Animal Behavior

Behavior is how an animal responds to its surroundings and to other animals. Biologists separate two kinds of cause. Proximate causes are how a behavior works right now; ultimate causes are why it evolved (see OpenStax 45.7).

In 1963 Niko Tinbergen split this into four questions that are still used today (see Bateson and Laland):

• Causation: how does it work, through nerves, hormones and senses?
• Development: how did it develop during the animal's lifetime?
• Function: what is it for, how does it help survival and reproduction?
• Evolution: how did it evolve over the history of the species?

Function and evolution are “why” questions; causation and development are “how” questions. Ernst Mayr's ultimate causes cover function and evolutionary history (see Bateson and Laland). A full answer needs all four.

Innate behavior is inborn: an animal does it the first time without learning. A reflex, like the knee jerk, is the simplest kind. In a kinesis an animal changes its speed or turning in response to a stimulus; in a taxis it moves straight toward or away from it (see OpenStax 45.7).

A fixed action pattern is a set series of actions triggered by a stimulus, and once started it runs to the end. A male stickleback attacks other males with red bellies, and in experiments it also attacks red-bellied objects that look nothing like a fish (see OpenStax 45.7). Niko Tinbergen used models like these to find the key stimuli that trigger behavior, and some exaggerated models worked even better than the real thing (see Nobel Prize 1973).

Habituation is learning to stop responding to a harmless stimulus that keeps happening. Prairie dogs sound an alarm call when a predator threatens them, but they habituate to the sound of human footsteps when no harm comes with it (see OpenStax 45.7).

Imprinting happens in a short critical period early in life. A newly hatched duckling follows the first moving object it sees, whether its mother, a cardboard box or a balloon. Konrad Lorenz studied imprinting (see Nobel Prize 1973); he wrote that he discovered it as a boy while keeping domestic ducks (see Lorenz).

In classical conditioning, a neutral stimulus is paired with one that already causes a response. Ivan Pavlov's dogs salivated at food; after a bell rang with each meal, the bell alone made them salivate. The food is the unconditioned stimulus and the bell becomes the conditioned stimulus (see OpenStax 45.7).

In operant conditioning, a behavior becomes more common when it is rewarded. B. F. Skinner put a rat in a box with a lever; the rat learned to press it when pressing released food (see OpenStax 45.7).

Cognitive learning uses something like reasoning. Wolfgang Köhler hung a banana too high for chimpanzees to reach; some stacked boxes, climbed up and got it. This suggests they could picture the result before acting (see OpenStax 45.7).

Animals send signals through four main channels (see OpenStax 45.7):

• Chemical: pheromones, used by social insects and others to attract mates, raise alarms and mark trails to food.
• Auditory: sounds, like bird song and whale song.
• Visual: displays, like a dog baring its teeth as a warning.
• Tactile: touch, like langur monkeys grooming each other.

Honeybees combine several. For food close to the hive a returning forager does a round dance; for food farther away she does a waggle dance (see Nobel Prize 1973). On the vertical comb, “up” stands for the direction of the sun. If the food is 40° to the left of the sun, the straight waggle run points 40° to the left of vertical. The farther the food, the longer the waggle run lasts: about half a second for food 200 m away and about 4 seconds for 4,500 m (see von Frisch).

Optimal foraging means feeding in a way that gets the most energy for the least effort (see OpenStax 45.7).

Animals choose mates in different mating systems (see OpenStax 45.7):

• Monogamy: one male and one female, as in gray wolves.
• Polygyny: one male with several females. In resource-based polygyny, males defend a territory with good resources, as honeyguide birds defend beehives. Elephant seals keep harems, and male sage grouse display together at a lek.
• Polyandry: one female with several males, as in pipefish and seahorses.

Sexual selection favors traits that help win mates. In intersexual selection one sex chooses, as peahens choose males with bright tails. In intrasexual selection members of one sex compete, as rams butt heads (see OpenStax 45.7). See how sexual selection fits into Evolution.

Altruism is behavior that lowers an animal's own fitness but raises another's (see OpenStax 45.7). Why would it evolve? Relatives share genes, so helping them can pass on copies of your own genes. This is kin selection, and an animal's total effect on its genes, through its own offspring and its relatives', is its inclusive fitness (see OpenStax 45.7).

W. D. Hamilton put this in a rule in 1964 (see Hamilton). Helping is favored when rB > C, where r is relatedness, B is the benefit to the relative and C is the cost to the helper (see Nielsen). A parent and offspring share r = 1/2, full siblings 1/2, half-siblings 1/4 and first cousins 1/8 (see Nielsen). Bees add a twist: because of their haplodiploid genetics, a female bee shares 75% of her genes with a sister but only 50% with her own offspring (see Nielsen).

Unrelated animals help each other too. In reciprocal altruism, one animal helps another now and is helped back later, as monkeys take turns grooming. It works only when the same animals meet again and cheaters can be punished (see OpenStax 45.7). Robert Trivers set out the idea in 1971 (see Trivers).

Migration is long-distance seasonal movement. Wildebeest travel over 1,800 miles a year to find food. Some migration is obligate, always happening; some is facultative, depending on conditions (see OpenStax 45.7). Finding the way takes cues: honeybees, as you saw, use the sun and polarized light (see von Frisch).

Watching animals became a science, ethology, when biologists began asking careful questions and testing them.