By Dr. Elizabeth Burgess and Dr. Rosalind Rolland, Anderson Cabot Center for Ocean Life, New England Aquarium
In 2010, 280 Florida manatees died during prolonged cold weather due to chronic metabolic stress, known as “cold-stress syndrome.” Since 2012, more than 150 manatees have died in the polluted Indian River Lagoon during a long-running “unusual mortality event,” the cause of which remains under investigation but appears to be related to a dietary shift following the drastic loss of seagrass in the region. Understanding the cause of these mortalities would provide key information about how insidious environmental changes are impacting manatees and their ecosystems.
Blood sampling is the traditional approach for determining the internal health of wildlife. However, the collection of blood samples typically requires capture and handling of animals out of the water, which can affect any hormonal stress signals being investigated. The elimination of hormones from the body in feces, however, provides a noninvasive means to collect valuable information on wildlife health.
With the support of a Christine Stevens Wildlife Award, the Anderson Cabot Center for Ocean Life at the New England Aquarium developed a methodology using fecal samples to assess the physiological condition of Florida manatees in the wild. Samples were collected by the Florida Fish and Wildlife Conservation Commission and the US Geological Survey during health assessments of free-ranging manatees and necropsies of carcasses. We focused on measuring levels of triiodothyronine (T3), a thyroid hormone, because it has profound influences on metabolism and is particularly responsive to nutritional deficits and extreme temperature insults. To date, we have analyzed 127 fecal samples from manatees across Florida, including apparently healthy individuals, animals who suffered cold-stress syndrome, and those who died during unusual mortality events. These data were used to establish a hormone reference database to assess the health of manatees in the future.
We found that manatees had elevated levels of T3 during spring-summer when wild populations experience heightened metabolic demands due to increased breeding activity and better food availability. Adult males, who jockey for position within “breeding herds” of a dozen or more males pursuing a single female, showed the greatest metabolic activity during this time of year. Thyroid levels were lower in winter, as manatees try to conserve their energy when ambient temperature is lower and food becomes scarcer. Manatees with access to naturally warmed artesian springs during winter had reduced metabolic activity compared to manatees overwintering in more disturbed habitats of industrial or secondary warm-water sites, such as power plant outflows. Manatees with symptoms of cold-induced stress and manatees who died during an unusual mortality event showed significantly elevated thyroid activity in winter compared to apparently healthy manatees. These findings demonstrate that manatee overwinter survival depends on good quality habitat—reliably warm waters close to robust foraging grounds—facilitating energy conservation at times when thermoregulatory challenges are greatest and food is least available.
This research demonstrates that using feces as a noninvasive technique to collect physiological data shows promise as an effective and humane way to monitor and assess wild manatee health and to aid in mortality investigations. The next innovation of this methodology will be its application to evaluate manatee health before deleterious consequences are observed (i.e., mortalities, low calving rates)—enhancing the opportunity for improved monitoring and earlier intervention to safeguard manatees in the wild.