But the hippopotamus remained a poorly understood animal. Herodotus’s ‘river horse” was the largest African mammal after the elephant, but its habit of lying in the water with just eyes and nostrils protruding made it difficult to study. Hippos were organized around a male. A mature male had a harem of several females and their offspring, a group of eight to fourteen animals altogether.

Despite their obese, rather humorous appearance, hippos were capable of unusual violence. The bull hippopotamus was a formidable creature, fourteen feet long and weighing nearly ten thousand pounds. Charging, he moved with extraordinary speed for such a large animal, and his four stubby blunted tusks were actually razor sharp on the sides. A hippo attacked by slashing, moving his cavernous mouth from side to side, rather than biting. And, unlike most animals, a fight between bulls often resulted in the death of one animal from deep slashing wounds. There was nothing symbolic about a hippopotamus fight.

The animal was dangerous to man, as well. In river areas where herds were found, half of native deaths were attributed to hippos; elephants and predatory cats accounted for the remainder. The hippopotamus was vegetarian, and at night the animals came onto the land, where they ate enormous quantities of grass to sustain their great bulk. A hippo separated from the water was especially dangerous; anyone finding himself between a landed hippo and the river he was rushing to return to did not generally survive the experience.

But the hippo was essential to Africa’s river ecology. His fecal matter, produced in prodigious quantities, fertilized the river grasses, which in turn allowed river fish and other creatures to live. Without the hippopotamus African rivers would be sterile, and where they had been driven away, the rivers died.

This much was known, and one thing more. The hippopotamus was fiercely territorial. Without exception, the male defended his river against any intruder. And as had been recorded on many occasions, intruders included other hippos, crocodiles, and passing boats. And the people in them.

MUNRO’S INTENTION IN CONTINUING THROUGH the night was two-fold. First, he hoped to make up precious time, for. all the computer projections assumed that they would stop each night. But it took no effort to ride the river in the moonlight; most of the party could sleep, and they would advance themselves another fifty or sixty miles by dawn.

But more important, he hoped to avoid the Ragora hippos, which could easily destroy their flimsy rubber boats. During the day, the hippos were found in pools beside the riverbanks, and the bulls would certainly attack any passing boat. At night, when the animals went ashore to forage, the expedition could slip down the river and avoid a confrontation entirely.

It was a clever plan, but it ran into trouble for an unexpected reason-their progress on the Ragora was too rapid. It was only nine o’clock at night when they reached the first hippo areas, too early for the animals to be eating. The hippos would attack the boats-but they would attack in the dark.

The river twisted and turned in a series of curves. At each curve there was a still pool, which Kahega pointed out as the kind of quiet water that hippos liked to inhabit. And he pointed to the grass on the banks, cut short as if the banks had been mown.

“Soon now,” Kahega said.

They heard a low grunting, “Raw-huh-huh-huh.” It sounded like an old man trying to clear his throat of phlegm. Munro tensed in the lead boat. They drifted around another curve, carried smoothly in the flow of current. The two boats were now about ten yards apart. Munro held his loaded shotgun ready.

The sound came again, this time in a chorus: “Haw-huh-huh-huh.”

Kahega plunged his paddle into the water. It struck bottom quickly. He pulled it out; only three feet of it was wet. “Not deep,” he said, shaking his head.

“Is that bad?” Ross said.

“Yes, I think it is bad.”

They came around the next bend, and Elliot saw a half-dozen partially submerged black rocks near the shore, gleaming in the moonlight. Then one of the “rocks” crashed upward and he saw an enormous creature lift entirely out of the shallow water so that he could see the four stubby legs, and the hippo churned forward toward Munro’s boat.

Munro fired a low magnesium flare as the animal charged; in the harsh white light Elliot saw a gigantic mouth, four huge glistening blunted teeth, the head lifted upward as the animal roared. And then the hippo was engulfed in a cloud of pale yellow gas. The gas drifted back, and stung their eyes.

“He’s using tear gas,” Ross said.

Munro’s boat had already moved on. With a roar of pain the male hippo had plunged down into the water and disappeared from sight. In the second boat, they blinked back tears and watched for him as they approached the pool. Overhead, the magnesium flare sizzled and descended, lengthening sharp shadows, glaring off the water.

“Perhaps he’s given up,” Elliot said. They could not see the hippo anywhere. They drifted in silence.

And suddenly the front of the boat bucked up, and the hippo roared and Ross screamed. Kahega toppled backward, discharging his gun into the air. The boat slapped down with a wrenching crash and a spray of water over the sides, and Elliot scrambled to his feet to check Amy and found himself staring into a huge pink cavernous mouth and hot breath. The mouth came down with a lateral slash on the side of the rubber boat, and the air began to hiss and sizzle in the water.

The mouth opened again, and the hippo grunted, but by then Kahega had got to his feet and flied a stinging cloud of gas. The hippo backed off and splashed down, rocking the boat and propelling them onward, down the river. The whole right side of the boat was collapsing swiftly as the air leaked out of the huge cuts in the rubber. Elliot tried to pull them shut with his hands; the hissing continued unabated. They would sink within a minute.

Behind them, the bull hippo charged, racing down the shallow river like a powerboat, churning water in a wake from both sides of his body, bellowing in anger.

“Hold on, bold on!” Kahega shouted, and fired again. The hippo disappeared behind a cloud of gas, and the boat drifted around another curve. When the gas cleared the animal was gone. The magnesium flare sputtered into the water and they were plunged into darkness again. Elliot grabbed Amy as the boat sank, and they found themselves standing knee-deep in the muddy water.

They managed to beach the Zodiac on the dark riverbank. In the lead boat, Munro paddled over, surveyed the damage, and announced that they would inflate another boat and go on. He called for a rest, and they all lay in the moonlight on the river’s edge swatting mosquitoes away.

Their reverie was interrupted by the screaming whine of ground-to-air rockets, blossoming explosions in the sky overhead. With each explosion, the riverbank glowed bright red, casting long shadows, then fading black once more.

“Muguru’s men firing from the ground,” Munro said, reaching for his field glasses.

“What’re they shooting at?” Elliot said, staring up into the sky.

“Beats me,” Munro said.

Amy touched Munro’s arm, and signed, Bird come. But they heard no sound of an aircraft, only the bursting of rockets in the sky.

Munro said, “You think she hears something?”

“Her hearing is very acute.”

And then they heard the drone of a distant aircraft, approaching from the south. As it came into view, they saw it twist, maneuvering among the brilliant yellow-red explosions that burst in the moonlight and glinted off the metal body of the aircraft.