**Context of the study** Thermoregulation represents all the processes necessary to maintain internal temperature. For endotherms, body temperature is mostly regulated by internal processes. Among them, warm-blooded animals have to maintain a higher temperature than the environment despite its variations. This is the case of humans who can produce heat by two processes: shivering (immediate, short-term response) and non-shivering thermogenesis (long-term response). Non-shivering thermogenesis mainly relies on a type of tissue which produces heat: brown adipose tissue or BAT. This tissue is relatively present in adult humans, but its physiological activity depends on external and internal factors [(Cohade et al. 2003)][1]. Another strategy to regulate body temperature is to use social thermoregulation. Indeed, to reduce the metabolic costs of individual thermoregulation, warm-blooded endotherms can rely on the heat of cooperative conspecifics. The behaviours enabling this “social thermoregulation” are associated with kleptothermy in animals, which refers to the behaviour of sharing thermogenesis with another animal. In the wild, it is observed in many species (for example, huddling in penguins). Humans also rely on others' temperature (for example, hugging or cuddling) to some extent. However, we don't know if kleptothermia can compensate for the two individual methods of thermogenesis. The potential trade-off between individual thermogenesis, especially non-shivering thermogenesis (BAT), and social thermogenesis has never been explored before. Social thermoregulation postulates that humans can regulate their temperature by relying on others' temperature which contributes to relationship maintenance and interpersonal attachments [(IJzerman et al, 2015)][2]. To assess to what extent people resort to social thermoregulation, a questionnaire on individuals’ desire to socially thermoregulate and risk avoidance was elaborated in 2019 [(Vergara et al. 2019)][3]. The results of this questionnaire supported the existence of interindividual differences in social representations of temperature. Moreover, they showed a correlation with attachment. This link between attachment and social thermoregulation is supported by results showing that coldness (respectively, warmth) makes people think about positive (respectively, negative) relationship experiences [(IJzerman et al, 2018)][4]. To counter temperature variations, the individual may want to thermoregulate socially based on their own attachments. This desire to thermoregulate socially may depend on individual thermogenesis. BAT thermogenesis could explain whether and how we rely on others to regulate our temperature. Therefore, individual thermoregulation might affect the desire for social thermoregulation and attachment styles. The present study explores a specific area: the contribution of individual differences in non-shivering thermogenesis, particularly brown adipose tissue (BAT) to these social behaviors. BAT produces heat especially in the postnatal period whenever the organism needs extra heat without using shivering, after a meal or when there are high body energy reserves. When there is reason to increase the rate of food combustion (decrease metabolic efficiency) or increase the rate of heat production, a signal is transmitted from the ventromedial hypothalamic nucleus (VMN) via the sympathetic nervous system to brown adipocytes. It releases norepinephrine (NE) which initiates the release of lipids in the blood [(Cannon & Nedergaard, 2004)][10]. In 2003, [Cohade and colleagues][11] found that a significant and variable amount of BAT (what they called “USA-Fat”) remains in adults in the supraclavicular area, a region extending from the an-terior neck to the thorax. There is variation in the amount of BAT people have: women are thought to have a greater mass of BAT than men. The amount of BAT detected decreases with age and, among older patients, with body-mass index [(Cypess et al, 2009)][12]. These results suggest that amount of BAT vary between individuals depending namely on sex, age and body-mass index. BAT is also highly correlated to the environment’s temperature and the photoperiod of the day indicating that BAT vary also within individuals [(Cohade et al, 2003)][13]. The individual dynamic of thermoregulation is intimately related to non-shivering thermogenesis for which BAT is responsible. To complement the activity of BAT, individuals can be prompted to count on others to coregulate their temperature. Indeed, people with more BAT may rely more on their own individual thermoregulatory mechanisms. Their thermogenesis is less dependent on others so they could be more secure about their relationships. The opposite relation may also exist: individuals with fewer BAT would struggle more to regulate their temperature and would tend to seek warmth from others. That is why we think amount of BAT could be linked to attachment style and desire to socially thermoregulate. The link between social behaviours and BAT has never been explored. The present study aims to estimate how the amount of BAT can account for individual social behaviours namely attachment style. Here are the main hypotheses: - The amount of BAT is negatively related to people’s reliance on others meaning that BAT is positively correlated to secure attachment style (H1), relatively low desire to socially thermoregulate and low sensitivity to temperature changes. *Auxiliary hypotheses* Since it is an exploratory study, none of the hypotheses above has been supported by previous research. However, our results may be able to support or not some previous results: - On average, BAT is more active in women [(Cypess et al, 2009)][14] - BAT activity depends on the outside temperature of the day and photoperiod [(Au-Yong et al. 2009)][15] [(Cohade et al. 2003)][16] [1]: https://pubmed.ncbi.nlm.nih.gov/12571205/ [2]: https://www.frontiersin.org/articles/10.3389/fpsyg.2015.00464/full [3]: https://www.researchgate.net/publication/337577827_Development_and_Validation_of_the_Social_Thermoregulation_and_Risk_Avoidance_Questionnaire_STRAQ-1 [4]: https://www.researchgate.net/publication/326126238_Socially_Thermoregulated_Thinking_How_Past_Experiences_Matter_in_Thinking_about_Our_Loved_Ones [5]: https://www.frontiersin.org/articles/10.3389/fpsyg.2015.00464/full [6]: https://psycnet.apa.org/record/2013-08589-013 [7]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2737341/ [8]: https://econtent.hogrefe.com/doi/10.1027/1864-9335/a000142 [9]: https://econtent.hogrefe.com/doi/10.1027/1864-9335/a000142 [10]: https://journals.physiology.org/doi/full/10.1152/physrev.00015.2003 [11]: https://pubmed.ncbi.nlm.nih.gov/12571205/ [12]: https://pubmed.ncbi.nlm.nih.gov/19357406/ [13]: https://pubmed.ncbi.nlm.nih.gov/12902417/ [14]: https://pubmed.ncbi.nlm.nih.gov/19357406/ [15]: https://pubmed.ncbi.nlm.nih.gov/19696186/ [16]: https://pubmed.ncbi.nlm.nih.gov/12902417/