Data from: Intraspecific competition and inbreeding depression: increased competitive effort by inbred males is costly to outbred opponents

A recent theoretical model suggests that intraspecific competition is an important determinant of the severity of inbreeding depression. The reason for this is that intraspecific competition is density dependent, leading to a stronger negative effect on inbred individuals if they are weaker competitors than outbred ones. In support of this prediction, previous empirical work shows that inbred individuals are weaker competitors than outbred ones and that intraspecific competition often exacerbates inbreeding depression. Here, we report an experiment on the burying beetle Nicrophorus vespilloides, in which we recorded the outcome of competition over a small vertebrate carcass between an inbred or outbred male resident caring for a brood and a size-matched inbred or outbred male intruder. We found that inbred males were more successful as intruders in taking over a carcass from a male resident and were injured more frequently as either residents or intruders. Furthermore, inbred males gained less mass during the breeding attempt and had a shorter adult life span than outbred males. Finally, successful resident males reared a substantially smaller brood comprised of lighter larvae when the intruder was inbred than when it was outbred. Our results shows that inbred males increased their competitive effort, thus contradicting previous work suggesting that inbred males are weaker competitors. Furthermore, our results shows that inbred intruders impose a greater cost to resident males, suggesting that outbred individuals can suffer fitness costs as a result of competition with inbred ones.近期一项理论模型表明，种内竞争（intraspecific competition）是决定近交衰退（inbreeding depression）严重程度的重要因素。其背后逻辑在于，种内竞争具有密度依赖（density dependent）特性，若近交个体（inbred individuals）相较于远交个体（outbred individuals）竞争力更弱，则会受到更强的负面影响。为验证这一预测，既往实证研究表明，近交个体的竞争力弱于远交个体，且种内竞争通常会加剧近交衰退。 本研究以斑埋葬甲（Nicrophorus vespilloides，俗称埋葬甲（burying beetle））为研究对象开展实验，记录了育幼种群中的近交或远交雄性定居者，与体型匹配（size-matched）的近交或远交雄性入侵者之间，针对小型脊椎动物尸体的竞争结局。实验结果显示，作为入侵者时，近交雄性更成功地从定居雄性手中夺取尸体；且无论作为定居者还是入侵者，近交雄性的受伤频率均更高。此外，相较于远交雄性，近交雄性在育幼过程中的体重增量更少，成虫寿命也更短。最后，当入侵者为近交个体时，成功守住尸体的定居雄性所抚育的幼虫种群规模显著更小，且幼虫体重更轻，相较于入侵者为远交个体的情况。 本研究结果表明，近交雄性提升了竞争投入，这与此前认为“近交雄性竞争力更弱”的研究结论相悖。此外，本研究结果显示，近交入侵者会给定居雄性带来更大的适合度代价（fitness costs），这表明远交个体在与近交个体竞争时，自身也会承受适合度损失。