Let's talk about dew.
On crisp spring mornings, you often find your lawn coated in it, but by noon, much of it has evaporated. Similarly, upon driving to work, you crank up the warm defrost to quickly disrupt the condensation on your windshield. Or - my personal favorite - you feel a brisk, cool gust of wind as you watch a dark thunderstorm approach.
These events highlight a pervasive interaction between temperature and humidity that we deal with every day; yet surprisingly, many people (including scientists) take it for granted.
The principle underlying all of these phenomena is that warmer air can hold more vapor. But what does that mean exactly? It means that as the air warms, more and more water molecules can remain as vapor - a gas. Conversely, as the air cools, the air loses its capacity to hold on to vapor, and the vapor condenses into water on your windshield or glass of water. In fact, the amount of vapor the air can hold increases exponentially as the temperature of the air warms (see figure above).
But what does this have to do with physiology? Many amphibians, like the salamanders that I study, depend upon wet skin to breath. Because of this, they often live in moist environments (however there are some exceptions). And as we know, moisture occurs more readily with cooler temperatures. To leverage this principle, many salamanders become active at night - when it is cooler. By limiting their activity to dark, cool nights, their skin remains wet and permeable to oxygen and carbon dioxide. Even though salamanders (probably) can't 'dew' the computation, they certainly live within the mathematical rules.
But you can understand the math. And when you see dew forming on your glass of ice water, you can thank salamanders for helping you to see that the ice in your glass cooled the thin layer of air surrounding your cool drink resulting in vapor condensing onto your glass.
So a round of applause for salamanders.
Don't panic - it's OK, just like your epidermis, your boundary layer has been showing for quite a while.
So what exactly is a boundary layer? In a sense, your boundary layer is where you interact with the air (or water) that surrounds you. It isn't there to help or hurt you, but rather, it is a consequence of the characteristics of your body (such as body temperature and the amount of moisture on your skin) as well as the characteristics of the air that surrounds you.
In fact, on a warm summer day, you can see the boundary layer between the hood of car and the air that surrounds it. The air surrounding your car is warmed by the hot hood, and being warmer than ambient air, the air rises and mixes the cooler ambient. This area - where the air mixes - is a boundary layer.
For amphibians, the boundary layer is an important barrier between their skin and the environment. This is especially important for lungless salamanders because they maintain moist skin in order to breath. However, by being leaky, salamanders risk drying out. Under certain conditions, the boundary layer can be (relatively) thick and play an important role in determining how quickly they dry out. The longer it takes the dry out, the longer a salamander can walk the forest floor in search of bugs to eat and a mate.
But how do you measure a boundary layer? As you can see below in the figure, it becomes ugly quickly. For most people, delving into dimensionless analysis is not necessary. But understanding boundary layers can help you make sense of the world around you.
Now go bundle up for the cold weather - and keep your boundary layer close.
What is physiological ecology? And why should I care?
Fortunately for me, these questions are easy to answer, and I will begin with an example.
Since the mid 20th century, the agricultural industry used dichlorodiphenyltrichloroethane (DDT) to control crop-hungry pests. In the following decades, bird populations dramatically declined, prompting Rachel Carson to write The Silent Spring in 1962.
Unbeknownst to many, DDT interrupted a vital protein, calcium ATPase, that is responsible for transporting calcium in organisms. Without calcium, bird eggs became soft and were crushed under the weight of adult birds trying to keep their eggs warm.
As a silver lining, The Silent Spring inspired policies to ban DDT in agricultural practices, and the story also underscored the link between physiology and ecology. Specifically, DDT changed how birds function (physiology) thereby influencing the abundance and distribution (ecology) of many avian species.
More recently, the collapse for bee colonies has been linked to an abundance of physiological mechanisms, including stress, exhaustion, and pesticides. But the link between these two fields is nothing new. Joseph Grinnell (1917) pointed to physiological tolerances as one of the mechanisms that determines why plants and animals live in some habitats but not others.
As environments change, so must the organisms that live there. Physiology provides a way to understand why nature changes and predict how life will change in the future.