Why Danish citizen scientists were on a quest to find the oldest European hedgehog
The beloved mammal is facing population decline in some habitats.
A citizen science project in Denmark helped researchers find the world’s oldest (or at least scientifically-confirmed oldest) European hedgehog (Erinaceus europaeus). At 16 years-old, Thorvald the hedgehog lived seven years longer than the previous record holder. On average, the six to 11 inch long animals typically found wooded areas, gardens, and parks and lives around two years.
The study on the life expectancy of European hedgehogs was published February 14 in the journal Animals.
While the European hedgehog is a beloved mammal, their populations have declined up to 30 percent in rural populations in the United Kingdom alone. Multiple projects have been launched by conservationists and researchers to monitor populations and inform initiatives that protect the animals in the wild. Citizen science is proving to be an ally in understanding how long these mammals live.
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In 2016, researchers from a citizen science conservation initiative called the The Danish Hedgehog Project, asked people in Denmark to collect data on any dead hedgehogs they encountered in an effort to figure out how long the mammals typically lived. Volunteers found 697 dead hedgehogs from all over Denmark.
The researchers determined the age of the hedgehogs by counting growth lines in thin sections of the hedgehogs’ jawbones, like counting growth rings in trees. Their jaw bones show growth lines because calcium metabolism slows down when they hibernate over winter. Bone growth will reduce or stop completely, resulting in one line that represents one hibernation.
The second and third place winners of oldest hedgehog were 13 and 11 years-old. The average age was only about two years and roughly 30 percent died before reaching one year old.
Most of the hedgehogs were killed while crossing roads. About 22 percent of the animals died at a hedgehog rehabilitation center following injuries from incidents like dog attacks, and 22 percent died of natural causes in the wild.
The male hedgehogs generally lived 24 percent longer than females (2.1 vs 1.6 years), but the males were also more likely to be killed in traffic. The team speculates that this is possibly because male hedgehogs come into contact with roads more frequently due to their longer ranges.
Road deaths also peaked during the month of July for both males and females. July is the height of mating season for hedgehogs in Denmark, and the increase is likely due to the hedgehogs walking longer distances and across more roads to search for mates.
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“Although we saw a high proportion of individuals dying at the age of one year, our data also showed that if the individuals survived this life stage, they could potentially live to become 16 years old and produce offspring for several years,” said Sophie Lund Rasmussen, a biologist from Oxford University’s Wildlife Conservation Research Unit (WildCRU) who leads The Danish Hedgehog Project, in a statement. “This may be because individual hedgehogs gradually gain more experience as they grow older. If they manage to survive to reach the age of two years or more, they would have likely learned to avoid dangers such as cars and predators.”
Rasmussen, also called Dr. Hedgehog on social media, also added that being a male hedgehog is “simply easier”—the animals are not territorial, they rarely fight. Not to mention that female hedgehogs also take on raising offspring alone.
To investigate if inbreeding influenced their lifespans, the researchers also took tissue samples. Previous studies have found a low genetic diversity in the Danish hedgehog population, an indicator of high degrees of inbreeding which can reduce the fitness of a population. Inbreeding allows hereditary, and potentially lethal, health conditions like lower offspring birth rate and reduced milk production, to be passed down to offspring.
Much to the team’s surprise, the tests showed that inbreeding did not seem to reduce the expected lifespan of the hedgehogs.
“Sadly, many species of wildlife are in decline, which often results in increased inbreeding, as the decline limits the selection of suitable mates. This study is one of the first thorough investigations of the effect of inbreeding on longevity,” said Rasmussen. “Our research indicates that if the hedgehogs manage to survive into adulthood, despite their high degree of inbreeding, which may cause several potentially lethal, hereditary conditions, the inbreeding does not reduce their longevity. That is a rather groundbreaking discovery, and very positive news from a conservation perspective.”
The results from this study will aim to improve conservation management for a “beloved and declining species.”