Two classes of hormones drive different responses to help vertebrates survive short‑ and long‑term stressors.

Introduction

A cat’s claws come out. A deer bolts across a field. An average human performs an extraordinary feat of strength to rescue someone in immediate danger. We are generally well versed in the stress response known as “fight or flight.” But what do we know about vertebrates’ other major stress response: the emergency state?

Osprey versus Eagle
Image: Andy Morffew / CC BY - Creative Commons Attribution alone

An osprey’s stress response has driven it to fight to retrieve a fish that a bald eagle has stolen.

Deer in captivity
Image: Kristijan Arsov / Public Domain - No restrictions

A deer peers through a fence perhaps feeling an emergency stress response due to captivity.

Cheetah’s hunting
Image: Antonio Marín Segovia / Public Domain - No restrictions

The unseen animals these cheetahs are chasing are likely experiencing “fight or flight” stress response.

The Strategy

In fight or flight, sensory organs hear a warning noise or detect danger and send impulses to the brain that scream, “Help!” The brain fires nerves connected to adrenal s located in the upper portion of the s. Within a few seconds of sensing the danger, these glands pump s into our blood.

Epinephrine (adrenaline) and norepinephrine course into our bloodstream. The hormones dock onto receptors dispersed on tissues throughout the body, unlocking various functions that ready us for battle or departure. Pupils dilate, improving vision. The heart races, blood pressure rises, and breathing rate quickens, increasing oxygen intake and distribution. Blood vessels to noncritical organs constrict while those around the brain and muscles dilate, diverting blood to where it’s needed most. These hormones also cause muscles to tremble, priming them for action. If an animal survives the immediate attack, the fight or flight response abruptly ends, and the body resumes normal function within a couple of minutes.

Longer-term stressors like severe weather, loss of a mate, habitat loss, or exposure to pollution cause the second type of stress response, putting the body in a state of emergency. The emergency response is a bit slower, taking from minutes to hours to ramp up. Also, the effects are longer-lived, lasting from hours to weeks, depending on the duration of the stressor.

As with fight or flight, hormones drive the emergency response. In this case, the brain sends signals to a different region of the adrenal glands, releasing a group of hormones called glucocorticoids. In fish and mammals, glucocorticoids consist primarily of cortisol, but in reptiles, birds, amphibians, and rodents, the main glucocorticoid is corticosterone. These hormones generally help regulate production, immune system function, and reproduction. They also affect migrating and feeding behaviors.

When a stressor increases glucocorticoid production, the emergency response shifts resources from long‑term investments to survival mode. 

When a stressor increases glucocorticoid production, the emergency response shifts resources from long-term investments to survival mode.

Hallmarks of the emergency response include a suppressed immune system and suspended reproduction. Animals in this state might seek refuge if “the storm” can be ridden out, or they might move to another area if resources are too scarce. Once the stressor subsides, the body resumes normal function.

In some cases, stressors are sustained over weeks or months, which prolongs elevated glucocorticoid levels, causing chronic stress.

Animals taken into captivity for farming, research, conservation, or other purposes may experience chronic stress due to separation from their natural habitat or due to forced interaction with humans. A 2018 study concluded that symptoms of chronic stress caused by captivity varied among species, but it found that weight loss, changes in the immune system, and suppression of reproduction were all common. If chronic stress occurs over long enough periods, animals may become ill or die.

The Potential

Studying the stress response in animals, especially chronic stress, could help us design fish farms, zoos, and research facilities to reduce animal stress. Larger cages, natural lighting, and appropriate temperature can all mitigate chronic stress symptoms. Considering behavioral needs such as social interactions and stimulating natural activities can also reduce chronic stress in captivity. Animals provide us much. Learning to reduce their stress in captivity can go a long way toward making our interactions more mutually positive.

Additionally, we humans are animals like any other, and much of our modern lifestyle involves conditions that are known to activate the emergency stress response: constrained quarters, separation from the natural environment, persistent questions about security or stability. The stress response is nature’s way of drawing attention to harmful conditions. Heeding its message could help us to create better living conditions for people in all levels of society in all sorts of environments, including shelters, office buildings, schools, or apartments, and even lead to better rehabilitation of those in prisons or recovery in hospitals.

Last Updated February 16, 2021