In 2020, the discovery of over 330 dead elephants in Botswana’s Okavango Delta during an aerial survey shocked scientists and the world. It was one of the most significant mass mortality events of large mammals in recent southern African history. The remote location and the peak of the COVID-19 pandemic hampered initial investigations, leaving the cause of the deaths unanswered. Now, scientists have confirmed a theory that was suspected by many at the time.
Little evidence pointed to anthrax or a virus. All theories, including death due to viral or bacterial infections, lacked sufficient evidence. All elephants had their tusks intact, ruling out poaching incidents. The Botswana government suggested the deaths could be due to environmental intoxication by cyanobacterial toxins, also known as cyanotoxins, but the theory lacked evidence.
Four years after the incident, new research provides the most comprehensive insight into this tragic event to date. Scientists took a groundbreaking approach, combining remote sensing and spatial analysis to reconstruct the elephants’ movements and link them to the environmental conditions that likely contributed to their deaths. This innovative approach works around the logistical challenges of traditional field-based investigations in remote areas.
The researchers, from Kings College in London and the Okavango Research Institute, focused on the eastern Okavango Panhandle, a region characterised by a complex network of permanent and ephemeral water bodies known as pans. They analysed data from 2015 to 2023 and used satellite imagery to map the location and water levels of approximately 3,000 pans.
The researchers integrated this data with the locations of elephant carcasses and live elephants, obtained from an aerial survey conducted in July 2020. This survey categorised carcasses based on their decomposition state: fresh (deaths within a month), recent (deaths within six months), and bones (older deaths). A key finding was a statistically significant difference in the spatial distribution of fresh/recent carcasses compared to the distribution of bones and live elephants, indicating that the die-off deviated from typical regional mortality patterns.
They revealed a striking correlation. Twenty pans near fresh carcasses showed a dramatic increase in cyanobacteria bloom events in 2020, exhibiting the highest average phytoplankton biomass recorded between 2015 and 2023. High levels of cyanobacteria produce potent toxins, posing a threat to the health of wildlife relying on these water bodies, and these findings suggest a heightened risk of cyanotoxin presence in these specific water sources. Elephants, which can drink up to 200 litres of water a day, could be particularly vulnerable due to consumption of large amounts of the toxins.
Scientists found that the estimated travel distance of the elephants before death, approximately 16.5 kilometres, was consistent with cyanotoxin poisoning.
The period between April and May 2020 showed the highest algae production, although high productivity and bloom events occurred throughout the year. On average, these pans had water only 11% of the time between January and July 2019, compared to 55% in 2020. There was a shift from a dry 2019 to a significantly wetter 2020, a change likely responsible for triggering the extreme algal growth in the pans.
While it might seem strange that blooms occur in wetter periods, Davide Lomeo, a member of the team from the Department of Geography at King’s College London, says there is an explanation.
“An increased amount of water being introduced into these systems does not suppress blooms. In the very short term, during the rainy period, bloom formation may be temporarily ‘suppressed’ due to the sustained water disturbance (by rain). In the short term, this may cause greater dilution but also leads to the re-suspension of sediments from dried-up waterhole beds and/or surrounding dusty areas.” Cyanobacteria, or blue-green algae, flourish in warm, stagnant waters rich in nutrients.
Coupled with the heavy rains, this led “to greater availability of nutrients within the water, that (especially) after the rains, when water is stagnant, increases the likelihood of bloom events,” says Lomeo.
The question of whether such events could have occurred in the past is a valid one. “Our satellite analysis (alongside others performed in areas around the Okavango Delta) showed that, at least for the past 20 years, there have not been years as dry as 2019,” according to Lomeo. He says 2015 also was very dry, but 2016 was nowhere near as wet as 2020. “This means that the drying up of these waterholes was not followed by such unprecedented flushing event,” he explains, “but we cannot know for certain what happened in the more distant past.”
In the same year, 35 elephants died in neighbouring Zimbabwe from an obscure bacterium in hot conditions. In that case, they suspected the bacteria, which live naturally in the tonsils of some animals, can pass into the bloodstream when temperatures go over 37 degrees Celsius (as in the case of antelopes dying in Kazakhstan in 2015), but this was not confirmed to be the case with the Zimbabwean elephants.
Does this mean events like this will become the norm with changing climatic conditions? “We cannot say for certain if these events will become more frequent, but we are seeing all around the world that climate change is leading to more frequent and extreme weather events, so everything suggests that we may be going in that direction,” says Lomeo.
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