Abstract
The adaptive significance of avian egg shape is poorly understood, and has been studied only in those species producing pyriform (pear-shaped, or pointed) eggs: waders and guillemots (murres) Uria spp., albeit to a limited extent. In the latter, it is widely believed that the pyriform shape has evolved to minimise their likelihood of rolling off a cliff ledge: the idea being that the more pointed the egg, the narrower the arc in which it rolls, and the less likely it is it will fall from a cliff ledge. Previous research also claimed that the rolling trajectory—the diameter of the arc they describe—of Common Guillemot U. aalge eggs is influenced not only by its shape but also by its mass, with heavier (i.e. larger) eggs describing a wider arc than lighter eggs. The finding that both shape and mass determined the rolling trajectory of Common Guillemot eggs (the shape–mass hypothesis) was used to explain the apparent anomaly that Brünnich’s Guillemot U. lomvia produce eggs that are less pointed, yet breed on narrower ledges than Common Guillemots. They are able to do this, it was suggested, because Brünnich’s Guillemot eggs are smaller and lighter in mass than those of Common Guillemots. However, since some populations of Brünnich’s Guillemots produce eggs that are as large or larger than those of some Common Guillemot populations, the shape–mass hypothesis predicts that that (1) larger (i.e. heavier) eggs of both guillemot species will be more pyriform (pointed) in shape, and (2) that eggs of the two species of same mass should be similarly pointed. We tested these predictions and found: (1) only a weak, positive association between egg volume and pointedness in both guillemot species (<3% of the variation in egg shape explained by egg volume), and (2) no evidence that eggs of the two species of similar mass were more similar in shape: regardless of their mass, Brunnich’s Guillemot eggs were less pointed than Common Guillemot eggs. Overall, our results call into question the long-held belief that protection from rolling is the main selective factor driving guillemot egg shape.
Highlights
The adaptive significance of avian egg shape is poorly understood (Barta and Szekely 1997) and, except for those species producing pyriform eggs such as waders and guillemots Uria spp., little studied
Previous research claimed that the rolling trajectory—the diameter of the arc they describe—of Common Guillemot U. aalge eggs is influenced by its shape and by its mass, with heavier eggs describing a wider arc than lighter eggs
For the Common Guillemot Uria aalge and Brunnich’s Guillemot U. lomvia, the pyriform shape of their single egg has long been considered an adaption to minimise the risk of rolling off the narrow cliff ledges on which these species typically breed (MacGillivray 1852; Belopol’skii 1957; Del Hoyo et al 1996; Gill 2007; reviewed in Birkhead 2016)
Summary
The adaptive significance of avian egg shape is poorly understood (Barta and Szekely 1997) and, except for those species producing pyriform (pear-shaped, or pointed) eggs such as waders and guillemots (murres) Uria spp., little studied. 1957; Del Hoyo et al 1996; Gill 2007; reviewed in Birkhead 2016) In support of this hypothesis, it has been shown that the guillemots’ pyriform eggs tend to roll in an arc, whereas the ‘elliptical-ovate’ egg of the closely related Razorbill Alca torda—which breeds as pairs in cavities with little risk of the egg falling—rolls in a much wider arc (Belopol’skii 1957; Ingold 1980). Ingold (1980) subsequently showed that the plaster eggs used in Tschanz et al (1969) study did not behave in the same the way as real eggs, and that, contrary to expectation, there was little difference in the rolling trajectories of real Common Guillemot and Razorbill eggs on natural substrates. Ingold (1980) concluded that: ‘It has to remain unanswered whether the form of the guillemot egg [has] evolved in response to the pressure of the risk of falling off.’ (translated from German)
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