Sudan was unable to stand up in the end. He was treated for age-related complications that led to degenerative changes in muscles and bones combined with extensive skin wounds. Veterinary experts took the decision to euthanize the animal." At the age of 45, Sudan was a very old man, well over 100 years old in human equivalent years," said the charity Helping Rhinos.

Rhino horns are used in traditional Chinese medicine and for dagger handles in Yemen. A poaching crisis in the 1970s and 1980s wiped out northern white rhino populations in Uganda, the Central African Republic, Sudan and Chad. The last wild population was in the Democratic Republic of Congo, but that succumbed to fighting in the region. At Ol Pejeta, Sudan was constantly under armed guard.

Given the danger that Sudan he would have been in when so much younger, he was among a group of northern white rhinos who were taken to a safari park in Czechoslovakia in the 1970s.

Sudan and a group of other northern white rhinos were moved back to Africa in 2009 in the hope that the move, in particular grassland at the Ol Peteja Conservancy in eastern Kenya, would give them more favorable breeding conditions.

The death of the only other northern white male, Suni, of natural causes in October 2014, further emphasized the need to urgently come up with alternative solutions for breeding.

Sudan eventually lived at Ol Peteja with the only other two members of his subspecies — his daughter Najin, and his granddaughter Fatu. Because of myriad health complications that mean the two cannot bear offspring, any future northern white rhino would have to be carried in pregnancy by southern white rhino female surrogates. However, their eggs would be used.

This white rhino subspecies made headlines last year following the death of Sudan, the last known male of his kind, making the species functionally extinct. Some scientists are cautiously optimistic that it could be brought back with the help of IVF technology, while others say time has simply run out for the subspecies, meaning it won't be long before it disappears.

The South China tiger is the most critically endangered of all tiger subspecies. No individual has been sighted in the wild since 1970, while the global captive population is likely to number less than 80. Some researchers have already declared the subspecies functionally extinct, but organizations such as Save China's Tigers (SCT) are persisting with their conservation efforts.

Fewer than 80 Amur leopards are believed to live in the wild, making it one of the rarest cats on the planet. Native to the forests of Southern China, Northern Russia and the Korean Peninsula, the amur leopard is threatened by poaching and deforestation. But there's still some hope — the leopard has found an unlikely refuge in the Korean Demilitarized Zone, which has become a haven for wildlife.

Also known as the gulf porpoise, the vaquita is the rarest marine animal in the world, with less than 15 thought to exist as of March 2018. The porpoise has never been hunted directly but it often becomes entangled in illegal gillnets intended to capture the totoaba, another critically endangered fish found in the Gulf of California. Efforts to stop illegal fishing in the area have largely failed.

Researchers fear the black rhino could suffer the same fate as its northern white cousin if conservation efforts aren't ramped up soon. The population currently numbers just over 5,000 individuals, although three subspecies have already been declared extinct. They are primarily threatened by illegal poaching, driven largely by demand for rhino horn on the black market.

With fewer than 30 remaining in the wild, the red wolf is considered critically endangered and is the focus of conservation efforts. It's actually a genetic mix between the gray wolf and the coyote, and was decimated in the 1960s thanks to intensive predator control programs. The shy subspecies is native to the southeastern US and Florida, but today is only found in eastern North Carolina.

First discovered in 1992, the elusive saola is often dubbed the 'Asian unicorn.' In fact, it's so rare that there have only been four confirmed sightings in the wild by researchers. They are found in the forests of Vietnam and Laos and are threatened by poaching and habitat destruction. The population is believed to have rapidly declined since its discovery, with no more than 100 likely to exist.

The largest living primate is critically endangered thanks to illegal hunting and deforestation. The lowland gorilla subspecies is more populous with 3,800 thought to live in the wild, while only about 880 mountain gorillas are thought to exist. Although conservation efforts have been lauded in recent years, there is still a long way to go before the population will be considered stable.