Last week, it was determined that there was no genetic evidence of the Moa declining at all before they were hunted to extinction. Unfortunately, the same does not appear to be true for woolly mammoths. According to a new study, many mammoths suffered severe birth defects in the form of cervical ribs as the species drew nearer to extinction. The research was performed by Jelle Reumer of Utrecht University and was published in the open access journal, PeerJ.
Mammoths first appeared on Earth around 5 million years ago and lasted until about 4,500 years ago. Humans are believed to have played a large role in their extinction, as early humans targeted the animals for their meat, bones, and fur. Smaller populations are more susceptible to succumbing to natural forces such as disease.
During fetal development, an error can occur and cause ribs to grow from cervical vertebrae. The severity of the ribs can range from thread-like projections to an actual full-sized rib. This can cause undue pressure on blood vessels or nerves with some pretty serious consequences. In humans, if the embryo even survives until birth (nearly 80% don’t), there is an 86% chance that it will die within the first year of life. Cervical ribs also have a strong correlation with the development of childhood cancers such as leukemia. Clearly, this is a trait that would be very harshly selected against.
Cervical ribs occur in about 1% of all human births. For Asian elephants, the closest living relative of the mammoth, the number jumps up to 3.6%. For mammoths living near the North Sea about 12,000 years ago, a jaw-dropping 33.3% of infants had cervical ribs. The sample size was admittedly small, as Reumer and his team analyzed only 16 mammoth cervical vertebrae. However, it was very striking that such a large percentage of those vertebrae would have ribs, especially ones as pronounced as they were.
According to the researchers, this influx of birth defects could have come about in two different ways. The genetic mutations could have arisen from inbreeding depression. As mammoths were reduced in number, genetic diversity would have plummeted and the number of mutations would have risen sharply. The other explanation offered states that expecting mothers would have been under considerable stress as the population dwindled. This prenatal stress could have had negative consequences for fetal development.
Cervical vertebra from a mammoth with cervical rib. Credit: Joris van Alphen