-moving animal and figures you must be poisonous, or at very least might be poisonous and it is not worth the risk. It goes off to look for other types of prey it has safely eaten in the past.

Blood in the Water

But what if you are bleeding. Then the shark sees an injured animal. Injured animals are often slow and clumsy even if they are not poisonous. So the fact that you are slow and clumsy no longer protects you and the shark attacks.

Scientists tell us that blood in the water will attract sharks from a great distance. I suppose so, the scientists are the experts. My hypothesis is concerned with why the shark would attack an injured person and leave the uninjured alone.

The Feeding Frenzy

This same type of reasoning concerning poison may help explain the feeding frenzy. If a shark sees another shark eating an animal they may take that as a signal that the animal is not poisonous and is safe to eat. This is particularly true if the other predator is a shark of the same species. An animal of a different species might be immune to the prey's poison, but if the other predator is of your species the prey is probably safe to eat. So once one shark begins to feed the others rapidly join in creating the well know feeding frenzy.

We know that we are not poisonous and it is easy for us to forget that the sharks and other predators do not. This maybe the reason we can swim in their world without being eaten.

Circling the Prey

Sharks circle humans. It has been argued that this is how they prepare for an attack, or that they are doing it for curiosity. Dr. Erick K. Ritter, a shark expert says it is curiosity, not preparation for an attack. I doubt the pure curiosity explanation. That sounds like anthropomorphizing. I suspect they are investigating the human as a possible food source. What confuses Dr. Ritter is that the sharks rarely move from circling to attack. If however, we assume that instinct tells the shark to avoid eating unfamiliar animals, particularly those that are slow and clumsy like people because they are probably poisonous, then the fact that the sharks do not attack is understandable. They are gathering information on a potential food source and the predictably conclude that it is not worth the risk.

If several sharks are circling, they may be hoping that one of the other sharks will take the risk of taking the first bite. If nothing happens they can join the feeding frenzy with little risk. If no shark attacks then they are instinctually using the experience of the other sharks. As none of the sharks has ever eaten a human none attack and the sharks take that as evidence that humans are likely to be poisonous, or have some other way of harming a predator that is not immediately apparent. Their instincts are not misleading them because humans are collectively very dangerous, even if we are individually defenseless.

Terestrial Predators

We flock to beaches, and some people swim out from shore far enough to be subject to a shark attack, but it is far more common for us to hike into forests and other wilderness areas where bears and mountain lions live and hunt. We often do this without weapons or other protection and yet only about three or four people in the United States and Canada are eaten by these predators every year. This is in spite of the fact that there are more than a third of a billion people in the United States and Canada, and about a million mammalian predators that when fully grown could easily kill us. Once again the story is probably that they do not know we are poisonous.

The stories of man-eating predators that kill many people while other animals of the same species leave the humans alone give further credence to this theory. The man-eater has learned that we are not literally poisonous, which is why it must be killed.

Generally, most predators with the exception of the Nile and saltwater crocodiles avoid eating us. You also might want to be careful about polar bears. I have written a web page that is principally about terrestrial predators here.

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