"No," he replied, swimming faster. Suddenly a distinctive hippo "bow wave" appeared on the surface. Hippos can propel themselves through the water by running along the bottom—in clear water you can see their submerged footpaths—or bounding off the bottom like a child in the shallow end of a pool. They can also hold their breath for up to five minutes. People who have studied hippos claim that in the animals' underwater travels they are remarkably graceful, not unlike those animated ballerinas in Fantasia. But they displace so much water that they create bow waves that reveal their position. Bristow, swimming for his life, watched such a wave in terror as it drew closer and closer, until, when it was just 10 yards from him, he was able to scramble onto shore. The hippo surfaced, blew air out of its nostrils, opened its huge mouth threateningly (hippos' jaws can spread 150 degrees) and then retreated to the depths of the river.
It is very rare, though not unheard of, for a hippopotamus to chase someone from the water onto dry land. Later, Bristow heard that a gigantic crocodile had been seen downstream with a baby hippo in its jaws, and Bristow theorizes that he and his guide were attacked by the enraged, grieving mother.
Hippopotamuses do not, however, require such concrete reason to lash out with the worst intentions, according to William Barklow, a biology professor at Framingham (Mass.) State College who has studied hippos in Tanzania since 1989. Territorial and suspicious by nature, "they generally will attack anything strange or new that comes along," Barklow says. That includes nosy scientists. By his own count, Barklow, who tapes the underwater vocalizing of hippos, has been charged five or six times. While setting up his microphones, he sometimes has to wade into the shallows, which is where he's most at risk. He tells his students to keep an eye out for bow waves and, if they see one, to yell, "Charge!" So far Barklow has been able to stay a step ahead of his subjects.
"I suspect hippos get blamed for a lot of deaths that aren't really their fault," he says, "like when they overturn a canoe and the people in it drown because they don't know how to swim. Most hippopotamus attacks are defensive in nature, unless two hippos are fighting each other. That's very dangerous because the loser will often charge anything nearby."
Nsanjama, who was director of wildlife in Malawi before assuming his post at the WWF, once saw a hippo redirect its aggression with tragic results. A pair of males were fighting to determine which would be the dominant bull in a territory—a common ritual that sometimes is waged to the death. They were standing side by side in the shallows of a river in Malawi, hooking each other savagely with their lower canines, which hippos wield like antlers. As the hippos fought, a canoe carrying women and children passed by on the deep-water side of the river. The hippo that was losing suddenly turned, gave chase and capsized the canoe. All the occupants drowned.
The ill-tempered beast no doubt then skulked away to lick its wounds, which do not heal easily in the muddy water in which hippos spend their days. To recover, hippos sometimes retire, for up to a week, to a thicket on dry land, with only periodic forays into the water.
In the water an adult hippo has no natural enemies other than man. On land, however, where hippos go at night to eat, they are more vulnerable, edgier and far more dangerous to encounter. It is there that young hippos—and even fully grown ones—can fall prey to hyenas and lions.
In Botswana, guides tell of the hippo-surfing lions. A lion leaps onto the back of a grazing hippo, and as the terrified hippo races for the haven of the river, the lion tries to bite through its back and sever its spinal cord. If the lion succeeds, the hippo will be paralyzed and die slowly by suffocation and dehydration. If the hippo reaches the water, however, the lion must bail out.
"When they are left alone, hippos are the most peaceful, mind-your-own-business animals," says Nsanjama. "The problems arise because where there's water in Africa, you find people and hippos. People grow crops, and hippos love to eat crops. To make matters worse, hippos feed mostly at night. When people try to protect the crops, they can be ambushed by a hippo."
Ambushed? By this mind-your-own-business, peace-loving creature? But ambushed is a pretty good word. Hippopotamuses tend to have pear-shaped territories of about 30 square kilometers, the narrow part being where the hippos leave and reenter the water. At dusk they leave the sanctuary of the river or lake where they have basked all day, and they walk to their grazing grounds along a trail that is easy to spot. Woe be unto the villager who strolls down such a path at night.