Bring on Spring! Animal physiology is as transformative as our seasons

Physiology is the study of how animals work and perform everyday functions like breathing, walking, and maintaining a normal body temperature. In many species, animal physiology responds to environmental conditions including the amount of available water, the temperature, and the time of year. A general rule is that when the environment changes then animal physiology responds.

It is important to note that changes in physiology and behavior often occur together. For example, a bird readying itself for migration will eat a larger amount of food in order to double the size of their fat stores that are crucial for providing fuel during long-distance flight. This behavior would be impossible if their physiology was inflexible, but luckily they remodel their digestive system to cope with processing all of this food.

An alternative tactic that animals use to fit into their environment is that they change only their behavior so that they experience a constant set of environmental conditions, and as a result they avoid changes in their physiology. Aquatic turtles use this strategy to keep their body temperatures fairly constant by switching between basking themselves in the hot sun and plunging themselves in the cooler water.

Whether or not animals change their physiology, their behavior, or their physiology and behavior depends on the species and on their environmental situation. For this post I'll summarize the types of changes in physiology that this blog will focus on.

An unfrozen wood frog ( Rana sylvatica ) found by a biology undergraduate student in Massachusetts, September 2017,  Photo credit: Kristen DeMoranville

An unfrozen wood frog (Rana sylvatica) found by a biology undergraduate student in Massachusetts, September 2017, Photo credit: Kristen DeMoranville

Physiology can change on a short-term timescale soon after their environments have changed, and these changes are reversible. This happens to us. Standing outside on a cold winter day in thin mittens, it only takes about 30 minutes to notice our hands are cold and beginning to hurt. In this situation, our blood flow has been rerouted to our center to keep our important organs warm. During the winter in North America a specific species of frog, the wood frog (Rana sylvatica), buries themselves beneath the soil in preparation for freezing temperatures. These frogs are unable to keep their body temperatures high enough so that their organ systems could properly function. These frogs embrace that shortcoming and allow their bodies to freeze nearly solid immediately as ice forms around them in order to survive the cold. Special physiological adaptations protect their organs during this freeze and help them to completely recover as soon as the temperatures warm and ice disappears.  The Dr. Richard Lee & Dr. Jon Constanzo lab investigates the physiology of these amazing frogs. An overview of Dr. Lee & Constanzo's research can be found here.
Spring is here and wood frogs are emerging from their frogsicle forms NOW! If you are in Canada or eastern North America then keep your eyes peeled and ears open.

A banded male North American cardinal ( Cardinalis cardinalis ) in Rhode Island, April 2018,  Photo credit: Steve Brenner

A banded male North American cardinal (Cardinalis cardinalis) in Rhode Island, April 2018, Photo credit: Steve Brenner

Animal physiology can change on a long-term scale either days, weeks, or months after their environments have changed, and these changes are reversible. We can relate to this too. Taking a long walk on the first scorching day of the summer we feel drained. After experiencing this heat day after day it seems easier to take this same walk in mid-summer. Our bodies adjust to the heat with repeated exposure, and as a result we can better endure hot weather. Songbirds that stick around for cold winters rather than migrate to tropical regions have to change their physiology so that they can stay warm enough to properly function and survive. Winter resident birds like the Northern Cardinal (Cardinalis cardinalis) keep warm by enhancing their ability to generate heat through a tactic similar to mammalian shivering, but without the muscle trembling that we experience. They also increase their fat and feather layers in the winter to improve insulation which is crucial for keeping that extra heat that they produce inside their bodies. These physiological adjustments do not happen after the first freeze or even with the first snowfall, rather it takes weeks at cold temperatures for birds to transform their bodies into fat, fluffy, heat generating machines. The Audubon society covers how birds stay warm in more detail here and Dr. David Swanson's lab focuses much of their research on cold hardiness.
Spring is here, and despite the recent snow, cold hardy birds like the Northern Cardinal and Black-capped Chickadee (Poecile atricapillus) are losing fat and shedding fluffy feathers in preparation for the breeding season.

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A hibernating 13-lined ground squirrel (Ictidomys tridecemlineatus) in Dr. Jim Staples lab at the University of Western Ontario, December 2015, Photo credit: Kristen DeMoranville

Animal physiology changes in repeating patterns either daily, monthly, or yearly under the control of an animal's internal biological clock (defined as the bodily components that keep track of time within an animal). Whether you are a night owl or an early bird, our nightly sleep habits are controlled by our biological clocks. Read more about how our biological clocks control sleep here
Mammals like the 13-lined ground squirrel (Ictidomys tridecemlineatus) are adapted to hibernate during harsh winters when temperatures are below freezing, days are short, and food is unavailable. The internal clocks of these mammals filter through the environmental cues (e.g., temperature, light levels, and food availability) to control physiology and make hibernation possible. Ground squirrels lower their body temperature (and actually feel cold to the touch! See above picture) and slow their metabolism* (defined below) so that they are barely using or producing any energy. This means that their heart rates drop from 200 beats per minute to 20 beats per minute. Their biological clocks use similar environmental cues to stimulate their metabolism and rouse them from hibernation. National Geographic gives more overview about hibernation here, and Dr. Jim Staples investigates the metabolism of hibernating ground squirrels.
The onset of Spring is inciting hearts to flutter and, as Owl would explain to Bambi and Thumper, squirrels are twiterpated. If you live central North America then go searching, and don't forget to count their lines.

Animal physiology responds to environmental changes either on a short-term scale, long-term scale, or periodically in a repeating pattern. These changes in physiology can be difficult to observe firsthand. Although, behavioral changes can often act as a flag that alerts us to unnoticeable changes within an organism. Next time that you observe a behavioral change in one of your favorite critters I challenge you to think about the types of physiological adjustments that your animal might require to make that behavioral change possible.

*Metabolism is a concept we will continue to revisit. This is how I like to think of the concept: All organisms require energy to perform their daily activities, and this so called fire for life is the product of chemical processes collectively referred to as an organism's metabolism.


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About the author:
Kristen J. DeMoranville @Kris10DeMo is a Ph.D. student researching the effects of diet and long-distance flight on a migratory songbird in Scott McWilliams lab at the University of Rhode Island