What’s the gardener saying?

When I opened my front door this morning I found a massive footprint on my door step.

It was a hippo's.

But this signature print was just the finale in a very weird performance.

Continuously, for about two hours last night, one of the local hippos performed a deafening serenade of growls, grumbles, wails, chortles and elephant-like trumpeting. Now I'm used to the hippos making a noise: their bellows reach 115 decibels (which is the volume recorded 5m from the speakers at a heavy metal rock concert). Nevertheless, it's only once every couple of months that a hippo indulges in these strange soliloquies. I have no idea why.

Local hippos.

After I retired to bed, the illustrious singer visited my garden. Of course, around here, it's not unusual for a hippo to crop your lawn, and I'm the first to admit that the waist-high grass in my front yard needs slashing. But last night's gardening activities were peculiar. The hippo awkwardly clambered down the three steps to my front door (where he left his print) and then chomped away a two metre-wide strip of grass, right along the wall of my house. He totally ignored the self-same grass growing everywhere else in the garden.

Was it just coincidence that my gardener 'performed' before coming to the house? Was he trying to warn me, or vocally intimidate me, to ensure undisturbed grazing? Or was his visit to my house, the final act of bravado in some macho display for a rival or potential mate?

OK, not crop-circles, but this awful photo shows the cropped strip on the left and the untouched grass on the right.

My ignorance of hippo vocalisations is a constant frustration. Unfortunately no-one (except hippos) knows what they're saying. For example, the classic hippo bellow that you'll have heard on wildlife documentaries is thought to be a territorial call because when one animal bellows, other hippos in the distance respond in kind. But after living by the river for two years, I'm sure this call is used to warn of intruders, and that the hippos 'pass it along'. They bellow whenever a human approaches the river (including me walking the dogs) and, for me, it's a fail-safe warning of someone being about.

But hippos have a problem. As you'll have noticed when swimming, the surface of the water blocks sound. When you're swimming underwater, you can't hear someone shouting from the bank, and they can't hear you scream (you can tell I live next to a crocodile-infested river). So if you're a hippo and half your pod is submerged, while the other half is drifting with their ears and eyes above the water, how do you warn them of an intruder?

Hippo facial features are designed so hippos can keep their ears and eyes above water while remaining safely submerged.

Photo by Lloyd Nel and borrowed from here.

Recent research by William Barklow, from Framingham State College in Massachusetts, has found that when hippos bellow, they snort the sound into the air through their nostrils (like an explosive sneeze), yet they simultaneously keep their mouth and lower jaw submerged, so as to call underwater too. Using underwater microphones, Barklow found that hippos are extremely 'talkative', but 80% of their calls are inaudible to us because they're given underwater. Hippos produce a huge range of sounds, including 'clicks' similar to those made by killer whales and, like elephants, they use infrasound (frequencies too low for our hearing) which carry for many kilometres.

Unfortunately, despite all these exciting discoveries, we still have no idea what hippos are actually saying to one another.

Photo by Neil van Asselt and borrowed from here.

Barklow, W. E. 1997. Some underwater sounds of the hippopotamus (Hippopotamus amphibius). Marine and Freshwater Behaviour and Physiology: 29, 237-249.

Barklow, W.E. 2004. Amphibious communication with sound in hippos (Hippopotamus amphibius). Animal Behaviour: 68, 1125-1132.