Among the most studied of such calls are those of bottlenose dolphins (Tursiops spp.) which are the type most commonly seen in aquaria, making them relatively easy to observe. Having said which, it was only in 1998 that scientists confirmed that there was more than one species of bottlenose, leading to some confusion as to which one lived where and, by extension, which one any given prior study referred to. And, as I blogged about when the announcement was made back in 2011, we now know of a third one as well.
Be that as it may, the most common call that bottlenose dolphins make when kept on their own in captivity is the 'signature whistle'. Even in the wild, these seem to be a very common type of call, suggesting that their primary use it to try and keep the group together.
It's been clear since at least the 1960s that signature whistles vary between individuals - hence the name - likely allowing the listener to identify a specific dolphin by its call. Having said which, it's interesting to note that on occasion, dolphins appear to deliberately mimic the call of another individual, especially if its one that they know well. This may be less common in the wild than in captivity, although it does still happen in at least some contexts. Since this doesn't seem to have anything to do with deception or trickery, it's been suggested that this may be a way of attracting the attention of the dolphin being copied, rather as we might call a pet dog by using its name.
However, these signature calls are by no means the only ones that dolphins make. Certain other calls seem to be made by several different members of the same group, and presumably have a common function in the manner of, say, a monkey's alarm call. For example, some calls might translate as "hey, there's some great food over here," or perhaps even provide some details as to its type or abundance. Significantly, when responding to these types of calls, the dolphins don't seem to care whether it's been made by a close relative or not, whereas they definitely pay more attention to signature calls from their own kin.
However, just because these calls do not uniquely identify the caller does not necessarily mean that they are inbuilt, and found in all members of the species, as an alarm call might be. Indeed, this level of sophistication would not be unique to cetaceans, since it has also been observed in other animals. Songbirds are a particularly common example here. Back in the 1980s, for example, it was shown that Australian magpies within a given social group share particular songs with one another that differ from those of birds further away. On the other hand, song sparrows not only share songs with their neighbours but directly imitate a rival as a means of escalating aggression - from a human perspective, it's almost as if they're mocking one another.
In the world of mammals, similar patterns of calls shared by groups but not the whole species are also seen among bats and primates, as well as at least some other cetacean species.
Why might animals evolve distinct versions of calls that differ from those of their neighbours? There are a number of possibilities, not all of which are mutually exclusive. They might signal membership of a group so as to exclude competitors for a food supply or mating opportunities, or they might help individual members of the group coordinate their activities without interference from outsiders. Another possibility is that, by making the call distinctive, it's easier for members of the group to pick out from background noise much as a human might suddenly notice their name being mentioned in a conversation on the other side of the room.
Given that there are clear advantages to both being able to uniquely identify an individual and to identify which group that individual belongs to beyond merely confirming that they're a member of your own species, we'd expect that, in social animals with a complex vocal repertoire we'd find both. So how well does that apply to bottlenose dolphins?
A recent study listened to the calls of a group of dolphins maintained by the US Navy at the Naval Base in San Diego. In addition to acoustic analysis of the calls, the researchers also used human observers, asking them to rate how similar the calls sounded. (This is evidently trickier than it might sound; the researchers note that about 20% of their observers sometimes rated calls as different even when surreptitiously fed duplicate recordings of the same one, and had to be excluded from the final analysis).
Consistent with previous studies, the five dolphins in this group produced signature whistles about 43% of the time, and these were easily assigned to specific individuals. A great many calls didn't fit any obvious pattern to human ears, but the most common non-signature calls were extremely similar across the group. These were quite a distinct form of whistle, notably different from both the signature calls and the miscellaneous ones. Even here, though, human observers were able to figure out which dolphin was making the call about half the time (with five dolphins, they'd still get it right 20% of the time if they were guessing randomly, but this is a good deal better than that). It's likely rather easier for the dolphins themselves, implying that the signature calls have a specific context beyond merely identifying an individual.
It's worth noting that the dolphins in the study weren't closely related to one another, at least so far as we know (they were caught years apart). They had been living together for over 20 years, however, and there have long been suggestions that dolphins living together develop a common dialect to identify one another even if they start out sounding different. Having said which, some recent studies have indicated that this might, at best, be an oversimplification, and, in this case, we have no proof either way since we don't know what they sounded like 20 years ago.
This is a preliminary study, since it gives no real clue as to what the non-signature calls are really used for and why they would be different to the signature calls if it's still possible to identify the specific individual making them. But it does imply that both similarity and distinctiveness are important to bottlenose dolphins when it comes to membership of a group and that their social lives likely have levels of sophistication that we're only beginning to understand.
[Photo by Gregory Slobirdr Smith, from Wikimedia Commons.]
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