Difficulty: Introductory to Intermediate | Prerequisites: Basic biology background (cell structure, DNA basics)
This material covers the foundational concepts of animal physiology: how organisms maintain stable internal conditions (homeostasis), how genes and environment interact to shape physiology, and how evolutionary forces explain why animals work the way they do. If you are coming in cold, you should already be comfortable with basic cell biology and the idea that DNA encodes traits. These concepts underpin every system you will study later in BIO 225, from thermoregulation to metabolism to neural signalling.
Animals use feedback loops and regulatory pathways to keep their internal environment stable, a process called homeostasis. A single genotype can produce different physiological outcomes depending on the environment (phenotypic plasticity), and both proximate (mechanistic) and ultimate (evolutionary) explanations are needed to fully understand any physiological trait.
Milieu interieur
The principle that living organisms preserve a distinct internal environment despite external changes. This idea, from Claude Bernard, led directly to the modern concept of homeostasis. Think of it as the body's commitment to keeping its inside world separate from whatever is happening outside.
Homeostasis
The process by which an animal initiates specific responses to control and regulate a particular variable (temperature, blood glucose, pH, etc.). In simple terms, it is the body's way of keeping things steady.
Conformers
Organisms that allow their internal conditions to change along with external conditions. Think of a fish whose body temperature matches the surrounding water.
Regulators
Organisms that maintain constant internal conditions regardless of what the environment does. Mammals holding a near-constant body temperature are the classic example.
Set-point
A preferred physiological state maintained by feedback loops, such as a target body temperature of roughly 37 degrees Celsius in humans.
Negative feedback loop
A response that sends a signal back to the stimulus, reducing its intensity. This is the most common regulatory mechanism in physiology. Think of a thermostat: when the room gets too warm, the heater switches off.
Positive feedback loop
A pathway that maximises changes in the regulated variable, beginning slowly but rapidly increasing in intensity until a signal stops the process at the proper time. Childbirth contractions are the textbook example. In simple terms, it is the opposite of negative feedback: the response amplifies the stimulus rather than dampening it.
Antagonistic controls
Two opposing pathways that fine-tune physiological responses by exerting opposite effects on the same variable. Think of the sympathetic and parasympathetic nervous systems working against each other to regulate heart rate.
Reflex control pathway
The most common mechanism used to maintain homeostasis. A change in the internal or external environment acts as a stimulus, which then triggers a response.
Circadian rhythm
Physiological processes that cycle on roughly a 24-hour schedule. Think of your sleep-wake cycle.
Central pattern generators (CPGs)
Groups of neurons that coordinate rhythmic behaviours such as breathing and walking, without requiring continuous input from higher brain centres.
Reductionism
The approach of studying physiological organisation by examining function at the next lower level. To understand an organ, you study its tissues; to understand a tissue, you study its cells.
Emergence
The principle that the whole is often more than the sum of its parts. Processes and characteristics of a system cannot always be explained by the component parts alone.
4 unifying themes in animal physiology
Physiological processes obey the laws of physics and chemistry.
Physiological processes are usually regulated.
Physiological phenotype is a product of both genotype and environment.
Genotype is the product of evolution through natural selection and other evolutionary processes.
Phenotype
The physiological properties of an animal, as determined by its genotype and environment. In simple terms, what the organism looks and functions like.
Genotype
The genes of the genome that determine and regulate physiological characteristics. Think of it as the blueprint.
Phenotypic plasticity
The ability of a single genotype to generate more than one phenotype depending on environmental conditions. One set of genes, multiple possible outcomes.
Polyphenism
A form of developmental plasticity in which development under different conditions results in alternative phenotypes that cannot be reversed. Unlike gradual plasticity, the switch is permanent.
Reaction norm
The range of possible trait values one genotype can produce across different environments, especially when the trait varies gradually.
Acclimation
The process of physiological change in response to a controlled environmental variable, such as in a laboratory setting.
Acclimatisation
The process of physiological change in response to natural environmental variation. Same idea as acclimation, but in the real world rather than a lab.
Proximate cause
The immediate or direct cause of a structure, function, or behaviour. Usually refers to the developmental or physiological mechanism. Answers the question: how does it work?
Ultimate cause
Why an organism has a particular structure or behaviour, understood through evolutionary advantage. Answers the question: why did it evolve?
Adaptation
The process of evolution through natural selection; can also be used as a synonym for acclimation in some contexts, which is a common source of confusion.
Genetic drift
Random changes in the frequency of particular genotypes in a population over time, as a result of chance events rather than selection.
Founder effect
A shift in genotypic distribution caused by a small number of individuals founding a new population, resulting in reduced genetic diversity.
Allometric scaling equation
Relates body mass (M), metabolic rate (y), a normalisation coefficient (a), and a scaling coefficient (b, typically 0.75): y = aM^b. This equation describes how metabolic rate scales with body size across species.
Ectotherms
Animals that regulate body temperature to match the environment. In simple terms, "cold-blooded" animals, though the term is imprecise.
Endotherms
Animals that regulate body temperature to stay constant regardless of the environment. "Warm-blooded" animals.
Protozoans
Early single-celled eukaryotes, now known as protists, that emerged before the three main eukaryote kingdoms.
Metazoan
A term that distinguishes multicellular organisms from single-celled protozoans.
The concept of homeostasis grew from Claude Bernard's idea of the milieu interieur: that organisms maintain a stable internal environment even as external conditions change.
Homeostasis is maintained primarily through reflex control pathways: a stimulus triggers a sensor, which signals an integrating centre, which activates an effector to produce a response.
Negative feedback loops are the workhorse of homeostasis. They reduce the intensity of the original stimulus, pulling the variable back toward its set-point.
Positive feedback loops amplify the stimulus instead. They are rarer and always require a separate termination signal to prevent runaway. Oxytocin release during labour is the classic example.
Antagonistic controls provide fine-tuning by placing two opposing pathways on the same variable, such as insulin (lowers blood glucose) and glucagon (raises it).
A set-point is the target value the body aims to maintain. It can shift, as with fever resetting the body's temperature target upward.
Conformers let internal conditions track external conditions. Advantage: low energy cost. Disadvantage: limited environmental range.
Regulators maintain internal constancy at the cost of energy expenditure. Advantage: wider habitat range. Disadvantage: higher metabolic demand.
Ectotherms (body temperature matches environment) and endotherms (body temperature stays constant) are the thermoregulation-specific versions of this distinction.
The genotype is the genetic blueprint. The phenotype is the observable outcome, shaped by both genotype and environment.
Phenotypic plasticity: one genotype, multiple possible phenotypes depending on conditions. This is not mutation; the DNA stays the same.
Polyphenism is a dramatic, irreversible form of plasticity, such as caste determination in social insects.
The reaction norm is the full range of phenotypes a genotype can produce across all possible environments.
Acclimation is plasticity in a controlled lab setting. Acclimatisation is the same process in natural conditions.
Every physiological trait can be explained at two levels.
Proximate: how does the mechanism work? (e.g., "Shivering generates heat through rapid muscle contractions.")
Ultimate: why did this trait evolve? (e.g., "Shivering was selected for because it improved survival in cold environments.")
Both levels are valid and complementary. Exams frequently ask you to distinguish between them.
Adaptation refers to evolutionary change through natural selection. Be careful: the word is also used loosely as a synonym for acclimation.
Genetic drift is non-selective, random change in allele frequency, most pronounced in small populations.
The founder effect is a specific case of drift: a small group colonises a new area and brings only a subset of the original population's genetic diversity.
Reductionism: understanding a system by studying its components at the next level down.
Emergence: the system has properties that cannot be predicted from the components alone.
Both approaches are needed. Physiology uses reductionism as a method but respects emergence as a reality.
Allometric scaling equation
y = aM^b
Where y = metabolic rate, M = body mass, a = normalisation coefficient, b = scaling coefficient (typically 0.75). This tells you that metabolic rate does not scale linearly with body size: a mouse burns far more energy per gram of body mass than an elephant does.
Homeostatic feedback is the principle behind medical devices such as insulin pumps, which monitor blood glucose and adjust delivery automatically, mimicking the body's own negative feedback loop.
The allometric scaling equation is used in pharmacology to estimate drug dosages across species of different sizes, and in ecology to predict the metabolic demands of animal populations.
Students often think positive feedback means "good feedback." It does not. Positive feedback means the response amplifies the stimulus. It can be dangerous if uncontrolled.
Students confuse acclimation and adaptation. Acclimation is a reversible physiological adjustment within an organism's lifetime. Adaptation is an evolutionary change across generations (though the word is sometimes used loosely as a synonym for acclimation, which adds to the confusion).
Students sometimes assume conformers are "primitive" and regulators are "advanced." Neither strategy is inherently superior; each is suited to particular ecological niches and carries its own energy trade-offs.
Students mix up proximate and ultimate explanations. A proximate cause is the mechanism (how). An ultimate cause is the evolutionary reason (why). Both are valid answers to different questions.
⚠️ Be able to distinguish negative feedback from positive feedback with a named example of each.
⚠️ Expect questions asking you to identify whether a given explanation is proximate or ultimate.
⚠️ The allometric scaling equation and the value of b = 0.75 are frequently tested. Know what each variable represents.
⚠️ Know the four unifying themes of animal physiology. These frame the entire course.
⚠️ Be prepared to distinguish acclimation from acclimatisation, and both from adaptation.
True or False: Positive feedback loops are more common than negative feedback loops in maintaining homeostasis. (False. Negative feedback is far more common.)
Fill in the blank: The scaling coefficient b in the allometric equation is typically ______. (0.75)
True or False: Acclimatisation occurs in a laboratory setting. (False. That is acclimation. Acclimatisation refers to natural conditions.)
Fill in the blank: A ______ cause explains the evolutionary reason for a trait, while a ______ cause explains the mechanism. (ultimate; proximate)
True or False: Conformers use more energy than regulators to maintain internal conditions. (False. Conformers expend less energy because they let internal conditions track the environment.)
Q: A lizard basks on a rock to raise its body temperature. Is this organism a conformer or a regulator, and is this an example of ectothermy or endothermy?
A: The lizard is an ectotherm that behaviourally regulates its temperature by seeking external heat sources. Ectotherms can still regulate, but they do so by adjusting their behaviour or environment rather than generating internal heat.
Q: A student argues that shivering exists "because muscles produce heat when they contract." Is this a proximate or ultimate explanation?
A: This is a proximate explanation. It describes the mechanism. An ultimate explanation would address why shivering was selected for through evolution, such as improved survival in cold environments.
Q: Explain why positive feedback loops require a separate termination mechanism.
A: Because positive feedback amplifies the stimulus rather than reducing it, the response would escalate indefinitely without an external signal to stop the process. In labour, for instance, the birth of the baby and expulsion of the placenta end the oxytocin-driven positive feedback loop.
Q: Two populations of the same species live in different climates. Population A has a broader reaction norm for cold tolerance than Population B. What does this suggest about their phenotypic plasticity?
A: Population A has greater phenotypic plasticity for cold tolerance, meaning its genotype can produce a wider range of physiological responses to temperature variation. This could reflect selection for flexibility in a more variable climate.
Q: What is the difference between genetic drift and natural selection?
A: Natural selection is non-random: traits that improve fitness become more common. Genetic drift is random: allele frequencies change by chance, regardless of fitness. Drift is most significant in small populations.
Homeostasis connects directly to thermoregulation, osmoregulation, and blood glucose regulation, which are all specific applications of the feedback principles covered here.
Proximate vs. ultimate causation reappears in every organ system: you will be asked to provide both types of explanation for neural, cardiovascular, and respiratory phenomena.
The allometric scaling equation links to metabolic rate and energetics topics later in the course, where body size becomes a key variable in comparing species.
homeostasis, negative feedback, positive feedback, set-point, conformer, regulator, ectotherm, endotherm, thermoregulation, phenotype, genotype, phenotypic plasticity, polyphenism, reaction norm, acclimation, acclimatisation, acclimatization, proximate cause, ultimate cause, adaptation, natural selection, genetic drift, founder effect, allometric scaling, metabolic rate, body mass, milieu interieur, Claude Bernard, circadian rhythm, central pattern generator, CPG, reductionism, emergence, reflex control pathway, antagonistic control, feedback loop, BIO 225, animal physiology, University of Michigan