8.1 Responses to the Environment

Living things constantly sense and react to their surroundings. Some responses are behavioral (moving, hiding, singing) and some are physiological (changes in body chemistry or growth). Examples you should recognize:

• Phototropism: plant shoots grow toward light, which lets their leaves capture more energy for photosynthesis.
• Photoperiodism: many plants flower in response to day length (really, the length of the night), so they bloom in the right season.
• Taxis: movement directed toward or away from a stimulus. A moth flying toward light shows positive phototaxis; bacteria swimming toward sugar show chemotaxis.
• Kinesis: a change in the speed of movement or how often an animal turns, without a set direction. Pill bugs (woodlice) move faster and wander more in dry places and slow down in humid ones, so they end up gathering where it's damp.
• Nocturnal and diurnal activity: some animals are active at night (nocturnal) and others in the day (diurnal), often matching when food is available or predators are less active.
• Fight-or-flight: a threat triggers hormones such as adrenaline that raise heart rate and release stored fuel, preparing an animal to fight or run.

Organisms exchange information that changes the behavior of others. Signals can be visual (a peacock's tail display, a firefly's flashes, warning colors), audible (bird songs, alarm calls), tactile (honeybees following a dancing forager in the dark hive), electrical (some fish sense and send electric pulses) or chemical (pheromones, scent marks).

Animals use signals to establish dominance, find food, mark territory and attract mates. A honeybee's waggle dance tells nestmates the direction and distance to food. Mammals mark territory with scent so rivals can avoid costly fights. Plants communicate too: some plants being chewed by caterpillars release airborne chemicals that attract wasps that attack the caterpillars. Signals that lead to more successful mating or food finding raise the signaler's reproductive success.

You won't be tested on the specific mechanisms behind any particular behavior or signal; focus on what the response does and how it affects fitness.

Innate behaviors are inherited and performed correctly the first time without experience, like a spider spinning a web. Learned behaviors change with experience, like a bird avoiding a bad-tasting insect after one try, or young songbirds learning their species' song by listening to adults. Natural selection favors both kinds when they increase survival and reproduction. Courtship displays, care of offspring by parents and efficient foraging all affect how many offspring an individual leaves.

Cooperation tends to raise the fitness of individuals and help the population survive. Wolves hunting in packs can bring down prey too large for one wolf. Herds, flocks and fish schools make it harder for predators to single out one individual. Honeybee and ant colonies divide labor among workers. Alarm calls warn others of predators.

Some cooperation helps relatives at a cost to the helper. This makes evolutionary sense through kin selection: relatives share many of your alleles, so helping them survive and reproduce can pass on copies of your alleles. Ground squirrels, for example, give alarm calls more often when close relatives are nearby, even though calling can draw a predator's attention.