Defensive Strategies of the Main Prey Items Available in the Environment

Under the pressure of predators, animals have evolved a formidable array of defensive strategies (Table 1). These generally involve both the structure of the animal, its external traits especially, and its behavior. These strategies are mainly proactive, that is, they reduce the chance of being found by a predator, or the amount of damage eventually suffered if captured, but may also include active means of reaction, sometimes comparable to those used by the predator in its attack. The latter circumstance is often explained by the fact that a predator, although bigger and possibly stronger than its average prey and, thus, of many other animals in the community, is also often a suitable prey to other, possibly still bigger and stronger animals: in this case, the animal's tools for offence and defense are literally the same.

The divide between defensive and nondefensive adaptations is anyway blurred. For example, living underground, as the moles do, obviously reduces the risk of being preyed upon by mammals or birds that do not dig into the soil, and do not wait until night, when a mole may briefly venture out of its subterranean world. But a mole's life style requires, in turn, the evolution of adaptations like strong digging forelimbs, which are not a defensive device per se, but are strictly required to implement the mole's defensive strategy.

Defensive strategies are obviously obtained at a cost to the potential prey. Therefore, it would be useful to evolve

Table 1 Specialized predators attacking preys with conspicuous defensive strategies

Prey's defensive

Prey

strategy

Predators

Jellyfish

Stinging cells

Fish, e.g., stromateids (pomfrets and butterfish)

Polyps

Stinging cells

Mollusks: many nudibranch sea-slugs; arthropods: many pycnogonids (sea-spiders)

Squids

Flexible arms with

Many large marine predators, especially toothed whales, including sperm whale

suckers; poison bite

Centipedes

Poison claws

Some snakes, especially the South African centipede-eaters (Aparallactus spp.)

Spiders

Poison fangs

Many insectivorous vertebrates, especially toads

Ants

Sting; biting claws;

Lizards such as Phrynosoma, Urosaurus, and Sceloporus (ants representing up to 50% of

collective defense

the lizard's diet); many birds, e.g., several woodpeckers (Picidae) and many antbirds

(Formicariidae), some of which exploit the formidably aggressive army ants; mammals

such as anteaters, pangolins, aardvard

Bees and

Sting; collective defense

Some birds, especially bee-eaters (Meropidae) where bees can represent up to 95% of the

wasps

diet

conditional defensive strategies, that is, the capacity to develop specific, costly defenses only when actually required. In fact, such a kind of inducible defenses has evolved in some predator-prey systems. For example, in the tadpoles of the gray tree frog (Hyla versicolor) adaptive changes in growth (reduced body size) and behavior (reduced foraging activity) are induced by the presence of dragonfly nymphs. Another example is a tiny crustacean of freshwater zooplankton, the water flea Daphnia pulex. If exposed to chemical stimuli signaling the presence, in the water body, of the predatory larvae of the phantom midge Chaoborus, Daphnia embryos still developing inside the mother's brood pouch will eventually give rise to water fleas provided with defensive teeth that will reduce the risk of being eaten by the predator.

Close interactions with specific kinds of prey over extended time spans have led to the evolution of specific counter-adaptations in their usual predators. In turn, the improved weapons of the predator may have been a cause for the subsequent evolution of still better defensive adaptations of the prey. This is a scenario of coevolution where we can expect an escalation in the evolution of both attack weapons and defensive devices. This has been documented in detail for many representatives of two groups of marine animals, one of which (the crabs) includes the main predators of many representatives of the other (mollusks, especially bivalves). Predation of crab on mollusks has been continuous in inshore environments since the Mesozoic Era. Quite likely, this history of coevolution spanning more than 100 million years provides the key to understanding the evolution of mollusks with increasingly thick shell (and, for gastropods, with increasingly narrow opening slit, as in the cowries, Cypraea) and, correlatively, of crabs with increasingly stout claws. To trace the origin of a defensive adaptation we must often go beyond the predator-prey system, to take additionally into account the relationships between the potential prey and other organisms in the community, in particular, those on which the prey feeds in turn. A great number of caterpillars are full of toxic compounds obtained from (and/or similar to those of) their food plants, and these compounds are generally conserved throughout the metamorphosis into the adult butterfly. That is, specific plant defenses may turn into animal defenses against predation. Similarly, this time however within the closed prey-predator system, prey defenses are sometimes saved from destruction during the predation act, and may turn into defenses used by predators in respect to their own enemies. The best example is provided by the nudibranchs, a group of shell-less marine gastropod mol-lusks, many of which selectively prey on cnidarian polyps, without suffering damage from the victim's stinging cells. To the contrary, when the polyp is eaten by the mollusk, these cells are saved from digestion and become eventually relocated at the tip of the mollusk's fleshy dorsal appendages, where these cells (called the cleptocnides, i.e., stolen stinging cells) can provide, for a while, a defense to the predator rather than to its victim.

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