river otter vocalisations_2014_11_18
Image: Steve Wilson, Flickr

Giant otters, the river- and lake-dwelling Amazonian cousins of the charismatic oceanic critters, are adorable and goofy and smell vaguely of fish. These endangered animals are fairly unique from other otters by their high level of sociality: they form family groups of as many as nine individuals, comprised of a breeding pair and their juvenile and sub-adult offspring from different breeding seasons. They're also quite the chatterboxes, known to be quite noisy.

University of Ulm researchers Christina A. S. Mumm and Mirjam Knörnschild report this week in the journal PLoS ONE that the most social of otter species is also the most vocally complex. Giant otter adults can produce 22 distinct vocalisations, while the infants have around 11 different types of calls.

To figure out how otters talk to each other, the researchers went to Peru and recorded the chatter of five otter groups from five different lakes and made detailed observations about what the otters were doing so they could match the behaviours up with each recorded bark, hum and scream. They also recorded the babble among the giant otters in three different German zoos.

They were able to work out, for example, that a whistle means something like "I'm hungry" while a growl means "nuh-uh, this fish is mine!". Snorts alert other otters about an approaching predator, such as a human or a caiman, while isolation calls might mean "hey, where'd everybody go?!" since they are used only when an otter has lost sight of its group. 

The calls all fall within five broad categories. There are the cohesion calls, which keep the group together and coordinated, the alarm calls, which are used either to alert others about a possible threat or to maintain the dominance hierarchy, and the begging calls, which are used to ask (or demand) others to share their food. Two other types of calls are emitted only in the contexts of mating or nursing.

Why do giant otters have such an impressive array of sounds? It might seem surprising, since neotropical otters only have four calls, Eurasian otters have around eight and the familiar sea otters have just ten. 

In 2012, psychologist Todd M. Freeberg put forward the social complexity hypothesis for communication. The basic argument goes that animals who negotiate complex social systems will require a more complex means of communication. So far the pattern seems to hold up: primate species that live in bigger groups or within more complicated social hierarchies have a wider variety of calls than those with simpler social lives. Humans are perhaps the most social and the most vocal of all primates. The pattern also holds up for canids (wolves, coyotes, dogs and their relatives) and even for the herpestids, animals like mongooses and meerkats.

So is this true for otters as well? The authors of the new study argue the otters represent a good suite of species to further test Freeberg's idea, because among the thirteen species still in existence there is wide variation in both social and vocal complexity.

But Mumm and Knörnschild note that large group size in an otter species doesn’t guarantee vocal complexity – sea otters, for example, gather in the thousands, but have only around ten different calls. Rather than the number of individuals in a group, it’s the strength of social relationships that matters here, the authors argue.

In other words, having lots of otters within a social group does not necessarily mean that their relationships are complicated. The massive aggregations of sea otters are made up just of males and only occur when they're not busy trying to mate with the females. Though many may turn up in the same place, their associations are simple and fleeting. The giant otters, on the other hand, might live in smaller groups, but their relationships are longer lasting and more nuanced. It makes sense, then, that they'd need more ways to talk to each other.

The researchers hope that an improved understanding of giant-otter chatter could help to facilitate conservation efforts for the endangered species. That could be thanks to a greater interest taken in the animals by scientists who study vocal communication. More practically, this information could prove extremely useful as researchers and conservationists attempt to evaluate individual otters' stress by listening in on their calls.

Top header image: Geoff Gallice, Flickr