Mate guarding in the Seychelles warbler is energetically costly and adjusted to paternity risk

Males may increase their fitness through extra-pair copulations (copulation s outside the pair bond) that result in extra-pair fertilizations, but also risk lost paternity when they leave their own mate unguarded. The fitness costs of cuckoldry for Seychelles warblers (Acrocephalus sechellensis) are considerable because warblers have a single-egg clutch and, given the short breeding season, no time for a successful replacement clutch. Neighbouring males are the primary threat to a male's genetic paternity. Males minimize their loss of paternity by guarding their mates to prevent them from havin g extra-pair copulations during their fertile period. Here, I provide exper imental evidence that mate-guarding behaviour is energetically costly and t hat the expression of this trade-off is adjusted to paternity risk (local m ale density). Free-living males that were induced to reduce mate guarding s pent significantly more time foraging and gained significantly better body condition than control males. The larger the reduction in mate guarding, th e more pronounced was the increase in foraging and body condition (accounti ng for food availability). An experimental increase in paternity risk resul ted in an increase in mate-guarding intensity and a decrease in foraging an d body condition, and vice versa. This is examined using both cross-section al and longitudinal data. This study on the Seychelles warbler offers exper imental evidence that mate guarding is energetically costly and adjusted to paternity risk.