Hormones are signals that help control body functions, such as stress or pregnancy. Different hormones are responsible for different aspects of physiology. For example, in humans and many other mammals, the hormone cortisol is released during stress and the hormones testosterone and progesterone function in fertility. These hormones usually travel around the body in the blood, but also can be deposited in the hair, feathers and nails of animals, which can tell the story about what has happened in the animal's body, sometimes several years in the past. One way to explore the stories that hormones can tell us is to study animals in zoos because they are already undergoing routine medical examinations and are trained to voluntarily participate in their medical care, such as having their nails clipped. Garrett Rich, a researcher at George Mason University in Virginia, USA, collaborated with colleagues at the Maryland Zoo in Baltimore, and the Smithsonian's National Zoo, both in the USA, to investigate hormones in elephant toenails and see whether nails can show researchers what is happening inside the elephants’ bodies.
To do this, Rich used toenail clippings taken from seven captive elephants (five females and two males, three of which were African elephants and four Asian elephants) over a year of medical examinations. Rich was interested in the hormones that indicated whether the animals were producing stress hormones and hormones associated with fertility, so he measured the cortisol, testosterone and progesterone levels in the dried and ground-up toenails with tests used to measure hormones in the tissues of other mammals. As this was the first time researchers had investigated these hormones in elephant toenails, Rich had to make sure the tests were measuring the hormone concentrations accurately. All the tests worked well, except for the testosterone measurement in male African elephant toenails, but Rich concluded that this was only a minor problem and likely because other hormones may be interfering with this test.
Another important part of Rich's study was to figure out how fast elephant toenails grew, so that researchers could estimate how long ago hormones entered the toenail clippings. Rich's colleagues, who were caring for the elephants at the zoos, measured the elephants’ toenail growth by marking the top of a toenail (near the skin) and measuring how far the mark travelled down the nail every 2 weeks. Interestingly, the team found that Asian elephant toenails grew a little faster (about 0.24 mm day−1) than African elephant toenails (about 0.18 mm day−1). The team then combined these growth rates with the toenail hormone concentrations that they had measured to create a year of hormone estimates for each elephant.
For the most part, cortisol increased in the toenails when fertility hormones increased, and decreased when fertility hormones declined, which makes sense as reproduction can be stressful in some animals. However, because the cortisol levels did not change drastically in the toenails over the year, Rich suggests that elephant toenails may be a better indicator of long-term rather than short-term stress. The team also noticed that testosterone increased in the toenails of male elephants during periods of musth, an elephant behaviour when males are ready to mate or become aggressive, which supports that toenails could tell researchers when elephants have high testosterone and may have previously been ready to reproduce. In contrast, females’ toenail progesterone levels were not as clear and only one female had high toenail progesterone levels that matched when the female became fertile; thus, more research is needed to figure out whether toenails can accurately measure elephant progesterone.
Overall, collecting elephant toenails for hormone measurements is an exciting step forward for elephant conservation, in both zoos and the wild. Rich proposes that toenails could be collected from wild elephants if they are tranquilized during field studies to look at stress or fertility, or from deceased elephants in museum collections to measure past hormone levels. In zoos, toenails can be collected during medical examinations and stored for years, giving researchers access to elephant hormone measurements taken over a very long time. These historical hormone concentrations, along with information about an elephant's life, may be able to show what type of events impact elephant stress or fertility, which is very important for the management of elephant populations in the future.
