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There was no variation in vascular topology observed between the two yellow-eyed penguin specimens. The plexus topologies of each penguin differed, but the preplexus and post-plexus topologies were essentially identical. The common arterial source to the wing of each penguin is A. axillaries, with A. brachialis replaced by a humeral plexus. The first division of A. axillaris is always A. profunda brachii, which supplies the dorsal surface of the wing. Vessels were traced on both the ventral and dorsal surfaces of each penguin wing, and we noted that the ventral surfaces were extensively vascularized compared with the dorsal surfaces. The reduced vascularization of the dorsal surface may benefit heat retention on land or reduce the severity of any laceration, as the extensively vascularized ventral surface can be pressed against the body (T. Ando, personal communication, 2011). The http://en.wikipedia.org/wiki/NK_cells number of humeral arteries ranges from two in little blue penguins to 15 in emperor penguins and includes some intraspecific variation (Trawa, 1970; Frost et al., 1975; Louw, 1992; Thomas and Fordyce, 2007). Heat is transferred where humeral arteries contact adjacent veins (Frost et al., 1975); increasing vascular contact area by increasing the number of arteries would elevate the efficiency of heat transfer. Heat-retention efficiency should therefore differ among penguins and should scale to compensate for heat lost through flippers (e.g., Chappell et al., 1989). The hydrodynamic function of http://www.selleckchem.com/products/Gefitinib.html flippers requires a svelte profile (Hui, 1988), and larger penguins need larger http://www.selleckchem.com/products/DAPT-GSI-IX.html flippers for optimal stroke rates (albeit with negative allometry; Sato et al., 2010). Increased heat-retention efficiency is therefore expected for penguins with larger flippers to compensate for the increased surface area exposed to cold water. Indeed, flipper surface area correlates well with the number of humeral arteries (exponential model, r2 = 0.91, P-value