Having gathered various types of sensory information, animals have to take appropriate action based upon the information gathered.
Effectors may be defined as anything capable of producing a biological response. This can range from muscles producing movement ― an overt biological response ― to endocrine glands secreting hormones to alter some aspect of metabolism, e.g. insulin reducing blood sugar levels. Many types of response are effected in response to sensory stimuli, some of which will be briefly mentioned now.
Some animals possess the ability to change the color of their skin. Such changes occur for a variety of reasons including camouflage and communication to other animals. Such signals may be to animals of the same species, for example, to obtain a mate, or to animals of other species, for example, to deter predators. The color change occurs because the skin of the animal contains pigment containing cells called chromatophores. Animals which possess such cells include squids, octopods, some flat fish (e.g. flounder), chameleons, frogs and snakes. Chromatophores are under neural or endocrine control, or in some cases both. They have the ability to alter the dispersal of pigment within the cell in minutes or seconds, thus altering the overall appearance of the animal. The mechanism by which this occurs differs from species to species. In cephalopods (e.g. squid and octopus), each chromatophore is bounded by muscle cells and the contraction and relaxation of these muscles alter the dispersion of the colored pigment. When the muscles contract, the chromatophore ‘enlarges’ and the pigment disperses. When the muscles relax, the chromatophore shrinks and the pigment becomes concentrated. Contraction and relaxation of these muscle cells is under neural control. In contrast, the chromatophores of amphibians function by simple dispersion of the pigment within the chromatophore. In many cases, this is under endocrine control.
Virtually all animals possess a system of glands whose secretions result in a variety of biological responses and which are activated under appropriate conditions. However, there are many other glands which serve more specialized functions. Some of these have already been mentioned, e.g. glands which release attractant pheromones. Other examples of such glandular secretions serve a more aggressive role ― For example, skunks release a foul-smelling substance when attacked. The bombardier beetle, Brachinus, sprays attackers with a fluid which has a temperature of 100°C. The tip of the abdomen of this beetle contains two separate chambers. One chamber contains the reactants necessary for the production of this hot fluid; the second chamber contains the necessary enzymes required for the reaction to proceed. The release of reactants into this second chamber allows the production of this hot fluid to occur and its release at potential predators. In addition to the two examples given above, there are many more subtle responses which are not observed. For example, the release of insulin into the bloodstream after a meal in order to maintain appropriate blood sugar levels is a response to disturbance of blood sugar levels.
However, as previously stated, perhaps the most obvious response that animals make is that of movement.