Data from: Sexual differences in head form and diet in a population of Mexican Lance-headed Rattlesnakes, Crotalus polystictus

Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape-limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, and sexual differences in diet in a population of Mexican lance-headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. Maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified while snout length was truncated with increased head size. In contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in our study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from preexisting body size differences rather than from character displacement for reducing intersexual competition.

与资源利用相关的表型性状（phenotypic traits）性别二态性（sexual dimorphism）在动物中普遍存在，其可能源于两性间的生态位分化（niche divergence）。蛇类作为张口受限的捕食者（gape-limited predators），且头部形态（head morphology）可直接反映其捕食猎物的大小与形态，因此被广泛应用于性别二态性的生态学基础研究。本研究针对来自墨西哥墨西哥州的墨西哥矛头响尾蛇（Crotalus polystictus）种群，对其体型与头部形态的性别二态性，以及饮食的性别差异展开了调查。雄性个体的最大吻肛长（snout–vent length）较雌性高出21%。雄性头部相对更大，且在剔除头部大小的影响后，其头部形态与雌性存在显著差异。此外，雄性响尾蛇的头部形态呈现正异速生长（allometry）：随着头部尺寸增大，头部宽度增加，而吻部长度相对缩短。与之相反，本研究数据未为雌性头部形态的异速生长提供明确证据。雌雄成体的主食均为鼠类与田鼠，但雄性个体同时会捕食更大比例的大型哺乳动物类群，且较少捕食小型猎物。饮食差异与头部形态的二态性相契合，为该研究种群中的两性间生态位分化（intersexual niche divergence）提供了证据。然而，由于两性的饮食重叠度极高，我们推测墨西哥矛头响尾蛇的饮食与头部二态性，更可能由预先存在的体型差异所引发的两性差异化选择压力所导致，而非为降低两性间竞争而发生的性状替换（character displacement）。