This common hippopotamus from Serengeti National Park, Tanzania, is yawning big but not because it's tired! In hippos, yawns like this are usually a threat display: a way to show off their giant canines, which can slice right through an opponent's thick hide. Hippos are a hugely important part of aquatic ecosystems in Africa. Since they eat grass on land at night and sleep, socialize, and most importantly, poo in the water during the day, they deposit nutrients into the water that feed aquatic insects and fish. Those nutrients then work their way up the food chain, to predators like fish eagles, mongooses, and even humans. Hippos are classed as Vulnerable on the IUCN Red List because they're declining throughout their range in sub-Saharan Africa. As the human population grows, hippos increasingly compete with humans for water, and the hippos normally lose out.