The Hippo – unlikely underwater communications expert
This week, the focus is on the distant cousin of whales and dolphins and what we can learn about the evolutionary journey of marine mammals by studying them;
What makes the Hippopotamus so interesting to marine biologists is that in them we can see a rudimentary version of the instruments marine mammals use to communicate underwater today.
For animals that spend long periods of time underwater, sound is a vital means of communication. In some areas of deep water less than 1% of sun light can penetrate but sound carries for miles regardless. Marine mammals therefore almost exclusively communicate by producing underwater sounds.
Scientists in Africa have discovered that while they are submerged, Hippos communicate below the water by producing sound in a similar way to marine mammals. Its been suggested that these noises, which are essentially “throat gargles” sent out through a fatty layer under the hippo’s chin, could show us the way that the ancestors of todays’ whales adapted from life on land to life under the sea.
The team studying this used underwater microphones called hydrophones to record the sound hippos produce whilst submerged in rivers, and they’ve interpreted them as two types of communication; statements of territory and mating signals. There is a lot of crossover with the signals that marine mammals convey to each other underwater.
Mysticetes (baleen whales) produce calls in a way not dissimilar to this. They use the equivalent of vocal chords in the larynx called the U fold, but have a specially adapted sac that they amplify the sounds through. It can be adjusted to change the frequency of the sounds and this is how the melodic mating calls of humpback whales are created.
The significance of the fatty layer under the chin becomes apparent when we look at the science of producing long distance sounds;
The most efficient mechanism of amplifying a noise is to produce it in the same environment as it is being sent into. This means that it doesn’t have to be transformed into a different form before it is emitted, it is already in the right form to travel easily through the environment.
For example, Humans live in a terrestrial environment surrounded by air, so we produce sound in our throats, an air pipe into the body. Humans use air flow over the vocal chords to make sound waves before sending them out into the surrounding air. Marine mammals live in an underwater environment surrounded by water, so its more efficient to produce sound waves behind a fatty liquid barrier before sending them out into liquid surroundings.
Hippos have a layer of sub cutaneous fat in front of their throats, and Bull hippos use this as a kind of transmitter. By using this fatty amplifier they don’t expend too much energy and can produce throat gargles that other hippos can hear for long distances underwater.
This can be thought of as a kind of ancient prototype for the toothed whale, or odontocete, communication system we see today. Over the millennia it has become a much more sophisticated system and toothed whales produce many more varied and complex sounds than hippos. The main differences are that odontocetes have a fatty amplifier in their foreheads instead of their chins, and they produce noises using an organ evolved from the nasal passage rather than their throats.
The fatty amplifier in their forehead, called the melon, is a fat-filled sack that can be flattened or condensed like a lens to focus noises in different directions. The sounds originate just behind the melon from an organ called the “phonic lips”. This is a narrow passage containing two membranes, like our nasal passages, that can be sucked in or slapped together to make the whistles and clicks that dolphins are renowned for.
Hippos produce territorial and mating calls underwater using a combination of their vocal chords and a layer of fat for amplification. It’s interesting that they seem to use a mixture of mechanisms that both families of whales use.
Hippos share an ancestor with whales and dolphins, so this is seen as evidence of how marine mammals evolved into the animals we see today. As their ancestors adapted to spending more and more of their lives underwater, their bodies changed and the tools for creating sounds underwater became more refined.
As distant cousins, Hippos and whales are quite similar on paper. Studying them further could tell us a lot about the journey marine mammals went on to become so well adapted to life under the sea.
Amphibious communication with sound in hippos, Hippopotamus amphibius, WILLIAM E. BARKLOW, ANIMAL BEHAVIOUR, 2004, 68, 1125–1132
Hippopotamus and whale phylogeny, Jonathan H. Geisler, Jessica M. Theodor, Nature volume 458, pages E1–E4 (19 March 2009)