After a decade as a recognizable attraction at the Houston Zoo, Jonathan the lion passed away Tuesday night after zoo keepers discovered he was suffering from a blood clotting issue.

Rescued from a private owner when he was just a few years old, Jonathan has been a major attraction at the Houston zoo since 2006. Jonathan and his pride have been used by the zoo to bring attention to the diminishing numbers of lions in the wild, due to conflicts with humans and loss of their natural habitat due to human expansion into the wild. Scientists believed the number of lions in the wild to be over 60,000 in the 90s, with the number down to maybe just 30,000 today.

Due to animals in captivity living longer lifespans than their counterparts in the wild, many become susceptible to geriatric complications, as Jonathan did. When zoo keepers found that the 18-year-old lion wasn't acting normally, they gave him a complete examination. Veterinarians found Jonathan to be suffering from a blood clotting issue and low white blood cell count, which is common in older animals. Though he spent the night in zoo's hospital center, Jonathan passed away during the night, leaving behind a lasting impression in the minds and hearts of zoo visitors and employees. According to Dr. Adrian Fowler, Vice President of Animal Operations at the zoo,

Jonathan’s impact on our keepers and guests will not be forgotten. Our team of keepers who have spent more than a decade caring and bonding with this incredible animal are grieving this loss, and we support them through this sad time.