How are the Animals Doing?

Climate change, human-wildlife interactions, and loss of habitat are altering our world. Such changes also impact animals and influence their behavior, population sizes, and physical health [1]. These changes can ultimately lead to the extinction of animals [1]. To understand how to help animals, scientists and veterinarians need to understand what is going on inside their bodies. However, wild animals cannot directly tell us how they are doing, and getting them to the veterinarian is often difficult. Instead, scientists can use traces left behind by animals to understand their health. These traces, like hair, blood, poop, and even earwax, contain chemicals, such as hormones, that help us understand animals, even when they cannot tell us what is going on with them.

The Bloodstream Works as a Highway

The circulatory system, led by the heart, pumps blood throughout the body. Blood contains many things, like oxygen-carrying red blood cells and disease-fighting white blood cells. These cells allow organs to function and help the immune system to fight off potential illness. Blood also contains hormones—chemical messengers released from organs and glands into the bloodstream. The bloodstream acts like a highway for hormones, allowing them to travel to other parts of the body (Figure 1). Once they have reached their intended destinations, hormones then interact with cells, tissues, and organs if those structures have a specific receptor for that hormone. Simply put, the hormone acts as a key and the receptor acts as a lock; only the right key can fit! Hormones bind to receptors, and this action causes a cascade of changes. Effects of hormones vary by situation but can affect the whole body through the release of other hormones and changes in which proteins the body’s cells make. Once hormones bind to their targets, hormones are broken down into metabolites and removed from the body.

Hormones are critical to life functions. For example, the hormone melatonin helps regulate the body’s internal clock; cortisol and corticosterone help with energy use and can indicate responses to stress; and progesterone is associated with pregnancy [1, 2]. Much like Goldilocks and the three bears, hormone levels need to be “just right”—too much or too little hormone may keep the body from functioning properly. Because hormones cause a cascade of changes in the body, consequences of high or low hormone levels may include increased susceptibility to illness or failure to reproduce [2]. To determine if hormone levels are too low, too high, or just right, scientists and veterinarians must also look at other symptoms, like diseases or changes in behavior.

How can we Find Hormones?

A doctor or veterinarian may do bloodwork on their patients to check hormone levels as part of a health exam. For example, if you are feeling unusually tired, your doctor may look at your thyroid hormone levels to see if your thyroid is functioning properly. Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormones. Hypothyroidism can also occur in animals, like a pet dog! Hormone testing from blood work can provide evidence of hormone problems and allow doctors and veterinarians to give patients the proper treatment.

However, scientists can also find hormones in other things that animals leave behind. This is particularly helpful if animals are rare or difficult to locate, if not enough blood can be safely collected, or if animals are very sensitive to human contact. Hormones can be found in keratinized materials like nails and hair [3]. As these structures grow, hormones are deposited in them from the nearby blood supply by passive diffusion (Figure 1) [3]. Hormones in their metabolite form can also be passed into excretions like feces (poop), urine (pee), and saliva (spit). For critters like frogs, scientists can even use the water they live in to measure hormones from the animal. Egg-laying moms also deposit hormones into their eggs, which can reveal a lot about both mom and baby. Scientists can use the same hormone analysis methods, regardless of whether the sample is blood, feces, or feathers. These different kinds of samples provide specific “snapshots” of hormone levels (Figure 2). For example, a blood sample represents hormones at the exact second of blood collection, a poop sample represents hormone levels within the last hours or days, and a nail sample represents hormone levels within weeks or months.

How Can Hormone Knowledge Help Us Conserve Animals?

When scientists collect samples from animals in the wild, they take these samples into the lab to measure hormone levels. By studying hormone levels, scientists can infer various things about the animal and the effects of its environment (Table 1). Hormone levels can reveal if animals may be affected by certain types of stress in their environments. The type of information gathered varies by animal and sample, but scientists have used this method in several ways to understand animal health. Knowledge of animal health can help scientists and conservationists to plan continuing conservation efforts and can alert scientists that animals may need help [2, 5, 7].

While this article discussed how animal traces are used to measure hormones as a way to understand animal health, scientists can also measure other things animals leave behind. For example, animal poop spreads seeds and provides nutrients for other organisms; microbes in tapir poop helped scientists understand forest health; and monarch butterfly wings helped scientists understand if parasites were related to wing deformities.

In summary, human impacts are changing the planet and affecting wildlife. Measuring hormones along with other indicators of individual and population health helps scientists understand how animals are coping in their environments. Hormones can be collected directly from the bloodstream or from the traces that animals leave behind, like scales, feces, and hair. Hormone research has helped improve the welfare of both wild and captive animals and it gives us information about which animals need our protection and how we can conserve them.

Glossary

Hormone: ↑ A molecule produced by a cell or gland and released into the bloodstream, acting as a chemical messenger and causing changes in the body.

Circulatory System: ↑ The system that moves blood throughout the body.

Metabolite: ↑ The leftover product of a hormone after it is broken down in the body.

Hypothyroidism: ↑ Condition that occurs when not enough thyroid hormone is produced in the body.

Keratinized Materials: ↑ Body structures that contain a substance called keratin, which is part of nails, hair, scales, and feathers.

Passive Diffusion: ↑ The movement of a substance, like a hormone, across cells or tissues without the need of energy, a special transporter, or regulation.

Excretions: ↑ Substances produced to eliminate wastes from the body, such as feces (poop), urine (pee), or sweat.

Acknowledgments

We thank Dr. Virginie Rolland, a Young Reviewer, and the Science Mentor for constructive feedback on previous drafts of this manuscript.

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Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.