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长期存在的经典研究问题。长期以来，学界普遍认为崖海鸦（Uria aalge）的卵呈梨形（pyriform），可使其“像陀螺般旋转”或“呈弧形滚动”，从而降低卵从繁殖岩架滚落的风险，但目前尚无证据支持上述两种机制中的任意一种。近期提出的两项替代性假说则认为，梨形卵可提升机械强度，并最大限度降低钝端受污物污染的风险。本研究提出全新假说：崖海鸦卵的梨形结构可使其在繁殖岩架上保持稳定，进而降低其滚落的可能性。我们通过测量不同形状的崖海鸦卵与刀嘴海雀（Alca torda）卵在20°、30°及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。结果显示，相较于卵形更趋近椭圆形的刀嘴海雀卵，崖海鸦卵的稳定性更强，且更易维持稳定状态；在崖海鸦卵中，梨形特征越显著的卵稳定性越高。从适应度（fitness）角度来看，崖海鸦卵在倾斜坡面上的稳定性似乎可带来生存优势，因此可能成为驱动自然选择青睐梨形卵形的重要进化压力。