Getting Started
This chapter explores how living organisms, from single cells to complex animals, perceive and react to their environments. We will examine the mechanisms that connect an external or internal cue to a specific physiological or behavioral response. The central problem organisms must solve is how to maintain internal stability and maximize their chances of survival and reproduction in a constantly changing world.
What You Should Be able to Do
After completing this section, you should be able to:
Explain how a specific environmental change triggers a behavioral or physiological response in an organism.
Describe how organisms use different types of signals to communicate information.
Connect an organism's behaviors, such as foraging or mating rituals, to its reproductive success.
Differentiate between innate and learned behaviors and explain how both can be shaped by natural selection.
Analyze how cooperative behaviors can increase the fitness of both an individual and a population.
Key Concepts & Mechanisms
Organisms are not passive inhabitants of their environment; they actively respond to it. The ability to detect a change and react appropriately is fundamental to life. This process can be understood as a pathway from stimulus to response, driven by the need to survive and reproduce.
Inputs & Preconditions: The Stimulus
The entire process begins with a stimulus, which is any detectable change in an organism's internal or external environment that elicits a response. For a response to occur, the organism must have a receptor capable of detecting that specific stimulus.
External Stimuli: These originate from outside the organism. Examples include changes in light intensity or duration (photoperiod), temperature fluctuations, the presence of a predator, or the release of a chemical by another organism.
Internal Stimuli: These originate from within the organism. Examples include a drop in blood glucose levels, a change in blood osmolarity (thirst), or the release of hormones that signal hunger or readiness to mate.
Key Steps / Mechanism: The Response Pathway
Once a stimulus is detected, a sequence of events translates that information into action.
Reception: Specialized cells or organs, known as receptors, detect the stimulus. Photoreceptors in the eye detect light, chemoreceptors in the nose detect odors, and thermoreceptors in the skin detect temperature.
Transduction: The receptor converts the energy of the stimulus into a signal that can be understood and transmitted by the body. This often involves a change in the membrane potential of a neuron or the release of a hormone into the bloodstream. This signal is then processed by the nervous and/or endocrine systems.
Response: The organism carries out a specific action. Responses can be broadly categorized as physiological or behavioral.
A physiological response is a change in the internal functioning of the organism. For example, in response to cold, mammals may shiver to generate heat or constrict blood vessels to conserve it.
A behavioral response is an observable action or activity of an organism. For example, a lizard moving into the sun to warm up is a behavioral response to the cold.
Outputs & Effects: Behavior, Fitness, and Selection
The ultimate purpose of any response is to increase an organism's fitness, defined as its ability to survive and pass its genes on to the next generation. Behaviors that successfully achieve this are favored by natural selection.
Innate Behaviors: These behaviors are genetically determined and do not require any prior experience or learning. They are sometimes called instincts. Examples include taxis, a directed movement toward or away from a stimulus (e.g., a moth flying toward a light), and kinesis, a non-directional change in activity level in response to a stimulus (e.g., sowbugs moving more in dry areas and less in humid areas).
Learned Behaviors: These behaviors are acquired or modified as a result of an individual's specific experiences. Learning allows an organism to adapt to unpredictable situations. Examples include a bird learning to avoid a brightly colored, toxic caterpillar after trying to eat one.
Communication and Cooperation: Many behaviors involve exchanging information between organisms. Communication can be achieved through visual, audible, tactile, electrical, or chemical signals. These signals can trigger responses that are critical for mating, warning of danger, or coordinating group activities. Cooperative behaviors, such as pack hunting or alarm calls, can increase the fitness of the individuals involved and the success of the entire population.
Regulation: The Role of Genes and Environment
Both innate and learned behaviors are shaped by evolution. An innate behavior persists in a population because the genes that code for it have historically conferred a survival or reproductive advantage. Learned behaviors are also subject to selection; the capacity to learn is a heritable trait, and individuals that learn beneficial behaviors (like a new food source) are more likely to survive and reproduce. Therefore, behavior is the product of a complex interplay between genetic predisposition and environmental influence.
Key Models & Diagrams
Organisms use a variety of signals to communicate, each with distinct advantages and limitations. The effectiveness of a signal depends on the environment and the sensory abilities of the receiver.
| Signal Type | Description | Example(s) | Advantages & Disadvantages |
|---|---|---|---|
| Visual | Information conveyed through gestures, postures, coloration, or light. | A male peacock's tail display; a dog baring its teeth. | Adv: Rapid, conveys complex information. Disadv: Requires light, can be blocked by obstacles. |
| Audible | Information conveyed through sound. | A bird's song; a whale's call; a squirrel's alarm chatter. | Adv: Travels long distances, works in the dark. Disadv: Can be detected by predators. |
| Tactile | Information conveyed through touch. | A honeybee's "waggle dance"; primates grooming each other. | Adv: Effective in darkness, private. Disadv: Requires close proximity. |
| Chemical | Information conveyed through molecules called pheromones. | An ant's trail to a food source; a moth attracting a mate from miles away. | Adv: Long-lasting, effective over great distances. Disadv: Slow to transmit, less directional. |
Key Components & Evidence
Stimulus: An environmental cue, such as light, temperature, or a chemical, that triggers a response.
Innate Behavior: A genetically determined, instinctual behavior that is performed correctly the first time without prior experience, such as a spider spinning a web.
Learned Behavior: A behavior that is modified by experience, such as a crow learning to use a tool to extract insects from wood.
Fight-or-Flight Response: A classic physiological response to a perceived threat, involving the release of hormones like adrenaline that prepare the body for intense physical activity.
Taxis: A directed movement in response to a stimulus. For example, phototaxis is movement toward or away from light.
Kinesis: A non-directional, random change in movement speed or turning rate in response to a stimulus.
Pheromones: Secreted chemical signals used by organisms to trigger a social response in members of the same species, crucial for mating and marking territory.
Communication: The transmission of information between organisms that results in a change in the recipient's behavior.
Cooperative Behavior: Group behaviors that increase the fitness of the individuals and the survival of the population, such as alarm calling in prairie dogs.
Natural Selection: The process whereby behaviors that enhance survival and reproduction become more frequent in a population over generations.
Skill Snapshots
Causation
Cause: A decrease in day length (photoperiod) in the fall. Effect: Triggers hormonal changes in many bird species, initiating migratory behavior.
Cause: A foraging honeybee discovers a rich source of nectar. Effect: The bee performs a "waggle dance" inside the hive to communicate the direction and distance of the food to other bees.
Cause: The presence of a predator is detected by a vervet monkey. Effect: The monkey emits a specific alarm call that causes other monkeys to take appropriate cover.
Comparison
Innate behaviors are developmentally fixed and consistent among individuals in a population, whereas learned behaviors are variable and depend on an individual's unique experiences.
Taxis is a directed movement toward or away from a stimulus, while kinesis is an undirected change in activity level in response to a stimulus.
Chemical signals (pheromones) are effective over long distances and in darkness, whereas visual signals are instantaneous but require a direct line of sight and sufficient light.
Change and Continuity Over Time (Evolution)
Baseline: Within an ancestral population, there is natural variation in a behavior, such as the boldness or shyness of individuals when foraging.
Change 1: An environmental pressure, like increased predation, selects against overly bold individuals who are more likely to be caught.
Change 2: Over many generations, the alleles associated with cautious foraging behavior increase in frequency, and the population as a whole becomes more shy.
Continuity: The fundamental drive to acquire energy through foraging remains a constant selective force, even as the specific strategies for doing so evolve.
Common Misconceptions & Clarifications
Misconception: "Innate" means simple and "learned" means complex.
- Clarification: Many innate behaviors, like the construction of a termite mound or the migratory route of a monarch butterfly, are extraordinarily complex. Conversely, some learned behaviors are very simple, such as habituation (learning to ignore a repeated, harmless stimulus).
Misconception: A behavior is either 100% genetic (innate) or 100% environmental (learned).
- Clarification: This is a false dichotomy. Nearly all behaviors are the product of an interaction between genes and the environment. A bird may have an innate predisposition to sing, but it must learn the correct species-specific song by listening to adults.
Misconception: Cooperative behaviors are purely altruistic and defy natural selection.
- Clarification: Behaviors that appear selfless often provide an indirect benefit to the individual. For example, giving an alarm call may endanger the caller but saves its relatives, who share many of the same genes. This increases the individual's inclusive fitness—the total success of its genes passed on to the next generation, both directly and through relatives.
One-Paragraph Summary
Organisms constantly respond to internal and external environmental cues through a combination of physiological and behavioral mechanisms. These responses, which range from simple movements like taxis to complex social interactions involving communication, are fundamental to survival and reproduction. Both innate (genetically determined) and learned (experience-based) behaviors are subject to natural selection. Behaviors that increase an organism's fitness—its ability to survive and pass on its genes—are favored and become more common in a population over time. Therefore, an organism's repertoire of responses is a finely tuned product of its evolutionary history, shaped by the challenges and opportunities of its environment.