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.

This is a time of huge physiological stress for the females, making them prone to disease and birth-related mortality

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.

They accompanied a research team organised by the Association for the Conservation of Biodiversity in Kazakhstan, which also contained researchers from the Royal Veterinary College, intent on monitoring calving to learn more about saiga ecology.

Soon a vast area stretching over hundreds of kilometres was littered with corpses

However, the saiga's strategy of intense birth effort, compressed in time and space, comes at a cost. This is a time of huge physiological stress for the females, making them prone to disease and birth-related mortality, and the weather is unreliable and calves often die from exposure.

Most years, things go well, but the ecological history of saigas is littered with mass mortalities from disease in the calving season. In fact, the reason why the student from the Royal Veterinary College was out monitoring calves was because of a large die-off in the calving area of another population only a few years before.

But nothing prepared us, or the camera crew, for what transpired in 2015.

As they gathered to give birth, an increasing number of females became weak and uncoordinated, dying in a matter of hours. Soon a vast area stretching over hundreds of kilometres was littered with corpses. The calves followed soon after; within any given aggregation of tens of thousands of animals, it appeared that every single animal died over a period of a few days.

This mass die-off was a terrible tragedy. It sparked a worldwide search for answers, some more outlandish than others – aliens were mentioned on social media a few times.

Why did these usually harmless bacteria become virulent?

As saiga scientists, we had mixed feelings; both a sense of personal devastation for the species which we care about, and curiosity to solve a fascinating scientific puzzle. What possible mechanism was there which could kill apparently all the individuals in a herd so very quickly? This is not how infectious disease normally works; infections spread through populations over time, and apart from anything else it is not in the parasite's interest to wipe out its entire host population.

This pointed to some non-infectious route; perhaps an environmental toxin or weather abnormality? But what kind of consistent environmental factor could affect so many animals almost simultaneously over a huge area (168,000 sq km; bigger than England and Wales combined), in an environment that is naturally variable in weather and vegetation at this time of year?

Thanks to a grant from the UK government's NERC Urgency Fund, together with generous donations from conservation charities and from individuals worldwide, we quickly got to work to form an international, interdisciplinary team to study the disease and its causes.

Led by Richard Kock at the Royal Veterinary College, with colleagues from the Kazakh government's Research Institute for Biological Safety Problems and the Association for the Conservation of Biodiversity in Kazakhstan, the universities of Oxford and Bristol, the UN's Food and Agriculture Organisation, and other institutions, we include ecologists, rangeland scientists, vets and spatial modellers. We sent a team into the field to collect samples from the environment and dead and dying saigas within a week of the first individuals starting to die.

There's no evidence for environmental toxins, other underlying infections or (as yet!) alien influence

On one level, we have now found the answer; the proximate cause of death was toxicity from infection by opportunistic bacteria found naturally in the animals' respiratory tract – Pasteurella multocida. But the next question is – why did these usually harmless bacteria become virulent? What was the environmental or internal trigger, either reducing the animals' immunity to these bacteria or triggering virulence in the bacteria, or both?

In exploring these questions, our research is a Russian doll; as we take off a layer of explanation we find more questions within. We have gone back to old field notes from the Institute of Zoology in Kazakhstan for 1988 when a similar mass mortality occurred; reviewed research on mass deaths in other species; looked for differences in the vegetation composition between the 2015 die-off and in other years; and built statistical models to explore changes in temperature and rainfall over a range of different temporal and spatial scales.

We also tested tissue and environmental samples for a wide range of toxins, as well as other disease-causing agents, in case some underlying infection was involved. So far, the evidence points towards a combination of short-term but landscape-scale weather variation and physiological stress from calving causing a cascading effect of virulence. There's no evidence for environmental toxins, other underlying infections or (as yet!) alien influence.

There has been huge public interest in this event, both within Kazakhstan and globally. People want quick answers and they want us to find solutions so that this will never happen again.

It seems, however, that we won't be able to give the comfort that is wanted; in fact, it is likely that with climate change these types of event will become more rather than less prevalent. However, we do have one clear and strong message: resilient and abundant populations of saigas are required, with strong protection from poaching.

This is a species that lives life on the edge, vulnerable to mass death but able to recover very rapidly. But this means it needs to be in large numbers in open rangelands to survive. This massive, very public, disaster has opened up new opportunities for us as saiga researchers and conservationists to make sure the saiga gets the protection it needs to flourish and keep providing the stunning annual spectacle which drew the BBC crew to its remote steppe home in the first place.

This article originally appeared on The Conversation, and is republished under a Creative Commons licence.