Some of these sounds - such as a cat's purr - are found only in a single group of related animals. (Not all wild cat species purr, of course, but many do). Others are, at least in general terms, produced by a range of different species, often in quite similar circumstances. One of these is the 'scream', an unusually loud call that is typically high-pitched. Screams also commonly include rapid, chaotic, changes in frequency and amplitude that we humans interpret as a harsh, inherently unpleasant, sound - and there's likely good reason for that.
The point of a scream is that it can be heard a long distance away, but why exactly an animal might want to do that can vary. It may, for instance, be a call for assistance, perhaps when an animal is frightened, injured, or lost, or they may represent efforts to drive away a rival or potential predators. More generally, it has been argued that loud, intense, calls, such as screams, tend to indicate that the animal in question is in a highly emotional state (although not necessarily a bad one), something that's quite obviously the case in humans.
So far, so not very surprising. But, whether a call is being used as a warning to others, or a call for help, it's helpful if the listener can have more information than 'something bad just happened'. If I'm going to come to the rescue of another animal in trouble, for instance, just how much difficulty am I about to get myself involved in, and is it worth my time? Come to that, is it genuinely a call for help, or is the other animal just having a temper tantrum?
In humans, it's generally not that hard to tell the difference between a scream of terror, a cry of triumph, a wail of despair, or a yell of rage, and many animals can pick up on similar distinctions in the screams of their own species. Spider monkeys, for instance, make subtly different screams when attacking a rival as they do when they are the rival being attacked (perhaps anger versus fear, in a human context). And, along with many primate species, vampire bats have individual 'voices' allowing listeners to distinguish who exactly it is that is doing the calling.
One of the primate species whose vocal repertoire has been particularly well studied is the vervet monkey (Chlorocebus pygerythrus). Vervet monkeys live across much of East Africa, typically in savannah and open woodland, rather than dense forest. They are highly sociable, with groups often containing several dozen members, and sound is a key part of how they communicate. We know, for example, that they have different alarm calls for different kinds of threat, and that they can identify specific individuals by the sound of their voice - for instance, when hearing an infant crying, unrelated females will turn to look towards that infant's mother, even if that isn't where the sound is coming from.
Vervet monkeys tend to scream when they are involved in aggressive interactions, but these can vary from relatively mild situations, such as being stared at in an intrusive manner, or just hearing a dominant male making an aggressive grunt, to those that are rather more serious, all the way up to violent biting attacks. Furthermore, both the attacker and the victim will be screaming at each other and, since the purpose of the scream is to alert other monkeys who might not be able to see what's going on, anyone wanting to intervene would surely find it useful to know which is which before putting themselves at risk.
So are there differences in the sounds that the monkeys make depending on the situation? A four-year study of a group of 26 wild vervet monkeys in KwaZulu-Natal recently showed that in such contests, the victim gave out longer and higher-pitched screams than their attacker. This in line with general principles across a number of animals - birds as well as mammals - that screeches of fear are typically higher-pitched than screams of anger or intimidation in the same species.
The same principle, however, would also suggest that screams would be longer and louder the more violent the fight became. If nothing else, the animal's emotional state would presumably be more intense in situations where it is genuinely in fear of being bitten or otherwise physically hurt. But the study seemed to show the opposite. While there's always the possibility that this one study is just a fluke, something similar has been previously shown in chimps. One plausible explanation is that the animals are simply getting out of breath due to the exertions of the fight.
Chimps being attacked are also more likely to scream loudly if they believe that there is a close relative nearby who is larger and more dominant than their attacker - the advantages of which are fairly obvious. In the case of the vervet monkeys, it's less obvious that this is going on. In around 20% of the fights, a third party did, indeed, join in on hearing the screams... but almost always on the side of the attacker.
In one sense, that's probably wise, since the attacker is likely to be the more dominant individual, making helping them out a much safer proposition than chasing them. But, while it may partly explain why the attacker is screaming (aside from intimidating their opponent), it does raise the question of why the victim is bothering. Of course, sometimes they do receive help, which could well turn the tide in their favour, so it may not be entirely fruitless, especially if they have close relatives nearby. It's also possible that screams of fear are a sign of submission, although, if so, it's obviously not a cast-iron one.
At any rate, it seems that altruistically helping out victims isn't high on a vervet monkey's list of priorities. Monkey generosity only goes so far.
[Photo by Joachim Huber, from Wikimedia Commons.]
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