Welcome to Comparative Physiology: An Introduction to How Animals Work The study of physiology is highly integrative. Thorough understanding of physiology must include knowledge of

• physics • chemistry • genetics • cellular & molecular biology • animal morphology • ecology • evolutionary processes

Approaches to Physiology While integrative, physiology can be approached from many different angles.

• Mechanistic physiology is the study of the actual "nuts and bolts" of a particular physiological process. • Evolutionary physiology is the study of the evolutionary origins of a particular physiological process. • Comparative physiology is the study of how different species perform similar physiological processes. (as opposed to just

"human physiology" or "plant" physiology) • Environmental physiology (aka physiological ecology) is the study of how organisms employ physiological processes to

meet the challenges of their external environment. • Integrative physiology is the study of how mechanisms at different levels of biological organization (e.g. the molecular,

cellular, and anatomical levels) drive physiological processes. Comparative Animal Physiology All living organisms are defined by, among other things, homeostasis, the ability (to a greater or lesser degree) to maintain a constant internal environment despite changes in their external environment. All living things

• are composed of one or more cells • grow via enlargement and/or division of cells • have an organized structure (anatomy) • acquire and use energy to build up and break down components (metabolism) • respond to internal and external stimuli • maintain an internal environment (homeostasis) • reproduce (asexually or sexually)

Physiology can be studied in any living organism, but in this course we will focus on animal physiology.

What is an Animal? In addition to the seven properties of life above, animals

• multicellular • are ingestive heterotrophs • store energy as glycogen (short-term) or fat (long term) • have Hox genes that determine the identity of developing body segments • have characteristic embryo development • have a nervous system • have a muscular system

Modern animals are the products of successful ancestors, and their populations are still evolving.

Animals must obey the laws of physics and chemistry, but they also can take advantage of those laws to maximize their physiological performance.

The Big Questions in Physiology: Proximate and Ultimate Mechanism: How do animals carry out their functions?

What molecules, cells, tissues, organs, etc. are involved in a particular physiological function, and how do these components make the function work?

example: HOW does a firefly produce light?

Origin: Why do animals have the mechanisms they do?

What are the reasons--ecological and evolutionary--that a particular physiological function exists? example: WHY does a firefly produce light?

1. To attract a mate (ecological reason)

(Click on the photo for firefly fun)

(Did you know that fireflies are disappearing?)

2. Because producing light was adaptive for the ancestors of modern species that use this mechanism (evolutionary reason).

Recall that natural selection is the evolutionary process by which genes encoding traits that increase an individual's likelihood of survival and reproduction increase in frequency in populations over generations. An evolutionary adaptation is a trait (including a physiological mechanism) that has been produced by natural selection. The adaptive significance of any particular trait is the reason why the trait is beneficial to the species in which it occurs (why it was naturally selected). example:

Luciferin-driven light production in fireflies is an adaptation. The adaptive significance of light production is the ultimate reason for natural selection: reproduction. It helps the fireflies acquire mating opportunities.

Mechanism and Adaptive Significance are Distinct You may know the mechanism of a physiolgical trait, but not know its adaptive signficance. Similarily, you may know the adaptive significance of a trait without understanding how it works (its mechanism). Many animals have arrived at very different mechanistic solutions to solve the same adaptive "problem" resulting in convergent evolution.

Note that animals will often use similar "raw materials" to solve the same ecological/evolutionary problems. This is because structures do not evolve de novo, fully formed. Evolutionary adaptations are the result of "repurposing" materials and structures that are already present and being used for other purposes.

"Nature is a tinkerer, not an inventor." -- Francois Jacob example: The crystallins that comprise the lenses in animal eyes evolved from chaperone proteins whose original function was to prevent denaturation of companion proteins during high temperature stress.

Foundations of Animal Physiology Over the course of this semester, you should keep in mind several major concepts which will be described here. Ever-changing Building Blocks Like all living things, animals are structurally dynamic. Throughout its life, the atoms that make up an animal's body are in constant, dynamic flux with those in their environment, swapping out constantly. Because of this, we can track the movement of atoms through the body of an organism and document where and how long they stay in the body, and--hopefully--what they do while they're in the body. The basis of this structural dynamism: Homeostasis Upon hearing the term environment most people automatically envision surroundings. But most of an animal's cells are surrounded by its other cells, and do not directly contact the external environment. Most cells, constantly bathed in bodily fluids and fed by the blood, experience only the internal environment (or milieu interieur, as it was first described by 19th century physiologist Claude Bernard): the conditions experienced by cells inside the body. This includes

• temperature • pH • water content • ion concentrations • etc.

An animal may keep some of these conditions of its internal environment distinct from its external environment, but not others.

Different species have evolved different abilities to control specific internal conditions, depending on the natural selection pressures to which they have been subjected. The maintenance of constant conditions in the internal environment is termed homeostasis, a term coined by Walter Bradford Cannon, though the concept was already nascent in Claude Bernard's work. Feedback: Key to Homeostasis A controlled variable is any parameter that must be kept constant in an animal's body.

example: body temperature The set point of the controlled variable is the level of that variable necessary for normal function.

example: 98.6oF for a human Feedback regarding the controlled variable tells the animal whether the variable needs to be regulated in some way.

example: If a human's environment is so cold that it loses heat and starts to go below the set point, the human will burn calories and shiver to maintain the set point.

Feedback from a controlled variable is facilitated by communication between body systems. • negative feedback - the system responds to changes in a controlled variable by bringing the variable back to its set point.

Negative feedback is the most common in living systems, and is nearly synonymous with homeostasis itself.

The basic chain of events:

• A stimulus produces a change to a variable (the factor being regulated). • A receptor monitoring the controlled variable detects the change. • Input travels away from the receptor to the... • control center, which determines the appropriate response. • Output from the control center travels to the... • effector, which delivers the response as directed by the control center.

For example...

• positive feedback - the system reinforces changes in a controlled variable. Positive feedback is relatively rare in living systems, though it is vital to certain functions such as

• nerve impulse transmission: a relatively small change in voltage across a neuron's plasma membrane will change the membrane to facilitate greater voltage change and the initiationof an action potential.

• mammalian birth: muscle contractions of the uterus stimulate hormonal signals that increase the intensity of the muscle contractions to expel the fetus.

Regulators vs. Conformers An organism's ability to survive extremes of its environment reflect its evolutionary history. Reproductive success is affected by ability to metbolically meet environmental challenges • regulators use metabolic means to maintain homeostasis in response to environmental changes. (e.g. - homeotherms, endotherms) • conformers are less able to metabolically maintain homeostasis. Their internal environment is governed primarily by the external environment. (e.g., poikilotherms, ectotherms) Example: osmoregulation • anadromous (fish that migrate from salt to freshwater habitats annually) and catadromous (fish that migrate from freshwater to marine habitats annually) maintain constant salt balance in their tissues via their renal systems, even when their environments vary. They are regulators

• Echinoderms, entirely lacking an excretory system, are strictly limited to marine environments. Their tissues have the same salinity as sea water. They are conformers

Regulation is more energy expensive than conformity. Regulation requires that an animal work against entropy to maintain a distinction between its body and its environment.

The Time Dimension of Physiology Physiology can be considered to change across five different time frames. Changes in response to external environment:

Acute changes Short-term changes in physiology in response to environmental changes. (An extremely sudden change is said to be peracute.) Acute changes are reversible.

Chronic Changes Long-term changes in physiology of an individual animal that take days, weeks, or months in response to environmental change.

• acclimation - long term adjustment to one or very few highly defined stimuli (lab situation)

• acclimatization - long term adjustment to a changed natural environment. Animals undergoing acclimation or acclimatization are exhibiting phenotypic plasticity. Chronic changes are reversible.

Evolutionary changes Changes in gene frequencies over the course of multiple generations exposed to new environments.

Changes internally programmed, irrespective of external environment:

Developmental changes Changes in physiology as an animal changes from an embryo to a juvenile to an adult and then to a senescent adult.

Biological Clock changes Changes that occur in repeating patterns on a daily, monthly, seasonal, annual, etc. basis due to internal biological clocks.

Size Matters Body size has a profound effect on an animal's physiology. In evolutionarily related species, many traits and physiological processes vary in predictable ways with body size.

Because these animals can be considered as "scaled up" and "scaled down" of an average body size, the study and description of the relationship between physiological processes and body size is known as scaling. Knowing the relationship between body size and a particular physiological trait can allow one to identify species that have specializations with respect to that trait. This is usually calculated via ordinary least squares regression. In the figure above, the Gray Duiker has a shorter gestation than predicted, whereas the Mountain Reedbuck has a longer gestation than predicted.

This allows the physiologist to ask that all-important question...WHY?

Environment and Limits of Tolerance An environment is the collective physical, chemical and biological component's of an organism's surroundings. An environment cannot be defined without considering the organism. Environments are relative.

• At the moment, this classroom is your environment. • When you step outside, that will be your environment. • You are currently the environment for a host of smaller organisms. • Try not to think about this too much.

Abiotic factors that affect animal physiology include

• temperature • oxygen levels • water and humidity • ionic composition and concentrations

Tolerance to various environmental challenges among species varies, and reflects millennia of natural selection. Animals' tolerance to environmental extremes affects their biogeography (i.e., where they live).

This figure shows that Swallowtail Butterflies are more diverse in warmer regions. (This may have to do not only with their temperature tolerance, but also with a reduction in flowering plant diversity and availability at colder latitudes.) Beyond specific levels of any given factor, a lethal range exists. This may differ significantly across species. Outside the lethal range, an individual organism may be able to adjust its metabolism or behavior to tolerate a challenging environmental stimulus. Eury- vs. Steno- • Organisms that tolerate a wide fluctuation of a given environmental parameter are known as eury-(parameter). • Organisms that do not tolerate wide fluctuations of a given environmental parameter are known as steno-(parameter) Examples:

• euryhaline - able to tolerate wide range of salinities

• stenohaline - can tolerate a relatively narrow range of salinities

• eurythermal - able to tolerate a wide range of temperatures

• stenobathic - can live at only a relatively narrow range of aquatic depths

• eurytopic - able to survive in a wide variety of habitats

...and so on.

Microenvironments A microenvironment or microhabitat is a small, specific area within a larger habitat. Its environmental conditions (light, moisture, pH, temperature range, etc.) combine to form a microclimate that may be quite different from that of the larger environment in which it exists.

The existence of microenvironments can increase the species diversity of an ecosystem, providing habitat for species with different environmental tolerance levels.

The horse and cowboy would experience temperatures ranging from 7oC (winter) to 50oC (summer). The Kangaroo Rat would experience temperatures ranging from 15oC to 32oC all year round. Animals Modify Their Environments Some animals actively change their environments in ways that can be subtle to dramatic.

• moving to a different microhabitat if the current one is not suitable • creating a microhabitat (burrow, house, etc.) • changing the nature of the microenvironment by its mere presence

o a mouse warming a semi-insulated burrow with its own body heat o a fish depleting the oxygen of a limited space such as a tide pool

The investigator taking physiological measurements must take such things into account.

Not All Traits are Adaptations Recall that natural selection is only one mechanism by which evolution can occur. An equally important process is genetic drift, which can result in populations having distinctive traits simply due to random sampling error. Consider variation in human populations of...

• eye color • skin color • body morphology

Are these necessarily a result of natural selection? Physiologists attempt to determine whether traits are adaptations in several ways.

• comparative studies of a particular function in closely and distantly related species • breeding laboratory populations under specific conditions for many generations • examining individual variation in a single cohort • inducing variation in a controlled population

o knockout technology o RNA interference

• population genetics • phylogenetic reconstruction (to determine when a trait appeared)

Evolution of Adaptations Depends on the Gene Pool Remember: Organisms do not evolve traits because they need them.

Traits can evolve in a population only if there is existing genetic information that can change and allow them to evolve.

And evolution can happen only if there is genetic variation within a population. Plenty of data exist to show that there are physiological "personalities" in various species' populations, from Deer Mice to Humans.

Thus, from the war of nature, from famine and death, the most exalted object of which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been

originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

--Charles Darwin And so our journey begins.