Abstract
We compared the shape and eggshell thickness of Great Auk Pinguinus impennis eggs with those of its closest relatives, the Razorbill Alca torda, Common Guillemot Uria aalge and Brünnich's Guillemot Uria lomvia, in order to gain additional insights into the breeding biology of the extinct Great Auk. The egg of the Great Auk was most similar in shape to that of Brünnich's Guillemot. The absolute thickness of the Great Auk eggshell was greater than that of the Common Guillemot and Razorbill egg, which is as expected given its greater size, but the relative shell thickness at the equator and pointed end (compared with the blunt end) was more similar to that of the Common Guillemot. On the basis of these and other results we suggest that Great Auk incubated in an upright posture in open habitat with little or no nest, where its pyriform egg shape provided stability and allowed safe manoeuvrability during incubation. On the basis of a recent phylogeny of the Alcidae, we speculate that a single brood patch, a pyriform egg and upright incubation posture, as in the Great Auk and the two Uria guillemots, is the ancestral state, and that the Razorbill – the Great Auk's closest relative – secondarily evolved two brood patches and an elliptical egg as adaptations for horizontal incubation, which provides flexibility in incubation site selection, allowing breeding in enclosed spaces such as crevices, burrows or under boulders, as well as on open ledges.
Highlights
The Great Auk Pinguinus impennis is extinct
Egg shape: We quantified egg shape using the methods described by Biggins et al (2018) that provides three indices of shape: (i) Elongation: the ratio of the length to the width at the widest point, (ii) Asymmetry (Pointedness): the length from the point where the egg is widest to the more distant end divided by the overall length, and (iii) Polar Asymmetry: the ratio of the diameter of the largest circle that will fit within the egg outline and touch the egg at its blunt pole to the diameter of the largest circle within the egg outline and touching the more pointed pole
We previously found a relationship between egg volume and shape (Birkhead et al 2017), in the present study we did not control for egg volume in our analyses of egg shape primarily because we were interested in using egg shape to infer something about the ecology and breeding site of the Great Auk in terms of the stability of their egg
Summary
The Great Auk Pinguinus impennis is extinct. What are thought to have been the last two individuals were killed on the island of Eldey, Iceland in June 1844 (Grieve 1885; Newton 1896; Fuller 1999). Since attempts have been made to reconstruct aspects of the Great Auk’s life history from two main sources: (i) anecdotal accounts of live birds observed at their breeding colonies only by those intent on harvesting the birds and their eggs rather than by scientists, and (ii) the ~80 skins, skeletal material and alcohol-preserved internal organs of two individuals, as well as ~70 eggs, most of which exist in museum collections (Bengtson 1984; Fuller 1999) This material is all that is available for reconstructing the life of the Great Auk — albeit with the help of new technologies To deal with this Biggins et al (2018) used a third index, Polar Asymmetry (see below)
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