Social tethers: density-dependent social tethers inhibit fitness

Jillian M. Kusch, Kristy Ferraro, Eric Vander Wal

Putative mechanisms affecting fitness that underlie why animals occupy a particular place are often in tension. A tension amplified in social animals, where individuals are often not free to make independent habitat selection or foraging decisions. The ideal free distribution (IFD) is a density-dependent emergent property describing how individuals should distribute themselves to maximize fitness on the landscape. However, IFD is agnostic to an individual’s propensity to be social, which is known to influence their habitat selection. Our agent-based model tested whether social behaviour explains undermatching, a distribution pattern where more individuals are found within poorer patches than predicted, under varying densities and competition. Low density and high sociality best explained undermatching. We reveal that in social animals, undermatching is driven by social tethering, which restricts an individual’s ability to escape undermatching due to conspecific attraction. Social tethering in small populations has the potential to inhibit population growth – or recovery.