11.3 Regulatory Mechanisms and Behavior

Responses to Internal and External Stimuli

All organisms detect and respond to stimuli. In plants the directed growth responses are called tropisms and are controlled by hormones, especially auxin.

In animals, responses include reflexes (involuntary, hardwired pathways such as the knee-jerk), instincts (inherited behaviors present without learning, such as a spider weaving a web), and learned behaviors (modified by experience, including habituation, classical conditioning, and imprinting).

Maintaining Stable Internal Conditions — Homeostasis

Homeostasis is the regulation of internal conditions within a narrow range despite external change. Major regulated variables include:

• Temperature — endotherms (mammals, birds) generate heat metabolically and use sweating, shivering, vasodilation, and vasoconstriction; ectotherms (reptiles, amphibians, most fish) rely largely on behavior — basking, seeking shade, burrowing — supplemented by some physiology.
• Blood glucose — insulin lowers blood glucose by promoting cellular uptake and storage as glycogen; glucagon raises it by stimulating glycogen breakdown in the liver.
• Water and solute balance (osmoregulation) — the kidneys filter blood, reabsorb water and salts under antidiuretic hormone (ADH) control, and produce urine of varying concentration; marine fish lose water and drink seawater, freshwater fish excrete dilute urine.
• Blood pH, oxygen, and carbon dioxide — regulated by breathing rate controlled by the medulla and by kidney bicarbonate buffering.

The structures that carry out these tasks include the nervous system (rapid electrical signaling), the endocrine system (slower hormonal signaling), the kidneys, and the skin (sweating and heat exchange).

Negative vs Positive Feedback

A negative feedback loop senses a deviation from a set point and activates a response that reverses the deviation, returning the variable toward the set point — this is the dominant homeostatic pattern. A positive feedback loop amplifies the original change, pushing the system further from its starting point — this is rare and usually requires an external event to stop it.

A typical thermoregulation loop: body temperature rises → thermoreceptors in skin and hypothalamus signal the brain → sweat glands produce sweat and skin blood vessels dilate → heat is lost by evaporation and radiation → temperature falls back toward the set point → sweating stops.

How Evolutionary History Affects Behavior

Many behaviors are inherited rather than learned, which means they have been shaped by natural selection over evolutionary time. A fixed action pattern (FAP) is an instinctive behavior triggered by a specific sign stimulus; once begun it runs to completion even if the stimulus is removed. The classic example is a greylag goose retrieving any egg-sized object rolled out of the nest — it completes the retrieval motion even if the object is taken away mid-sequence.

Courtship displays in birds of paradise, stickleback fish, and many insects are species-specific and often act as reproductive isolating barriers, preventing mating between species. The structure of these displays reflects ancestry: closely related species share display elements, and differences accumulate with divergence time. Behavioral adaptations such as migration timing, hibernation, and cooperative hunting all carry the signature of the environments in which a lineage evolved.

Plant Hormones Beyond Auxin

Auxin is the most familiar plant regulator, but the exam also expects the other major hormones.

Ethylene is unusual because it is a gas, so one ripening fruit accelerates ripening in neighbors. ABA and gibberellins act as opposites in seed dormancy: ABA maintains it while gibberellins break it.

Learned Behaviors

Learned behaviors are modified by experience, adding flexibility within the framework evolutionary history provides. Four categories appear most often on the exam:

• Habituation — a decrement of response to a repeated, harmless stimulus. Urban birds ignore frequent loud noises that would startle a naive bird.
• Classical conditioning — associating a neutral stimulus with a meaningful one. Pavlov's dogs salivated at a bell once it predicted food.
• Operant conditioning — trial-and-error learning reinforced by consequences. A rat presses a lever for food and repeats rewarded actions.
• Imprinting — a rapid, usually irreversible attachment during a critical period. Konrad Lorenz's greylag goslings imprinted on the first moving object they saw.

Circadian Rhythms and Biological Clocks

A circadian rhythm is an internal cycle of roughly 24 hours that persists even under constant conditions, reflecting an endogenous biological clock. Examples include the mammalian sleep-wake cycle, nightly leaf folding in legumes, and the dawn release of cortisol. In mammals the master clock is the suprachiasmatic nucleus (SCN) of the hypothalamus, entrained by light from the retina.

In plants, photoperiodism links day length to development: long-day plants flower when nights are shorter than a critical length; short-day plants flower when nights exceed it. The pigment phytochrome detects red and far-red wavelengths and times these responses. Circadian rhythms and photoperiodism thus connect environmental signals to regulatory hormones and behavior, closing the loop between stimulus and response.