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.
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.