Social interactions, especially those related to the defence of resources or kittens from interlopers and predators, often involve sound. Indeed, Red squirrels are known to produce both loud and soft “chucking” noises, a vehement “wrruhh-ing” as well as various moans and teeth chattering. Piercing screams have been documented during particularly aggressive encounters and Red kittens produce “shrill piping calls”. While studying Greys on the Auburn University campus in Alabama between April 1978 and June ‘79, Robert Lishak recorded 5,000 vocalisations from 82 squirrels and grouped them into 11 different call types. Adult alarm calls consisted of four distinct types: buzz, kuk, moan and the repetitive barking quaa. Lishak noted that some hybrid calls were produced, including the buzz-quaa that was “not unlike a chicken’s clucking” and modulated quaas or quaa-moans. Lishak also described vocalisations that started and ended abruptly, with a mean note duration of 0.05 seconds; these were the mating calls and varied according to the stimuli the squirrel was exposed to.
The "chuck" or "quaa" call of a Grey squirrel. - Credit: Marc Baldwin
The short, soft "chukking" call of Red squirrels during a chase. - Credit: Marc Baldwin
When turning his attention to nestling squirrels, Lishak could decipher a further five calls: loud (72dB) distress calls in the form of squeaks that travelled considerable distance; growls; screams that were also sometimes emitted with growls; tooth chatter that was associated with aggression and often accompanied by both screams and growls; and a range of what he termed ‘solicitation calls’, that were non-vocal and involved the nestlings smacking their lips or vibrating their nasopharyngeal membranes. Squeaks and lip-smacking were the first calls to be produced, while growls, muk-muk and screams didn’t materialise until the nestlings were four weeks old, and tooth chattering began with the eruption of the incisors at about eight weeks old.
The agonistic and alarm behaviour in squirrels is complex and, using a model of a grey squirrel that ‘barked’ (probably the 'muk-muk') and flicked its tail, biologists in the USA demonstrated that it was the combination of these actions that the squirrels responded to most often; they generally paid little attention either signal on its own. More recently, however, data have suggested this response may vary across populations, depending on the habitat.
In a recent series of experiments, scientists based in Massachusetts found that squirrels in urban areas were more active and responded more to tail flicking than to barking, while those in more rural settings used both cues. This, the biologists suggested in their 2010 paper to the journal Current Zoology, could indicate a shift by urban squirrels to rely more on visual cues than audible ones, which could be attenuated in the noisy environment of the city.
Squirrel kittens may vocalise during play and in his book, Charlie Brown, about a hand-reared Grey squirrel, Mike Towler wrote:
“Young Charlie had quite a vocabulary, although he did not ‘talk’ a lot. Very different sounds, unlike most animals where it is variants on the same theme. And he could be LOUD! He got all excited playing on the floor and let out one of the most piercing yells imaginable.”