Because otherwise animals could kill and eat them. Which still does happen, because humans do hunt hippos for meat. Much less nowadays, but largely because both hunting and habitat destruction have lowered their numbers.
Lions still can and occasionally do take down hippos, and in fairly recent evolutionary history there were a lot of other animals capable of doing so. So it was advantageous for hippos to establish that they are not to be trifled with.
also keep in mind how hippos are built. They are very slow on land, and they have no real way to hurt anything that is attacking them from behind. So when they go into a fight or flight response because they feel threatened, fighting gives them a far better chance of survival than trying to run away.
All animals large enough to not give a fuck about predators are aggressively territorial, because they can be. What makes hippos so different compared to e.g. rhinos or even a big bull moose, is that it’s almost impossible for a human to accidentally get in the personal space of the latter, while it’s super easy with a hippo, because of how concealed they are. That’s why in Africa, hippos kill like 5x the people compared to all other wild animals combined.
EDIT, from my other comment, but should go here too: sorta analogous to how various Adders aren’t aggressive, but result in a lot more snake bites than other species, because people step on them a lot due to how they live. Same with hippos, people get close to them, or even bump them(in boats) waaay more often than they do with other large animals.
Everything is territorial. The difference is hippos have a very high rate of success at telling things to F off. If you keep winning you are more sure of yourself pushing others around. See humans, elephants, polar bears.
Now if there is a chance shit goes wrong for you. You might try but you might just skip it all together. See most everything else.
Have you seen what they share the waterways with? Have you seen what is prowling around on the shore banks?
Hippos don’t have much for defenses other than bulk and teeth, and they don’t have the numbers to form dense herds, nor the speed to escape predators, so their survival tactic is that the best defense is a good offense. Nothing will hunt you if they are scared to even pass by you.
Aggression is one of the “schools of thought” for herbivore survival. There’s being fast, there’s being hard to see, there’s being big, there’s being poisonous/unappetizing, and there’s being so angry and aggressive nothing wants to risk trying to hunt you. Most herbivores use one or more of these strategies. Many geese species are famously the same way. Hyper aggressive and defensive.
Predators are always living right on the edge. Even a small wound can be lethal if it makes it harder to hunt. They won’t want to hunt something if there’s a high risk they could get hurt in the process.
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