Because they, like the impala, were spooked when the sky went black. Astronomer Paul Murdin basks in the spectacle of the 21st century's first total solar eclipse and describes its effects on the wildlife of the African bush.
I sat on the right bank of the Zambezi River, looking across from Zimbabwe to the mountains of Zambia. It was a spectacular view in spectacular circumstances. We were minutes from a total solar eclipse - the first of the 21st century.
With 250 others, I was in the African bush, in Mana Pools National Park in northern Zimbabwe.
As the eclipse progressed towards totality in a cloud-free sky, the sun looked like a thin horseshoe hanging upside down. Projected through small gaps in the leaves into the dappled shade below the trees, the sun's image showed as multiple crescent shapes. The intensity of daylight reduced and the colour of the scene paled towards the silver of moonlight.
The warmth of the sun had gone. In the chill, the hairs on my forearms and the back of my neck stood up. It was unusually silent with few birdcalls or insect noises. Birds had reacted to the reduced light as if to nightfall. Butterflies descended from the acacia flowers, settled and folded their wings. A swarm of bees ceased buzzing as they withdrew into a hive in the mudbank.
The sudden dusk, the chill and the silence were eerie. How did the animals react?
Awash on a river sandbank about 25 hippopotamuses slept in a huddle. As a control, I had watched this same group at sunrise and sunset for the past few days. This was a maternal pod - females and juveniles - and one grossly fat dominant male. His hide was curiously pink and reminded me of the fleshy, crowded sultan's harem in Ingres's painting The Turkish Bath .
In the increasing dusk, the hippos stood up - presumably breakfast was on their minds. They waddled into the river, bottom-walking towards the grassy banks.
The crescent sun disappeared in a flare of dying glory, the last piece of the sun's surface showing brightly in the "diamond ring" effect. In the three-minute period of total eclipse, the sun's atmosphere stood out as the corona, shining white and vibrant with radial streaks. Because the sun is at maximum activity this year, its atmosphere is at its densest for 11 years and the corona was the brightest I have seen. Standing above the black, circular outline of the moon were crimson solar prominences. These arches of solar material are spun above the sun's surface by its magnetic fields as the surface boils, driven by the immense power from its interior.
Around the horizon, above the mountain tops, was a pink twilight. I was in the middle of the patch of the earth from which no part of the sun's surface could be seen - the totality zone, 150km wide. Outside this shadow, the sun continued to illuminate the earth, creating 360 degrees of twilight. The twilight reflected from the river and I could see the hippos' backs breaking the surface as they strode towards the banks.
Frogs started calling. Nocturnal mosquitoes bit unlotioned eclipse watchers.
The sudden blaze of sunlight, when totality ended, stopped the hippos in their tracks. Night had seemingly fallen - time for breakfast. But suddenly daylight had returned - bedtime. The hippos remained indecisively in the river, eyes and ears twitching above the surface.
Elsewhere, the restored light revealed that the impala, which were browsing as darkness approached, were now apprehensively drawn together, defensive, alert. Like me, the impala and the hippos appeared spooked by the eclipse.
There swelled a loud dawn-like chorus of turtle-doves, iridescent blue starlings, bulbuls and weaverbirds. The frogs ceased calling, the mosquitoes resettled.
Waterbirds had also reacted to the eclipse. Flights of water birds - egrets, oxpeckers and ibis-returned to roost, one U-turning back towards the feeding grounds after daylight returned.
In a riverside acacia glade the next morning, we reported what we had seen as, behind us, the hippos straggled back to their sandbank much later than usual, their daily routine upset. Lower-order species, such as insects, and cold-blooded creatures had responded to the eclipse immediately like thermometers. Because of their strong diurnal cycles, birds had also reacted instinctively. Strong-willed and single-minded species such as lion, buffalo, baboon, crocodile, eland, warthog and elephant had taken the eclipse in their stride. Hippos and impala lie intermediate in this spectrum, reacting more than most mammals because they are more sensitised to the light and warmth of their environment.
However, in contrast, man showed the greatest reaction of any species, migrating great distances to be in the eclipse's path.
Paul Murdin is head of astronomy at the UK's Particle Physics and Astronomy Research Council.
HOW THE CONTINENT GOT DARKER
The first total solar eclipse of the 21st century attracted large numbers of scientists and spectators to Africa. But it wasn't just the sun - or lack of it - that kept them on their feet.
Rather, for 250 wildlife enthusiasts, the eclipse offered the chance to take part in a unique project.
Over the centuries, accounts of eclipses have repeatedly mentionesd altered animal behaviour - gulls taking to the skies, insects falling silent - yet there have been few, if any, large scale scientifically robust studies of animal eclipse behaviour. Until now.
The June 21 eclipse saw the first such study. Camped in the Mana Pools National Park in Northern Zimbabwe were 250, mostly amateur, wildlife enthusiasts, many of whom were members of the voluntary group Wildlife and Environment Zimbabwe. With lions, hippos and vultures for company, they were right in the path for the eclipse.