Data from: The pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces

The adaptive significance of avian egg shape is a long-standing problem in biology. For many years, it was widely believed that the pyriform shape of the Common Murre (Uria aalge) egg allowed it to either “spin like a top” or “roll in an arc,” thereby reducing its risk of rolling off the breeding ledge. There is no evidence in support of either mechanism. Two recent alternative hypotheses suggest that a pyriform egg confers mechanical strength and minimizes the risk of dirt contamination of the blunt end. We present a new hypothesis: that the Common Murre egg's pyriform shape confers stability on the breeding ledge, thus reducing the chance that it will begin to roll. We tested this hypothesis by measuring the stability of Common Murre and Razorbill (Alca torda) eggs of different shapes on slopes of 20°, 30°, and 40° above the horizontal. Common Murre eggs were more stable, and easier to stabilize, than the more elliptical Razorbill eggs. Within Common Murre eggs, more pyriform eggs were more stable. From a fitness perspective, the stability of the Common Murre egg on a slope seems likely to confer an advantage and thus may be a strong force of natural selection favoring the pyriform shape.

鸟类卵形的适应性意义是生物学领域中长期存在的研究难题。多年来，学界普遍认为普通海鸠（Common Murre, Uria aalge）的梨形（pyriform）卵要么能够“像陀螺般旋转”，要么能够“呈弧形滚动”，从而降低卵从繁殖崖边滚落的风险，但目前尚无证据支持这两种机制中的任意一种。近期提出的两项替代假说认为，梨形卵可提升结构机械强度，并最大程度降低卵钝端被污物污染的风险。本研究提出全新假说：普通海鸠卵的梨形结构可使其在繁殖崖面上具备更高稳定性，进而降低其初始滚落的概率。我们通过测量不同形状的普通海鸠卵与刀嘴海雀（Razorbill, Alca torda）卵在20°、30°、40°倾斜坡面（与水平面夹角）上的稳定性，对该假说进行了验证。实验结果显示，相较于更接近椭圆形的刀嘴海雀卵，普通海鸠卵的稳定性更强，且更易维持稳定状态；在普通海鸠卵中，梨形特征越显著的卵稳定性越高。从进化适合度的视角来看，普通海鸠卵在倾斜坡面上的稳定性大概率能带来生存优势，因此可能是驱动自然选择青睐梨形卵形态的重要选择压力。