Contrasting selection pressure on body and weapon size in a polygynous megaherbivore

Sexual size dimorphism (SSD) is a common morphological trait in ungulates, with polygyny considered the leading driver of larger male body mass and weapon size. However, not all polygynous species exhibit SSD, while molecular evidence has revealed a more complex relationship between paternity and mating system than originally predicted. SSD is, therefore, likely to be shaped by a range of social, ecological and physiological factors. We present the first definitive analysis of SSD in the common hippopotamus (Hippopotamus amphibius ) using a unique morphological dataset collected from 2994 aged individuals. The results confirm that hippos exhibit SSD, but the mean body mass differed by only 5% between the sexes, which is rather limited compared with many other polygynous ungulates. However, jaw and canine mass are significantly greater in males than females (44% and 81% heavier, respectively), highlighting the considerable selection pressure for acquiring larger weapons. A predominantly aquatic lifestyle coupled with the physiological limitations of their foregut fermenting morphology likely restricts body size differences between the sexes. Indeed, hippos appear to be a rare example among ungulates whereby sexual selection favours increased weapon size over body mass, underlining the important role that species-specific ecology and physiology have in shaping SSD.

Methods
The data were collected in Queen Elizabeth National Park (QENP), Uganda from 1961–1966 by Dr Richard Laws (Nuffield Unit of Tropical Animal Ecology) and his research team. Detailed morphological measurements (i.e. body mass, height, length and lower jaw weight) were taken from 2994 hippos culled in and around Lake Edward. All of the hippos were sexed and subsequently aged using jaw characteristics, tooth replacement and tooth wear as described in Laws (1968). The canine data were extracted from figures 12 & 13 in Laws (1968).

Laws RM. 1968 Dentition and Ageing of the Hippopotamus. Afr. J. Ecol. 6, 19–52.

性体型二态性（Sexual size dimorphism, SSD）是有蹄类动物中常见的形态学特征，一雄多雌制被认为是雄性体重与武器结构更大的主要驱动因素。然而，并非所有一雄多雌制物种都表现出性体型二态性，且分子生物学证据显示，父本亲缘关系与交配系统之间的关系比最初预测的更为复杂。因此，性体型二态性很可能受到一系列社会、生态及生理因素的塑造。本研究利用采集自2994只已知年龄个体的独特形态学数据集，首次对普通河马（Hippopotamus amphibius）的性体型二态性进行确定性分析。研究结果证实河马存在性体型二态性，但两性平均体重差异仅为5%，相较于多数其他一雄多雌制有蹄类动物而言相当有限。不过，雄性的下颌与犬齿重量显著高于雌性（分别重44%与81%），这凸显出演化选择对更大武器结构的强烈压力。河马以水生生活为主的习性，结合其前肠发酵形态（foregut fermenting morphology）带来的生理限制，可能限制了两性间的体型差异。事实上，河马似乎是有蹄类动物中罕见的类群，其性选择更倾向于增大武器结构而非体型，这强调了物种特异性的生态与生理特征在塑造性体型二态性过程中的重要作用。

方法
研究数据由理查德·劳斯博士（Richard Laws）及其研究团队于1961年至1966年间，在乌干达伊丽莎白女王国家公园（Queen Elizabeth National Park, QENP）的爱德华湖及其周边区域采集自被控制性捕杀的河马。研究团队对2994只河马开展了详细的形态学测量，包括体重、体高、体长以及下颌重量。所有个体均完成了性别鉴定，并依据劳斯（1968）所述的下颌特征、牙齿替换与牙齿磨损情况进行了年龄鉴定。犬齿相关数据提取自劳斯（1968）的图12与图13。

参考文献
劳斯 R·M·. 1968 河马的牙齿与年龄鉴定. 非洲生态学杂志（Afr. J. Ecol.）6, 19–52.