Learning from Death

Roadkill can be a valuable resource for researchers seeking to understand the behaviors of creatures great and small.


Squished squirrels. Dismembered deer. Flattened frogs. Eviscerated elk. Mangled moose. That’s roadkill – the blood, guts, feathers, and bones of animals that fall victim to our roads and highways. Motorists swerve to avoid it. Bicyclists wrinkle their noses and hold their breath when pedaling past it. Joggers veer off-course. Rats and raptors feed on it.

photo of a dirt road, dead rabbitphoto Blake ThornberryThere are few estimates about the full toll of our roads on wild and domestic animals, but the Humane Society of the United States believes that some 1 million vertebrates die every on US roads alone.

From deer and moose carcasses in roadside ditches to bugs and butterflies encrusted on our windshields and headlights, the mind-numbing profusion of roadkill has direct implications for everything from species survival to traffic safety. Animals are vulnerable in congested urban areas in Europe where they are squeezed into smaller and smaller tracts of open lands and woodlands surrounded by city streets. They are definitely vulnerable in rural parts of North America bisected by two-lane country roads. And they are increasingly vulnerable in developing countries in Africa, Latin America, and Asia, where expanded road systems are revving up traffic density and allowing people to drive faster to more places, including through once-remote and protected areas.

Scientists have been collecting data on roadkill since at least the 1920s – back when collision victims were sometimes referred to as “flat meat” due to the likelihood that they would end up on dinner plates – but the research is pretty patchy. To this day, there’s no precise data about just how many animals fall victim to cars and trucks across the globe. What is clear is that the vast scope of the death toll is almost impossible to grasp.

Take these figures for the United States. The US Department of Transportation estimates anywhere between 1 and 2 million large animals – like deer, moose, elk, and bear – are killed every year along the country’s nearly 4.1 million miles of roadways. Accidents with large animals are the most commonly reported – they can cause injury and death to drivers and passengers, as well as substantial vehicle damage, and as a result, are more likely to prompt insurance reports. But they are far from the only fatalities. The US Fish & Wildlife Service and Smithsonian Conservation Biology Institute estimate that 80 to 340 million birds also die annually on US roads. It’s hard to find even a rough estimate about the combined toll for all animals – including mammals large and small, wild and domestic, as well as birds and reptiles. But the Humane Society of the United States has suggested that some 1 million vertebrates die every day on US roads.

And even that number doesn’t include bugs. One team of scientists calculated that hundreds of thousands of pollinating insects such as wasps, bees, and butterflies die annually along a single two-kilometer stretch of highway in Ontario, Canada. From that, they estimated the butcher’s bill at a potential hundreds of billions of pollinating insects per year in North America alone.

“It is gloomy,” says biologist David Lesbarrères of Laurentian University, who worked on the Ontario project. “We don’t have a very good application for what is out there. Yes, it could be billions, but how many insects are out there? Is it a drop in the bucket or is it an [ecological] impact?” he says, referring to the fact that insect population estimates are scarce.

But is there an upside to all these deaths? For scientists, yes. Tragic as they are, roadkill casualties can help tell important tales. Researchers from Florida to Ethiopia are turning to traffic victims as a valuable resource, seeking out those tales to devise ways to curb the spread of diseases, better understand the behaviors of creatures great and small, and even to reduce roadway mortality.

Nobody claims environmental research is easy, and studies using roadkill often require boots on the ground. But scientists gotta do what scientists gotta do, and roads provide a relatively uncontroversial source of study material. As Julie Melotti, a lab technician at the Michigan Department of Natural Resources (DNR) puts it, “Nobody’s going to complain if you’re collecting roadkill deer.”

Roadways also provide a ready-made survey method. “Roads themselves could be viewed as a sampling tool that operates continuously, depending on traffic volume, thus functioning in much the same way as do automated trail/wildlife cameras,” a Canadian-Australian research team observed recently.

But unlike cameras, using roadkill for research means getting out gathering animal carcasses. Lesbarrères has done road ecology studies for a dozen years. “I’m immune to the gruesomeness of it,” he says. “Especially after a rainy night, it’s gruesome to walk or bicycle in a relatively nice environment, but the asphalt is paved with flattened frogs and we remove them from the asphalt and find keys to identify their species and gender.”

The process can take persistence as well. For instance, Ontario researchers conducted more than 200 bicycle surveys – over four years, from 2008 to 2011 – along the 1000 Islands Parkway adjacent to the St. Lawrence River, counting dead vertebrates along a stretch that bisects an Algonquin-to-Adirondacks international conservation corridor. Some researchers get outside help. English and Welsh eco-groups collected more than 650 Eurasian otters – about 90 percent of them road-killed – put them in freezers and sent them to Cardiff University in Wales for a study of parasites. “We built a relationship with environmental organizations, as well as private individuals and conservation charities,” bioscientist Elizabeth Chadwick says of the team’s 2013 study.

What happens once animals have been collected? For a look at what researchers do with some of their accrued roadkill, photographer Tony Cepak and I visited the Michigan DNR facility responsible for monitoring the well-being and health of the state’s wildlife. The agency’s Wildlife Disease Laboratory is housed at Michigan State University’s Veterinary Diagnostic Laboratory. Bodies come in year-round and are frozen pending necropsy. “If someone finds a dead animal and wants to know how it died, they come here,” says Melotti, who showed us through the facility.

Through a second-floor observation window, we looked down into the lab where a pathology technician was pulling lymph nodes out of a white-tailed deer’s head to check for chronic wasting disease, or CWD, a contagious neurological disease affecting deer, elk, and moose. Below us, I counted two moose heads, a table with about sixteen eagles, three dead raccoons, and a cart stacked high with more deer heads. The bird family was well-represented – crow, barred owl, Canada goose. So were mammals – possum, bobcat, gray fox, gray squirrel, woodchuck. A wolf’s feet were visible as well.

“A lot of states are looking for roadkill for CWD,” Melotti says. “We’re getting a lot of young animals and fawns. They’re young and inexperienced,” and thus less wise in the ways of avoiding cars.

Chronic wasting disease, which can devastate populations of cervids such as deer, elk, and moose by causing weight loss and death, is far from the only animal disease attracting attention among researchers. There is a litany of diseases to study, watch for, and sometimes fear. Take avian botulism, a paralytic disease that birds contract by ingesting a toxin produced by bacteria found in the soil. Or bovine tuberculosis, a bacterial disease that primarily affects cattle but can also hit white-tailed deer, elk, fox, mink, raccoons, and bears. Mosquito-borne West Nile virus infects both birds and people. Then there’s the deadly white-nose syndrome that’s wiping out bats across North America. Add to that list rabies, which impacts animals like raccoons and coyotes, fungal diseases that impact snakes, and lead poisoning of waterfowl that ingest lead ammunition, and there’s more than enough to keep wildlife managers busy.

Finding ways to reduce adverse impacts of roads on wildlife is essential to species protection.

Roadkill is helping to fill in crucial knowledge gaps with respect to many of these diseases, and more. A road-killed black howler monkey recently became the first male of its species in Mexico confirmed with a pinworm called Trypanoxyuris pigrae. A study of road-killed lesser grisons in Brazil identified six species of helminth parasites (parasitic worms), more than previously found in the country. And an Ontario study of kidney samples from road-killed and hunter/trapper mammal carcasses identified the first beaver and the first possum in the province that were infected with the pathogenic bacteria leptospirosis. Leptospirosis is of particular concern because it can infect people and domestic animals such as livestock.

In other cases, scientists are using roadkill to make the connections between disease and behaviors that might increase the risk of animal-vehicle collisions. Iranian researchers obtained the carcasses of three Persian leopards, an endangered subspecies on the IUCN Red List, killed along the Asian Highway in Golestan National Park. (That highway “has a large negative impact” on the felines’ habitat and is responsible for many deaths from collisions, the researchers noted in a study reporting their findings.) They hypothesized that infectious diseases like rabies and acute toxoplasmosis in big cats contribute to collisions by blinding animals, altering their behavior, or due to neurological disturbances resulting from infection. To test their hypothesis, they dissected the leopards and tested tissue from brains, kidneys, and lungs, among other things. Two of the cats tested positive for toxoplasmosis.

In the Cardiff University Otter Project, Chadwick’s team found one-quarter of the tested animals were infected with a parasitic protozoan that can harm many types of warm-blooded animals, including humans. The disease, the researchers wrote, “is notorious for its role as a host manipulator, with infected rodents and even primates becoming more risk-taking and active,” including their possible willingness to take more risks in crossing roads. In other words, the disease may make otters less careful and, thus, more vulnerable to becoming roadkill.

Roadkill-based studies can provide valuable insight into animal behavior, distribution patterns, population trends, and ecosystem-based threats to wildlife. Curious which animals make their home in northern Ethiopia? A survey of 330 miles of road provided a snapshot of biodiversity in the region, finding 20 species of wildlife, including laughing doves, speckled pigeons, black kites, black-backed jackals, white-tailed mongoose, and spotted hyenas. Wonder what anteaters eat in Argentina? So did scientists. Roadkill helped satisfy their curiosity and fill in some blanks about the previously unknown feeding habits of two species: the terrestrial giant anteater of the grasslands and the southern tamandua, which feeds on the ground and in trees. Examination of the stomach contents of roadkill and feces revealed dozens of species of ants, many more than those fed on by the anteaters of Brazil, Colombia, and Venezuela.

In Florida, researchers used roadkill to estimate the statewide population of Florida panthers, an endangered panther subspecies. By combining information about the number of panthers killed in car collisions with traffic volumes and data from a small number of radio-collared cats, the team found that panther numbers may be slowly increasing. But they also estimated that the population never surpassed 150 between 2005 and 2012, well below the 240 required to remove the big cat from the endangered species list.

photo of a person performing necropsies in a labphoto Tony CepakResearchers at Michigan DNR’s Wildlife Disease Laboratory study roadkill to learn more about how diseases impact wildlife. Around the globe, vehicle-collision victims are proving valuable for examing everything from what animals eat to how endangered species are faring.

And in western Uganda, scientists have studied roadkill to glean information about vulnerable species. “While highly undesirable, roadkills provide valuable information on the health and condition of endangered species,” researchers at the Budongo Conservation Field Station in Ethiopia’s Hoima District wrote after conducting a necropsy of a road-killed chimpanzee named Olive. One member of that team, Matt McLennan, told me, “While the road accident we reported was highly unfortunate – the unlucky chimp was a prime-aged breeding female with a newborn infant from a small group of just 21 chimps – it was a good opportunity to investigate the chimpanzee’s health status.”

Researchers at the University of California, Davis, are engaging citizens across California in documenting the roadkill they spot, and using this information to learn more about both native and nonnative species. Using citizen-uploaded data, they have observed that the nonnative eastern gray squirrel and eastern fox squirrel are increasingly encroaching on the native western gray squirrel’s habitat, for example. (Roadkill has similarly been used to track the spread of invasive Burmese pythons in Florida.) They also believe that roadkill patterns hold insight into the toll of California’s recent five-year drought, capturing the movement of animals in search of water and food in the early years of the drought, and the ultimate decline in mammal and amphibian populations after several dry years.

Roadkill can also highlight species’ broader vulnerabilities to the roads themselves. In Assam, India, the discovery of a road-killed small Indian civet, a protected species, spotlighted threats posed by of habitat fragmentation. The animal died while crossing a recently constructed highway bypass from one patch of scrub forest to another. The forestland there had previously been contiguous. “Although incidents such as this may be viewed as just another road fatality, it actually presents a picture of the ground reality of the threats that the urban wildlife is facing due to habitat fragmentation,” researchers from the Nature Conservation Foundation and Wildlife Institute of India wrote in a study.

Finding ways to reduce adverse impacts of roads on wildlife is essential to species protection amid the reality of ever-expanding development and road systems. When it comes to wildlife-vehicle collisions, that requires understanding how and when animals are most vulnerable, and which mitigation measures are most effective. Studies involving traffic victims help scientists and their collaborators like highway engineers, planners, and transportation agencies identify roadkill hotspots or “biodiversity collision blackspots” in manager-speak – think of them as death zones – and assess preventative measures such as wildlife underpasses and overpasses, culverts, warning signs, roadside barriers, and lower speed limits. Findings can also inform government and public lands managers about landscape connectivity issues and other environmental challenges.

Thankfully, scientists have tackled these issues, too. Research has shown that animals are most vulnerable to roads in the early morning and late evening, likely due to high traffic volumes combined with the inability of drivers to spot animals in the dark. Vulnerability also varies with season. For example, collisions with deer – by far the most commonly hit large mammal in the US – spike in the autumn, during mating season. Certain attributes may also put animals at increased risk. For example, cold-blooded reptiles are attracted to heat-absorbing roads. They are also slow-moving, which means it takes them longer to cross a highway, and turtles’ behavior of retreating into their shells when vehicles pass may increase their exposure time on the road.

There’s also the matter of where animals breed and feed. In Spain, the ability of European rabbits to thrive in verges – strips of terrain adjacent to roads – is both good news and bad news for red fox and other carnivores that love to chow down on them: Rabbits provide them with an abundant food source, but also lure them close to roads, and thus closer to death-by-vehicle. Roads also often attract gulls, raptors, and members of the crow family to scavenge on roadkills, making them potential victims themselves, while white wagtails are at risk as they forage for insects next to roads.

Science has shown that some fortunate critters are smart enough, or well-wired enough by instinct, to quickly adapt their behavior and improve their odds of survival. For example, a Norwegian scientist used roadkill and field observations to conclude that a significant proportion of hooded crows and western jackdaws started flying at higher elevations after a road was newly opened to car traffic, thus reducing the odds they’d be hit.

Other species need some assistance avoiding cars. With that in mind, researchers in Ontario set out to study roadkill reduction methods. They combined roadkill survey data with motion-activated cameras and antennas to evaluate the effectiveness of fencing along a causeway to funnel reptiles towards culverts for safe crossing under the road, thus reducing fatalities. Fencing in the study area was installed along “one of the deadliest roads in North America” for at risk species, including the endangered Blanding’s turtles. The researchers found that reptiles were in fact using the culverts to safely move from one side of the road to the other, and unsurprisingly, that full fencing was much more effective at directing them towards the tunnels than was partial fencing.

The study also helped draw attention to roadway risks. Conservation biologist Chantel Markle of McMaster University, explains, “We were able to show the damage to the species [from the road], and the different populations being hit. A lot of people were concerned and wanted to do something.” While fencing and culverts are often recommended, they are “pretty costly projects,” she adds. “We want to make sure the tactics are having an effect.” Based on the findings, she says the researchers concluded that “while partial fencing is cheaper” it was also less effective, indicating that any decision about mitigation “shouldn’t be strictly a monetary one.”

Transportation officials in Los Angeles, CA, plan to use roadkill information to similarly evaluate the effectiveness of habitat connectivity efforts. Wildlife advocates there have been pushing tirelessly for an overpass across a stretch of the 101 Freeway, spurred by concerns over inbreeding and roadkill deaths of mountain lions living in and around the Santa Monica Mountains. If and when the ambitious project is constructed, they plan to monitor roadkill, among other things, to assess the value of the overpass to regional habitat connectivity efforts.

Meanwhile, inventors are jumping into this niche market, patenting roadside alarm systems, thermal and infrared sensors, warning lights, and guardrail designs intended to lower the traffic death toll. Elsewhere, concerned communities are getting creative, painting reindeer antlers and horse torsos with glow-in-the-dark paint to help them stand out at night. Yet high-tech wizardry is costly, and painting animals that shed their fur annually isn’t quite practical on a large scale.

Roadkill ecology tales don’t end with happily-ever-after. However, they may provide essential insight into how to best combat wildlife disease, protect at-risk species, and reduce carnage on the roads. They also may raise public awareness that roadkill means more than yucky, malodorous splatter, and that the sooner we figure out effective ways to reduce these tragic deaths, the better for biodiversity, our safety, and that of the many animals we share our roads with.

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