Hippos’ famous huge tusks and wide gaping mouths mean they are unable to properly chew the grass they love to gorge on, according to new research.
The study of zoo animals and museum specimens suggests modern hippos may have lost their ancestral side-to-side jaw motion so they could become better fighters.
Their prominent tusks and wide gape restrict grinding jaw movements – making their chewing inefficient, according to the Swiss study published in the journal PLoS One.
Co-author Professor Marcus Clauss, of the University of Zurich, said: “Most herbivores grind their food by moving the jaw sideways.
“Hippos have given up on this because they need a rigid jaw for fighting.
“Hippos have the widest gape of any mammal but are very inefficient at chewing, which limits how much they can eat.
“A possible explanation is that the size and arrangement of their teeth prevents them grinding their jaws side-to-side.”
The Swiss team observed chewing in common hippos (Hippopotamus amphibius) and pygmy hippos (Choeropsis liberiensis) using video footage of zoo animals.
They also measured the size and arrangement of teeth in the skulls of museum specimens from 86 common and 21 pygmy hippos, and looked for signs of wear that indicate how the animals chewed their food.
The team found little difference for either species in the width of their upper and lower jaw or cheek teeth, suggesting a mainly vertical chewing motion.
Prof Clauss said: “The size and positioning of their canine teeth don’t prevent side-to-side chewing, but long lower canine teeth limit the extent of this grinding action.
“Most importantly, common hippos have interlocking upper and lower front teeth which almost completely prevent side-to-side motion.”
Video footage and wear analysis confirmed that pygmy hippos use a slight “side-to-side” grinding motion when chewing, whereas common hippos rely almost exclusively on vertical jaw movements.
Prof Clauss said: “The slight side-to-side chewing movements used by living hippos suggest that the ancestors of both modern species relied more extensively on grinding jaw movements.
“Why modern hippos lost this ability remains unclear, but chewing efficiency may have been compromised in favour of evolving a rigid jaw and a wide gape, which is an asset during fights with other hippos.”
He added that Inefficient chewing might have restricted common hippos to a semi-aquatic lifestyle.
Produced in association with SWNS Talker
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