ORIGINAL ARTICLE Amy Haigh • Ruth O’Riordan • Fidelma Butler Variations in aggression and activity levels amongst squirrels inhabiting low and high density areas Received: 12 April 2017 / Accepted: 30 August 2017 Ó The Ecological Society of Japan 2017 Abstract Variation in an individual animal’s behavioural traits has been observed in a wide range of species and is believed to have important fitness consequences. As part of a larger study, it was observed that squirrels occurred at different densities on a 315 ha island, and this sub- sequently led to variations in faecal cortisol metabolite levels. This study aimed to examine whether living at different densities would also lead to variations in activity and aggression levels and in their survival probability, breeding and body condition. In order to examine variations in activity and aggression, beha- vioural tests (open field test, mirror image stimulation, breath rates and struggle rate tests) were conducted on 32 individuals (15$, 17#), a total of 69 times. Activity in the wild was investigated through radio tracking. There was a significant correlation between the time a squirrel was active and in their aggressive behaviour during the behavioural tests, with more aggressive individuals also being more active. Squirrels in the low density area spent a greater proportion of time active and engaged in aggressive behaviour in the open field test but also moved further in the wild and had a lower breath rate and higher struggle rates. Squirrels in the high density area were the least aggressive, had a smaller home range and higher breath and lower struggle rates. However, this was found to have no effect on survival probability, breeding or body condition. These variations may be the result of sampling dispersing squirrels. Keywords Aggression Æ Activity Æ Home range Æ Fitness Æ Stress Æ Tolerance Introduction Behavioural traits are thought to have important fitness consequences (Boon et al. 2007; Minderman et al. 2009) and may be reflected in many aspects of an animal’s behaviour and ecology, including habitat use, interac- tions with conspecifics, and willingness to take risks (Boon et al. 2008). Such traits can determine how indi- viduals cope with various environmental challenges by behavioural means (Møller, 2010), for instance, less aggressive individuals might do well in situations where low aggression is favoured, but poorly in competitive situations (Sih et al. 2004). The correlation of different behaviours mean that an individual’s behaviour is not infinitely flexible. ‘‘An individual cannot adjust its be- haviour to perfectly match each situation, but behaves in a similar manner in many different types of situations’’ (Korpela et al. 2011). Even if most individuals exhibit limited plasticity and a poor ability to cope with envi- ronmental change, a species can still respond adequately to the change, if the species harbours substantial varia- tion in behavioural types so that at least some individ- uals respond well (Sih et al. 2004). Therefore, no single specific behavioural phenotype is favoured by selection, as changing and unpredictable environmental conditions will favour different phenotypes. As a result, heteroge- neous populations, with individuals of both bold and shy personalities exist, allowing for adjustment to vari- able selective forces (Møller 2010). Sih et al. (2004) define behavioural syndromes as a combination of correlated behaviours. Behavioural syndromes may reflect a selection of combinations of behaviours that work well together in a certain habitat (Penke et al. 2007). Adriaenssens and Johnsson (2011) found support for behavioural syndromes in brown trout (Salmo trutta) with less explorative individuals being less aggressive and showing more flexible beha- viour. Similarly, Both et al. (2005) suggested that slow exploring great tits (Parus major) may be better parents, because they respond more easily to environmental Electronic supplementary material The online version of this article (doi:10.1007/s11284-017-1506-8) contains supplementary material, which is available to authorized users. A. Haigh (&) Æ R. O’Riordan Æ F. Butler School of Biological, Earth and Environmental Sciences, Univer- sity College Cork and Fota Wildlife Park, Carrigtwohill, Cork, Ireland E-mail: amyjohaigh@yahoo.com Tel.: 00353 86 1536327 Ecol Res DOI 10.1007/s11284-017-1506-8