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的性别二态性在动物中普遍存在，且可能源于两性间的生态位分化。由于蛇类属于受张口幅度约束的捕食者，且头部形态可直接反映其所捕食猎物的体型与形态，因此蛇类已被广泛应用于性别二态性生态基础的相关研究中。本研究针对采自墨西哥墨西哥州的墨西哥矛头响尾蛇（Crotalus polystictus）种群，探究了其体型与头部形态的性别二态性，以及饮食方面的两性差异。雄性的最大吻肛长（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，头部宽度显著扩增，而吻部长度的增长则受到限制。与之相反，本研究数据并未为雌性头部形态的异速生长提供明确证据。两性成体均主要捕食鼠类与田鼠；但雄性个体捕食较大型哺乳动物的比例更高，而小型猎物的占比更低。饮食差异与头部形态的性别二态性相契合，为本研究种群中的两性生态位分化提供了证据。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性别二态性，更可能源于由预先存在的体型差异所带来的两性不同选择压力，而非为减少两性间竞争而发生的特征替换（character displacement）。