A modern day health problem! Congenital defects in elephant seals, such as spinal deformities and cleft palates, reveal a 100-year-old story that goes back to when this species had a brush with extinction!
April 13, 2012
TVA, a northern elephant seal weaner rescued earlier this year, rests inside a rubber carrier equipped with a heating pad on a wet Thursday morning at The Marine Mammal Center in Sausalito. She is suffering from malnourishment due to not hunting effectively after having been weaned from her mother. Beachgoers found this skinny pup alone and in a bad location with humans around, so the Center rescued her. Luckily, at The Marine Mammal Center she’ll get the nourishment and medical care she needs to fatten up and become strong enough for release back to the ocean. In many ways she is like the other thousands of seals and sea lions that have been rescued by the Center since 1975.
But what’s particularly interesting about TVA (nicknamed for where she was rescued in Tennessee Cove, a spot within the Tennessee Valley in Marin County, CA) is that she has a deformity, or birth defect, which gives her a “hunchback” due to an out of place bone in her spine. As a matter of fact, rates of congenital deformities in northern elephant seals are higher than in any other marine mammal species we see at the hospital, according to Dr. Bill Van Bonn, Director of Veterinary Science at The Marine Mammal Center.
“Our records show that approximately 1.0% of the elephant seals we’ve seen at our hospital (some 3,300 since 1975) have birth defects.” Amongst animals seen at the Center, California sea lions (more than 10,000 have been admitted to the Center since 1975) have a lower rate of birth defects, at 0.09%, while Pacific harbor seals (the Center has admitted more than 2,200 since 1975) are at 0.4%.
So why are elephant seals so prone to birth defects? The answer may lie with this pinniped’s brush with near extinction hundreds of years ago. Fossil records show that northern elephant seals have been around for millions of years, yet by the end of the 19th century, experts say there were only about 100 seals left as a result of over-hunting by humans, who harvested the animals' valuable blubber in order to make oil. The population slowly recovered and the current population of elephant seals is estimated at about 170,000. However, with only a small gene pool to draw upon, a genetic bottleneck was created, potentially causing the present-day problems of congenital defects such as cleft palates, spinal deformities and heart, brain and eye deformities.
“Science leads us to expect that over time, these seals will “re-evolve” and these problems will become less and less, but that could take thousands of years," said Dr. Van Bonn. “We're fortunate at the hospital to be able to get up close to these animals and learn from them because they really are so fascinating from a comparative physiological standpoint.”
While volunteers and medical staff are helping TVA, along with Faith, another elephant seal with a spinal deformity, get better, both animals are helping researchers from the Center gain a deeper understanding of spinal deformities. The researchers plan on comparing the spines of these and other recent patients to see how and when bones form, and document what should be normal for the species. They hope to publish their findings within a year.
Congenital Defects in Northern Elephant Seals Stranded Along the Central California Coast - Journal of Wildlife Diseases, 33(2), 1997
Persistent Right Aortic Arch and Cribiform Plate Aplasia in a Northern Elephant Seal (Mirounga angustirostris) - Journal of Wildlife Diseases, 44(2), 2008