Helpers in a cooperative breeder pay a high price to stay: effects of demand, helper size and sex

'Pay to stay' is a mechanism generating cooperation in some highly social vertebrates. However, it is unknown which behaviours subordinate helpers preferentially use to pay for the commodities provided by resource owners. Helpers could either provide 'cheap' service to save costs or trade expensive actions for the benefit of being tolerated. In the cichlid Neolamprologus pulcher, unrelated helpers pay to stay in a safe territory of dominant breeders by performing a range of behaviours including direct brood care, territory maintenance and defence. We investigated which behaviours helpers differing in size and sex showed in response to increased demands. By presenting high or low perceived intruder pressure we tested (1) whether helpers adjust their level of payment to the intensity of competition, (2) which behaviours are used for payment, and (3) whether the response to the treatment differs between helper size and sex. All helpers performed more defence behaviour in the high-density situation and large helpers put significantly more effort into digging and defence than small helpers did in both treatments. In the low-density treatment, large helpers reacted to breeder aggression by increased submissiveness, whereas in the high-density treatment they responded by increased territory defence, as predicted by the pay-to-stay hypothesis. Helper sex did not influence their behavioural responses. Our results suggest that helpers respond to demand and large helpers pay generally more than small ones to remain tolerated in the breeders' territory.