Australian Journal of Zoology Australian Journal of Zoology Society
Evolutionary, molecular and comparative zoology

The heterothermic loophole exploited by penguins

Daniel B. Thomas A B and R. Ewan Fordyce A

A Department of Geology, University of Otago, PO Box 56, Dunedin 9054, New Zealand.

B Corresponding author. Email:

Australian Journal of Zoology 55(5) 317-321
Submitted: 9 October 2007  Accepted: 16 January 2008   Published: 8 February 2007


Penguins are generally smaller than the predicted minimum body size for marine endotherms. Anatomical observations of the little blue penguin (Eudyptula minor) suggest that extant penguins actively defeat the lower size threshold using regional heterothermy. The wing arteries in the little blue penguin form a humeral plexus, a series of three parallel arteries (including the marginal artery) that stem from the axillary artery, replacing a single brachial artery. Each artery of the plexus is associated with at least two veins to form a counter current heat-retention system. The humeral plexus thus restricts heat from entering distal areas of the wing and dissipating into seawater, ultimately reducing the bulk mass required for heat production and insulation. Humeral plexi are confirmed as a synapomorphic character of the Spheniscinae, with the humeral plexus of E. minor most similar to that of the African penguin Spheniscus demersus. The humeral plexus represents a loophole in body size minima of marine endotherms and offers insight into the ecology of living penguins.


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