Dominance hierarchies are known in sciurid society, with pecking orders observed in both Reds and Greys. In Red squirrels, inter- and intrasexual (i.e. between and within sexes) dominance has been recorded; males are often, but not necessarily, dominant to females. Dominance or subordination seems to be based on a combination of size and age. Typically, the larger and older the squirrel, the more dominant it is likely to be and, frequently, the larger its home range.
A study of Grey squirrel behaviour in woodland on the Virginia Polytechnic Institute Farms site in the USA, led by forestry biologist James Pack, found that immature squirrels showed less social aggressiveness than older animals. Social rank was also observed to increase with age, and males were dominant over females in 79% of 323 interactions (66% when comparing equivalent age classes). In their chapter on the social structure of the Grey squirrel in the 2016 compendium The Grey Squirrel, Colin Lawton, Craig Shuttleworth and Robert Kenward note that juveniles aged between 3.5 and 6 months are dominant over non-resident juveniles and react aggressively to these intruders, while behaviour amicably towards littermates. Similarly, mothers are amicable towards their kittens, but will chase away un-related young.
In his study of the Grey population living on the campus of Kansas University, Koprowski observed amicable and agonistic interactions among adults, with adults chasing other adults being the most common of the latter type. Most chases end with the aggressor giving up and returning to their range or feeder when they feel they’ve made their point, while 18 (3%) of the 568 chases Koprowski witnessed ending in physical violence. In his 1996 paper, Koprowski noted that Grey squirrels form a well-defined and relatively complex social system, with females in particular being organised into interactive social units that formed as a result of females frequently remaining in the area in which they were born; so-called natal philopatry.
Interestingly, Koprowski observed a pattern in amicable behaviours, whereby close relatives were more likely to be shown amicable behaviour—i.e. grooming, playing, nose-touching, nest sharing—while more distantly-related animals, grandmothers and aunts for example, were treated much the same as strangers. Indeed, 75% of all amicable behaviour recorded was between sisters, while only about 12% was between grandmother and granddaughter.
Koprowski doesn’t mention any examples of grooming, playing or nose-touching between males, although he did observe them sharing nests, with ‘bachelor groups’ accounting for just over one-third of all nest sharing incidents. Similarly, Erkki Pulliainen and Paivi Jussila observed Reds nest sharing on the Värriö Nature Reserve in Lapland during the winter of 1991/92. In a 1995 paper, the zoologists described how two individuals of different colour phases lived in the same area of pine forest, frequently fed together and, on nine occasions, were observed sharing one of the four dreys located in the area. This study also observed regular nest sharing by two squirrels outside the breeding season when empty dreys were available in the vicinity, suggesting some individual preference in the behaviour rather than it being controlled solely by environmental factors.
During a detailed study of the Grey squirrels living in a 4.5 ha (450 acre) wood at Silwood Park, Berkshire over a 12 month period starting in November 1964, M.A.F.F. biologist Jan Taylor documented their agonistic behaviour. Taylor noted that there was a well-defined scale of precedence at feeding sites, where adults were dominant to juveniles as well as separate hierarchies within and between sexes; it seems that the dominant male took precedence over all females and the dominant female fed preferentially to males and females lower in the social order.
In a paper to the Symposia of the Zoological Society of London during 1966, Taylor described the dominant and submissive body language she observed when two squirrels met. A narrowing of the eyes, ears brought forward (exposing the white 'puff' behind the ear), hackles raised, tail hair standing on end, flapping of tail, padding of hind feet and chattering of the teeth were all considered dominance signals. Conversely, wide eyes, the covering of the white fur behind the ears, avoidance (often abruptly altering course), squealing upon capture, and high-pitched growling accompanied by swiping with forepaws were all submissive behaviours.
The use of ear puffs and tail hairs during aggressive encounters is apparently most important to females while they're raising kittens (and thus defending their young and competing with other females for access to food); Taylor notes how: “...summer-breeding females are the last to lose the tail and ear hairs which is so important in displays of aggression.” Territorial disputes among squirrels can result in injuries incurred from falls and bites. Indeed, injuries to the back, head and ears are occasionally sustained and bites to the tail appear relatively common; in some cases, sections of the tail may be lost altogether. Indeed, I have seen completely tail-less squirrels.
Tail loss may come about through accident (near misses with cars or predators), fights with other squirrels (damage to the base of the tail can cause the skin to peel away, drying the vertebrae out and causing the tail to drop off) or could potentially be congenital. The tail will not grow back and one might consider its loss would be a considerable hindrance to the squirrel. Squirrels use their tails as a counterbalance and for improved aerodynamics while moving through the treetops, so those lacking a tail might be expected to be less adept in their arboreal activities, although this has not been my experience.
The tail also plays an important thermoregulatory role; used to catch sun while sunbathing, to shade the squirrel during intense heat and is wrapped around the body during cold weather, acting as a blanket. In the base of the tail is a network of blood vessels that form a counter-current heat exchange system (CCHES). Squirrel tails have a large surface area to volume ratio, which means they lose heat rapidly, even despite the fur. In hot weather the CCHES can be bypassed and the squirrel pumps warm blood straight to the tail; heat is radiated to the environment, cooling the squirrel. In colder conditions, by contrast, warm blood from the heart goes through the CCHES where it passes close to cooler blood coming back from the tail; close enough to allow the heat to radiate into the cooler blood and be returned to the body. This prevents valuable heat being lost to the environment when the weather’s cold. Tails are also used during social interactions, with vigorous tail flicking accompanying many vocalisations, and when scent-marking.
These factors suggest that tail-less squirrels should be at a disadvantage and thus should have reduced survival; there is, however, little evidence of this. In his 1954 study on the biology of the American red squirrel (Tamiascurius hudsonicus) in New York, James Layne found that 20% of the adults he caught had injuries to the tail, ranging from minor nicks to half the tail missing, while only 8% of the immature individuals had damaged tails. Among the adults, there was a discrepancy between the sexes, with 25% of the males and 14% of the females sustaining damage to their tails. Layne noted how: “Bob-tailed individuals are not visibly handicapped by the shorter length of this member”. I have not found any comparable data for squirrels in Britain, but the tail-less animals I have seen did not appear to be in particularly poor condition. That said, all tail-less animals were in a city park and the superabundance of food may be a consideration here.