On the remote steppes of central Kazakhstan, a truly extraordinary – and tragic – event unfolded in May 2015. Female saigas gathered in huge numbers to give birth on the open plain over a period of just 10 days – and a BBC camera crew and the research team they were with watched them die in their hundreds of thousands in the space of just a fortnight. The animals are captured in the latest episode of BBC nature documentary Planet Earth II.

But why did this mass death happen? By gathering like this, for as short a time as possible, the saigas swamp their main predator, wolves, with food so that each individual calf is less likely to be eaten. The calves are born large and well-developed – in fact, saigas have the largest proportional birth weight of any wild ungulate – so that they can outrun a predator within just a few days. They also need to give birth in a short time in order to coincide with the peak of lush grass before the summer heat of this harsh continental plain dries the vegetation.

An example of this incredible spectacle was filmed by another BBC camera crew for their pioneering programme about nature in the former Soviet Union, Realms of the Russian Bear, shown in 1994. The enormous herds of the time can be seen here.

But much has happened in the interim. The saiga was poached to near-extinction in the early 2000s for their horns and meat as the Soviet Union collapsed, and was listed as Critically Endangered on the IUCN Red List in 2001.

However, by 2015, conservation work by governments, scientists and NGOs was paying dividends; overall numbers had risen from its nadir of an estimated 50,000 in the early 2000s to around 300,000 in early 2015. One central Kazakhstan population, in particular, was responsible for the vast majority of this increase – and this is where the Planet Earth II camera crew headed for their shots of the calving spectacle in 2015.