Revisiting the “visible burrow system”: The impact of the group, social rank, and gender on voles under owl attack

• Corticosterone was higher in voles after than before life-threat (owl attack).
• Corticosterone levels were also higher in females than in male voles.
• Contagious fear may account for higher corticosterone levels in grouped voles.
• Voles with higher body mass had lower corticosterone than low-mass voles.
• Being less stressed by the owls, high-mass voles lead and stabilize their groups.

In the present study, corticosterone levels and behavior were compared between voles (Microtus socialis) that were attacked by a barn owl (Tyto alba) and voles that did not experience such attack. Both female and male voles were exposed to the owl either together with their groupmates or when socially isolated. As hypothesized, corticosterone levels were higher in voles after the owl attack, and were higher in females than in males. However, blood corticosterone was higher in voles that experienced the attack in groups compared with the socially-isolated voles. The latter result seems enigmatic, since group members usually benefit from the “social buffering” conferred by their group-mates. It is suggested that contagious vigilance among group members accounts for the higher mean corticosterone level in grouped compared to socially-isolated voles, over-shadowing the possible impact of social buffering. We also found a negative correlation between body mass and corticosterone level, with more high-mass voles showing low corticosterone levels compared with low-mass voles. This finding accords with a previous study in which the behavior of high-mass voles was less affected by owl attack compared to low-mass voles. The novelty of the present results therefore lies in supporting, at the hormonal level, past behavioral findings in rats and voles, and in demonstrating that high-mass voles, by virtue of their physical strength and perhaps also their life experience, are less stressed by the owl attack and become the leaders and stabilizers of their groups.