Selection for the Plexus
Insulation and vascular heat-retention mechanisms allow penguins to forage for a prolonged time in water that is much cooler than core body temperature. One of the key structures that penguins evolved long ago is the humeral plexus, a system of arteries and veins that form a counter current heat exchanger. My colleagues Dr. Daniel Thomas and Dr. Ewan Fordyce have been collecting data on this fascinating structure from different penguin species and recently published the results of a comprehensive study. They dissected the flippers of two genera of penguins that had not been examined for plexus morphology before (learn more about the 6 genera here). Don’t worry, no penguins were harmed – all specimens were birds that died in the wild and were salvaged from beaches. One of the interesting results is that the plexus is not identical in every species – larger species tend to have more arteries in the plexus, even though they have the same number of basic arteries in other parts of the flipper. Little Blue Penguins have only 2 plexus arteries, while Emperor Penguins have 15 plexus arteries.
So why do larger penguins have more arteries in the plexus? Well, this is a tricky question to answer. One the one hand, larger penguins will have larger wing surface areas. Like elephant ears, penguin flippers have a high surface area to volume ratio, and so easily shed heat. Thus, larger penguins might need to increase the number of humeral arteries to compensate for heat loss. On the other hand, penguins that live in colder areas would benefit more from additional humeral arteries because heat will be lost more rapidly in colder water. The tricky part is that the largest penguins also tend to live in the coldest environments. Thus, you can fit a nice curve to data showing a relationship between size and artery number, and can fit a similarly nice curve to data showing a relationship between environmental temperature and artery number. The authors of the study tested the strength of these relationships and found that humeral arteries has a stronger correlation with wing surface area than with sea water temperature. However, both probably play some roll – in the end its all about keeping warm whether its preventing heat shedding though a big flipper or fighting sub-zero temperatures.
Thomas, D.B. and R.E. Forydce. In Press 2012. Biological Plasticity in Penguin Heat-Retention Structures. Anatomical Record.