Britton Robert H, Moser Michael E
Station Biologique de la Tour du Valat, Le Sambuc, F-13200, Arles, France.
Department of Zoology, University of Durham, South Road, DH1 3LE, Durham, UK.
Oecologia. 1982 Jan;53(2):146-151. doi: 10.1007/BF00545657.
Sex-ratios of Gambusia affinis populations in freshwater marshes in the Camargue (Rhône Delta), are highly biased in favour of males, whereas the sex-ratios in ditches are close to unity. Studies of the diet of free living birds and experimental studies on prey size selection in captivity show that the abnormal sex-ratios in marshes can be attributed to differential heron predation. Ditches are relatively free from predation. Mature female Gambusia are larger, and have an energy content 5-25 times greater than that of mature males. Handling times of Grey Heron (Ardea cinerea) and Little Egret (Egretta garzetta) are only slightly longer for female Gambusia than males. Hence, females represent a much more profitable prey.Analysis of nestling regurgitates show that Gambusia makes up a considerable proportion of the diet of four species of Camargue herons, and that the majority of Gambusia taken are females. Under experimental conditions, captive herons consume almost exclusively female fish, even when offered in ratios where they are heavily outnumbered by males.The relevance of these results to optimal diet theory is discussed.
卡马尔格（罗纳河三角洲）淡水沼泽中食蚊鱼种群的性别比例严重偏向雄性，而沟渠中的性别比例接近1:1。对自由生活鸟类饮食的研究以及圈养条件下猎物大小选择的实验研究表明，沼泽中异常的性别比例可归因于鹭的捕食差异。沟渠相对较少受到捕食。成年雌性食蚊鱼体型更大，能量含量比成年雄性食蚊鱼高5至25倍。苍鹭（苍鹭属）和小白鹭（白鹭属）捕食雌性食蚊鱼的处理时间仅比捕食雄性食蚊鱼略长。因此，雌性食蚊鱼是更有利可图的猎物。对雏鸟反刍物的分析表明，食蚊鱼在卡马尔格四种鹭的饮食中占相当大的比例，且被捕食的食蚊鱼大多数是雌性。在实验条件下，圈养的鹭几乎只捕食雌鱼，即使提供的雄鱼数量远远超过雌鱼。本文讨论了这些结果与最优饮食理论的相关性。
