Glenn T. Crossin (Simon Fraser University and the Natural Environment Research Council), Phil N. Trathan (Natural Environment Research Council), Richard A. Phillips (Natural Environment Research Council), Alistair Dawson (Natural Environment Research Council), Fabrice Le Bouard (Natural Environment Research Council), and Tony D. Williams (Simon Fraser University)
(176:September)
Two eggs over easy? Not really. Macaroni penguins, migration, and the cost of egg size dimorphism
One of the longest standing mysteries in avian biology has been that of the extreme egg size dimorphism observed in crested penguins. Macaroni penguins spend 6 long winter months at sea, navigating Antarctic Polar Front waters, and in spring they return with great synchrony to breeding colonies to begin egg-laying. But macaroni penguins are unique among birds in that females lay one small A-egg, which never hatches, and then one larger B-egg, which does. Questions about the physiological causes (the "how") and functional significance (the "why") of this intriguing reproductive pattern have been unanswered for over 60 years. Now Canadian and British researchers have provided answers to the "how" question by revealing links between migration, reproductive physiology, and egg size.
The degree of A:B egg size dimorphism varies among individual female penguins. Some females produce an A-egg that is only about half the size of the larger B-egg, while others can produce an A-egg that is almost equal in size to the B-egg. At the same time, it is also known that some females begin egg-laying almost immediately upon their arrival at the colony while others arrive and spend about two weeks there before egg-laying. The researchers hypothesized that variation in egg size dimorphism might be the result of a "physiological conflict" due a conflict between migration and reproduction: some females must have been producing eggs while they were still actively migrating back to the colony, and perhaps these females produce the smallest A-eggs and most dimorphic clutches. Physiological sampling of arriving females confirmed that concentrations of the protein vitellogenin, the primary constituent of developing eggs, were lowest in penguins that had the greatest overlap between migration and egg formation. In other words, these females appeared less "reproductively ready." Migration is thus a key determinant of egg size dimorphism in macaroni penguins, and females who avoid physiological conflict by finishing migration before initiating egg production can produce two eggs more equal in size.
The degree of A:B egg size dimorphism varies among individual female penguins. Some females produce an A-egg that is only about half the size of the larger B-egg, while others can produce an A-egg that is almost equal in size to the B-egg. At the same time, it is also known that some females begin egg-laying almost immediately upon their arrival at the colony while others arrive and spend about two weeks there before egg-laying. The researchers hypothesized that variation in egg size dimorphism might be the result of a "physiological conflict" due a conflict between migration and reproduction: some females must have been producing eggs while they were still actively migrating back to the colony, and perhaps these females produce the smallest A-eggs and most dimorphic clutches. Physiological sampling of arriving females confirmed that concentrations of the protein vitellogenin, the primary constituent of developing eggs, were lowest in penguins that had the greatest overlap between migration and egg formation. In other words, these females appeared less "reproductively ready." Migration is thus a key determinant of egg size dimorphism in macaroni penguins, and females who avoid physiological conflict by finishing migration before initiating egg production can produce two eggs more equal in size.
After migrating throughout the southern latitudes for more than six months, macaroni penguins arrive each spring at a breeding colony in Bird Island, South Georgia. The inset shows the dimorphism of A-eggs and B-eggs, which is the result of a physiological conflict imposed by migration. (photograph by Glenn Crossin ©)
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