Aug. 21, 2009 -- If you think hippos are the couch potatoes of the animal kingdom -- enormous, sure, but otherwise docile and harmless -- think again. They might look like bathtubs with legs, but the hippo is one of the most aggressive animals on earth.
On our visit to Lake Edward, we were coasting along when suddenly a male hippo emerged from the water. The dominant male of his group, he wasn't too happy to have company.
"That seems to be a male trying to protect the group. If this was a small boat, that one would overturn the boat," explained our guide, Peter, a grizzled veteran hippo watcher.
It's said that hippos kill more humans every year than any other mammal. In fact, according to Peter, a hippo had killed a woman a week before we arrived in Uganda.
This aggression is somewhat surprising, given the fact that hippos are also vegetarians. So if they're not eating other animals, then why all the hostility?
"They kill just for defense purposes," Peter explained.
The sheer size of the animals, as well as their huge, sharp teeth make them a formidable enemy. Humans are no match for even a single chomp from these herbivores. "If [a hippo] gets you in the middle, it can cut you in two," Peter said.
At their largest, hippos can be 17 feet long and weigh nearly 10,000 pounds. Although they look a little bit like overgrown pigs, their closest animal relative is actually the whale.
Amazingly, despite their girth, they can run between 18 and 30 miles an hour, way faster than humans. In the water, they have astounding speed as well.
"They know how to swim very well," Peter said. "Whether you try to swim and they are faster than you."
The famous hippo yawn is actually a defense mechanism: Hippos ward off other animals by opening their mouths wide. "That's a way of scaring. That is a way of marking the territory and scaring any enemy away ... of even warning others that there is a problem here, there are some intruders," Peter said.
Hippos Enjoy Lazy Days in Uganda
Despite their ferociousness, hippos are also desperately camera-shy.
Every time "Nightline's" crew approached a group of hippos -- known as a "school" -- they either dove under the water and then peered out warily, or rumbled up onto the shore.
"These ones are now scared. They have seen us, they have seen the camera. So they have to move out of the water," Peter said.
Hippos spend most of their day lolling about in shallow water or wallowing in mud, which helps protect them from overheating. Lazy days are crucial for the animals; with their thick skin, they can easily overheat in the daytime sun, and even die.
Hippos also mate and give birth in the water.
Only at night, when the air is cooler, do they finally emerge from the water to feed, putting away up 150 pounds of grass a night.
Then, as dawn breaks, the animals return to water for another day of wallowing and resting.
Compared with other countries, Uganda is a veritable paradise for hippos. Despite occasional run-ins with the humans in the area, the hippos peacefully co-exist with other large herbivores like elephants and water buffalos.
"They are friendly because they are all grass-eaters. The buffalos cannot even fight a hippo; the hippo also cannot fight a buffalo, because they are all grass-eaters. You know it is better to fight a predator than fight your fellow friends," Peter said.
However, they have been known to tangle with predators like lions or crocodiles on occasion.
Hippos are a key part of an ecosystem that includes a variety of creatures, from goose-stepping cormorants to regal eagles to dancing antelopes.
On our visit to Uganda, we also encountered an aggressive male water buck.
These animals might seem mellow, but they aren't afraid to use their huge, intimidating horns to attack. Our guide and his colleague throw rocks to get him to move, but he wouldn't budge. We had no choice but to opt for an alternate route.
Carnage in Congo: Civil War Leads to Hippo Massacre
While wildlife in Uganda is thriving, right across this lake from where the hippos and their animal friends are living in relative peace is the Democratic Republic of Congo, where an all-out civil war has resulted in the massacre of not only people, but also hippos.
"Their numbers have reduced dramatically over the last 35 or so years. We've seen a reduction of 95 percent," said Kimunga Mugo, who works with the World Wildlife Fund. "It's basically a slaughter. It's a massive slaughter."
There were 30,000 hippos in Congo in 1974. Now, there are only 800.
They're disappearing because they're being eaten by competing bands of marauding militias.
"The armies are living off of hippo meat, because basically when they're out in the bush, they don't have any other source of food. They have to get their protein, their meat, from somewhere. They're basically decimating the population," Mugo explained.
Civilians are also eating the hippos, an appetite fueled by desperation and also by an ancient belief that eating hippo meat will make women fertile and provide more children.
The slaughter of the hippos isn't just bad for the animals themselves, it has also had a devastating impact on the environment, creating a vicious cycle of starvation. Hippo dung is a food source for the fish in this lake. As the hippos disappear, so do the fish, a major source of food for people around here.
Conservation groups like the World Wildlife Fund have been working feverishly to stop the killing of Congo's hippos, but it may be too late. They have helped move refugees out of national parks and out of the hippos' habitat.
But at least in Uganda, there are so many hippos that we literally bumped into one with our boat. Wildlife officials view the healthy population here with mixed feelings. There is a sneaking suspicion that the only reason there are so many here is because they are running away from the carnage in Congo.