- Scientists commonly analyze the stress hormones found in scat to evaluate the impact of environmental disturbances on animal health — looking, for example, at the effect of logging on owls, ecotourism on mountain gorillas, and shipping traffic on right whales.
- One past assumption held that stress hormone levels found in individuals are representative of their species. But findings show that glucocorticoid (GC) stress hormone levels can vary due to factors such as sex, age, and reproductive phase.
- Julia Greenberg is using 20 years of data collected by the Mara Hyena Project to test the limits of fecal GC stress level measurements, and look for long-term patterns. She wanted to know: Could GCs be used to show general stress trends in an animal population to help improve wildlife management planning?
- So far, she has found that individual hyenas, with high fecal GC levels early in life, don’t live as long. On the other hand, she could not correlate early high GC levels with reproductive success later in life.
Poop: picking it up and studying it is one way scientists determine stress levels in animals.
But it is not an ideal tool: “I quickly learned that interpreting fecal hormone measures is a lot more complicated than it first appeared!” says Julia Greenberg, a PhD candidate in the Zoology Department of Michigan State University, in East Lansing. She is studying stress hormone levels in African spotted hyenas.
Without getting too close for anyone’s discomfort, scat can be analyzed for glucocorticoids (GCs), the hormones released by the adrenal glands in response to life’s many pressures. In the short term, this hormone boost is considered good — it helps raise blood sugar levels and stave off muscle fatigue during “fight or flight” events.
But chronically elevated glucocorticoid levels, raised by constant low levels of stress, can weaken the immune system and wear the body down; an effect seen in humans suffering from Post Traumatic Stress Disorder. In wild animal populations, this chronic response could shorten an animal’s life span or reduce reproductive success.
Ideally, tracking hormone levels in scat gives researchers a noninvasive way to evaluate the impact of stress on a wide variety of species. This information can help conservationists assess the effect of human disturbances or other stressors on animals. Studies that have measured stress hormones in scat, have looked at the impact of logging on spotted owls, ecotourism on mountain gorillas, and shipping traffic on right whales.
But the technique isn’t foolproof. Not every individual or species expresses stress the same way.
“There was a point when everyone started using this powerful tool, and then maybe ten years into it, we see there are some issues with it,” says Greenberg. “We still haven’t been able to use stress markers to figure out general trends across an entire species.”
Yet scientists and wildlife managers continue investing a lot of time and money in monitoring species’ stress hormones. Greenberg wondered: Is there a way to use these tests to project population trends, in order to develop better wildlife management plans?
Mongabay: Why monitor stress hormones in spotted hyenas?
Julia Greenberg: My interest was piqued by their response to encroachment from increasing cattle grazing relative to other large carnivores in Kenya. Compared to lions and cheetahs, the spotted hyenas (Crocuta crocuta) haven’t had a big demographic drop. I wanted to know how they managed to adapt in this changing world.
I bet that the hyenas’ really long developmental period had something to do with shaping their flexibility [and resilience]. The pups don’t wean until 14 months, and they aren’t hunting proficiently until five years of age. We think that long time with their mom is because their powerful jaws take years to develop and, until then, they need mom to defend them at highly competitive carcasses and kills.
The hyenas are also long-lived. In our study clan, Navajo is the longest-lived hyena on record, at 24 years of age. With our long-term data set, GCs are a fundamental biomarker that I thought could help me look at the fitness implications of elevated stress hormone concentrations.
I also thought — naively — this is lab work, so everything will be straightforward: I’ll have really objective biological measures… but I quickly learned that interpreting fecal hormone measures is a lot more complicated than it first appeared.
Mongabay: How did stress hormones relate to hyenas’ survival and reproductive success?
Julia Greenberg: First, I had the humbling privilege of opening a data set that spans almost 30 years. The Mara Hyena Project started in 1988 at the Masai Mara National Reserve in southwestern Kenya with Kay Holekamp, professor in the Department of Integrative Biology at MSU [Michigan State University]. Since then, many people willing to observe and pick up these animals’ poop have worked on the project.
Every day, we’ve followed individuals [in the field] by their spot patterns, ear damage or collars (using telemetry or GPS). So we have a good understanding of the relationships and the strictly matrilineal social hierarchy of the clan. The power of these long histories means we can relate individual behaviors to the hormone levels in the feces we collect. It’s only now, after all these years that we can start to see how these factors play out in a long-lived species.
With scat collected from 1993 to 2013, we looked specifically at individuals where we had samples from the first year of their life. The hypothesis was that early behaviors and physiological functions would have a more pronounced effect on who later lives or dies. That time period is also when we lose about 50 percent of individuals to mortality.
I correlated stress hormone levels with how long each hyena lived, and which females had at least two litters. We found that individuals with high fecal GC levels early in life don’t live as long. On the other hand, there was no relationship with reproductive success. In this study, these values don’t appear to be useful for predicting cub numbers, but may help managers assess individual longevity.
Mongabay: What’s next?
Julia Greenberg: Another factor that we often assume is true is that individual stress levels are a trait, meaning that GC concentrations we get on Monday will be the same on any other day. In our data set I’d like to see if these “stress metrics” are repeatable. Are there high-stress individuals? We don’t really know.
We also have the luxury of anesthetizing our hyenas to get blood tests, so we can measure glucocorticoid concentrations that way, too. Some people prefer blood samples over fecal samples. I’d like to compare patterns between the two. Then we can see whether we are missing something in studies where we can only get fecal samples.
As more studies are conducted, hopefully general trends will emerge that help us make more useful predictions with this test.
For more on the topic:
Kay Holekamp’s Hyena study blog:
Kay Holekamp’s webpage on hyena work:
Julia Greenberg’s blog