Avoiding Predation

Most animals get their food from preying on other organisms, and much of the life of animals involves eating and avoiding being eaten.

• increase the effectiveness of predation and
• minimize the risk of being preyed upon.

Camouflage (cryptic coloration)

The motionless twig caterpillar shown here (courtesy of Muriel V. Williams) complete with "buds" and "lenticels" escapes detection by birds (but pays for its cleverness by occasionally having some other insect lay eggs on it by mistake).

• the peppered moth
• the winter flounder

Chemical Defenses

• Millipedes secrete hydrocyanic acid when disturbed.
• Some beetles squirt potential predators with such mixtures as 85% acetic acid or 40% formic acid.
• The discharge of the skunk is another familiar example.

Aposematic Coloration

This is the larva of the monarch butterfly; an example of aposematic coloration. There is no question of camouflage here. Rather this creature is advertising its presence.

The milkweed leaves on which it is feeding contain several substances that are toxic to vertebrates. The larva stores these within its body and thus becomes unpalatable to vertebrate predators.

The chemicals remain in the body even after metamorphosis, so that adults are unpalatable as well.

In these photographs (provided by Lincoln P. Brower) a blue jay eats a portion of a monarch butterfly (left) that had fed (in its larval stage) on poisonous milkweed. A short time later, the blue jay vomits (right). Following this episode, the blue jay refused to eat any other monarch offered to it.

Mimicry

Batesian Mimicry

If an animal is not noxious to potential predators, why not look like an animal that is?

Some examples:

• A number of harmless snakes closely mimic the bright warning coloration of the coral snake — the most poisonous snake in the United States.
• The harmless robber fly (right) resembles the bumblebee (left) even though the two are not closely related. The robber fly is a dipteran, with only a single pair of wings, while the bumblebee is a hymenopteran with two pairs.
  
• The viceroy butterfly (bottom) contains no toxic substances in its body and presumably is quite palatable (one entomologist declared it tastes like dried toast). If so, the viceroy's striking resemblance to the monarch (top) enables it to capitalize on the monarch's unpalatability. (Photos courtesy of Tom Eisner.)

Müllerian Mimicry

Some unpalatable animals closely resemble other equally unpalatable species. Such mimicry is called Müllerian mimicry (in honor of the German zoologist Fritz Müller, who studied it). Presumably each species gains a measure of protection from the occasional, but educational, losses of the other species to predators.

Lincoln Brower, who has studied the monarch and viceroy, believes that the viceroy is as unpalatable to potential predators as the monarch, and thus is really an example of Müllerian mimicry.

Aggressive Mimicry

Some carnivores have evolved devices with which they mimic the prey (or potential mate) of other (usually smaller) predators. They use these devices as lures.

Two examples:

• The angler fish (Antennarius) displays a lure resembling a small fish. The lure is a development of the spine of the first dorsal fin. This species of anglerfish, which was found in the Philippines, is 9.5 cm long. Note its use of camouflage: its texture (and color) closely resemble the sponge- and algae-encrusted rocks found in its habitat.

  (Photo courtesy of David B. Grobecker from Pietsch, T. W. and D. B. Grobecker, Science, 201:369, 1978.)

• Fireflies use their flashes to attract mates. The pattern differs from species to species. In one species, the females sometimes mimic the pattern used by females of another species. When the males of the second species respond to these "femmes fatales", they are eaten!

  Link to discussion of how fireflies control their flashing.

Group Behavior

• Grazing ungulates are usually organized so that the stronger are on the outside of the herd, the weaker within. In this photo (courtesy of Ted Grant, National Film Board of Canada), musk oxen have responded to a threat by forming a circle with the females and young in the center.
• Both herds of ungulates and flocks of birds often have members who are extra-watchful, ready to give the alarm if danger threatens.
• When alarmed, smelt (a kind of fish) release a pheromone into the water that warns other members of the school.
• When a honeybee stings an intruder, she release isoamyl acetate, which excites other bees to join the attack.