Exaptations in Action – A Warm Origin for a Cool Structure in Penguin Wings

A few posts back, we talked about the penguin rete mirabile, or arterial plexus.  This network of blood vessels helps living penguins keep their core body temperature warm by shutting down heat flow to the tip of the flipper when they are in foraging in cold water.  A new paper published today in Biology Letters tracks the origin of this feature deep into the fossil record.  How is this possible?  After all, blood vessels, don’t usually fossilize. As a co-author of the study, I can explain how we drew our conclusions.

This research project started all the way back in 2006, when lead author Dr. Daniel Thomas and I were both still graduate students.  Daniel was studying the evolution of penguin counter current heat exchange systems through dissections and isotope studies, while I was focusing on the evolution of the penguin skeleton and trying to understand what each bump, groove, and ridge on the flipper bones actually meant.  In many cases these structures mark the places where muscles attach, but there were a few “mystery grooves”.  As Daniel and I shared penguin dissection data, we realized one deep groove across the humerus, the main bone of the flipper, was particularly important.  This groove sat right below the vessels of the arterial plexus.  In fact, the groove was formed by the vessels being pressed against the bone.  While this might seem like a mere “fun fact”, it had big implications for us – this bony mark (or osteological correlate in paleontology jargon) provided a way to determine whether fossil penguins might have had the an arterial plexus as well.  Over the next few years (in between finishing graduate school many other projects) we surveyed nearly every penguin humerus in museums worldwide.  Together with Dr. R. Ewan Fordyce, a cetacean expert who has also discovered many important penguin fossils, we used our data to map where the plexus first appeared on the penguin evolutionary tree.

The bony groove on the penguin humuerus (sulcus) houses the arterial plexus. This groove lets us infer whether the plexus was present in fossil penguins, even though the vessels themselves don't fossilize. Image from Thomas et al. 2010.

Now, if we only knew about today’s penguins, we might naturally assume the plexus arose in association with penguins moving into cold climates.  Fascinatingly, the fossil record tells us the opposite.  As we surveyed fossil penguin wing bones to check whether they preserved the groove for the blood vessels, we found that the oldest penguins lacked it.  These species lived in New Zealand about 60 million years ago.  The first penguins that show signs of the sulcus start popping up all over the Southern Hemisphere at about 45 million years ago.  The twist is that they appear during one of the hottest times in Earth history, during an interval when there were no permanent polar ice sheets.

Of course, the plexus is also very useful for penguins in Antarctic environments.  In fact, they might not even be able to survive without it due to the intense physiological demands of swimming in  nearly freezing water.   However, the fossil record shows that the plexus did not evolve in response to global cooling, but instead probably  appeared to help penguins spend longer periods feeding at sea, where heat leaves the body faster than in air.  Having inherited a feature evolved by warm-weather ancestors tens of millions of years in the past, modern penguins were more than ready to invade sea ice shelves over their more recent history.  This is an example of exaptation – a feature that evolved for one purpose and then came to serve another.  In this case, the plexus evolved to allow longer periods of foraging and was later perfect for preserving warmth during long marches across ice sheets and weeks of huddling in gale force winds at nest time.

 

Reference:

Thomas, D.B., D.T. Ksepka and R.E. Fordyce. 2010. Penguin heat-retention structures evolved in a greenhouse Earth. Biology Letters (published online before print December 22, 2010, doi:10.1098/rsbl.2010.0993)

Guest Article at Scientific American

If you’ve been reading March of the Fossil Penguins diligently, you probably already know most of the “Five Things You Never Knew About Penguins” in this article, but please check it out if you need a penguin evolution fix before the holidays:

Penguins are perhaps the most popular birds on Earth, thanks in equal measure to their incredible life cycles and charming tuxedo-clad appearances. Among their long list of superlatives, penguins can survive sub-freezing temperatures and gale force winds, dive over 1600 feet deep, hold their breath for more than 15 minutes, and survive with no food for weeks by living off stored fat [1]. These facts are so often repeated that they sometimes lose their initial wonder. Talking to K-12 schools as a guest speaker, I’ve found that half the classroom often knows many of these bits of penguin trivia before the presentation even starts, thanks to popular books, television specials, and the movie “March of the Penguins”.

To read the rest of the article, click here: http://www.scientificamerican.com/blog/post.cfm?id=five-things-you-never-knew-about-pe-2010-12-20

One Tough Penguin

Here is a picture from Cape Town’s local penguin colony.  South Africa is today home to one species of penguin, Spheniscus demersus (often called the Jackass Penguin for its loud braying call). In the past, several other species lived in South Africa, perhaps as many as four at a time.  While these fossils tell a great story, I wanted to post the image of this tough living penguin today.  This fellow in the picture has only one leg.  Yet, it is marching only slightly slower than normal penguin land speed.  It made it up from the water, across the beach and over to a burrow just fine.  Most likely, this penguin lost its leg to a shark attack.  While this type of encounter may sound like it would be deadly, one legged penguins are not all that rare.  Since the penguin can trod along the short distance to its burrow, one leg will do.  Based on its healed injury, good plumage and level of fat stores, it seems the one-legged trooper has been making due for at least a few weeks.  A one-winged penguin would be completely out of luck though, and would not be able to swim to collect food.   So, in the sense that a shark attack can ever be considered “lucky” this penguin was fortunate.

 

Penguins in Africa

For the next few months, part of my research is going to be focused on South Africa’s fossil penguins.  These species are much younger than most of the others we’ve talked about in this blog, like Inkayacu and Paraptenodytes.   They come from Pliocene age rocks about 5 million years old, and most of the bones look similar to modern species.  Although each type of penguin from the African fossil record has been given a different genus name, they may all belong in the tuxedo penguin genus Spheniscus.  Figuring that out is one part of the research project.  Another is figuring out where each of these species came from.  Penguins can travel long distances, and species like the King Penguin which live very far from Africa occasionally show up by accident.  It is possible that a handful of wayward birds orignally founded the first penguin colony in Africa.  Understanding how each species is related to others from around the world can help tell where they might have originally come from.  As this project progresses, I’ll post more of what our team finds.

 

Jackass Penguins lounging indolently in the hot sun at Boulders breach, Cape Town

 

 

Today, only one species of penguins lives on the continent, in southernmost Africa.  Penguins of the  Jackass penguin species (Spheniscus demerus)  live along the coast right up against human dwellings.  So close that a fence keeps them aways from the road and some special plastic “burrows” are provided to make up for lost habitat.   In the parking lot, a sign urges drivers to “Check Under Car for Penguins” before driving off.  As the photo above reminds us, these and many other penguins prefer warm climates to Antarctica.

Penguins “Cheat” to Stay Warm in Icy Waters

Many penguin species love cold environments, and even those that live in tropical latitudes often feed in very cold currents offshore.  This presents a special challenge for small species. Theoretically, an endothermic (“warm-blooded”) animal needs to be about 7kg or larger to survive in cold water without some type of special heat retention mechanism.  This is because volume increases faster than surface area as animals get larger, leading to slower heat transfer in larger animals and faster heat transfer in smaller animals.   In theory at sizes below 7kg, the rate of heat loss to the surrounding cold water becomes too great and the animal will enter hypothermia.  The smallest penguin alive today, the Little Blue Penguin (Eudyptula minor) is only about 1kg.

So how do these penguins survive?  Regional heterothermy is a strategy in which an animal allows some parts of the body to cool down in order to preserve heat for the core.  For penguins, this means letting the flippers cool down and keeping the brain and vital organs warm.  As a means of propulsion, the penguin flipper is a marvel of evolution, perfectly suited for underwater “flight”.  However, it is also a terrible heat sink.  The flattened wing bones and tightly attached skin and feathers give the flipper a very high surface area to volume ratio, which means it will shed heat to surrounding air or water at a high rate.  Immersed in icy water, a penguin-shaped object would cool fairly rapidly.

Penguins have a trick to keep this from happening.   Blood vessels of the wing in penguins form a “rete mirabile”, a plexus of arteries and veins.  This term means wonderful net in Latin, and it is indeed a wondrous evolutionary novelty.  Arteries carry oxygenated blood from the heart of the penguin to the extremities, and veins return the de-oxygenated blood back again.  In cold water, the blood from the arteries is hot, but the blood returning from the tips of the flippers and toes can be quite cold.  As the normal arteries of the penguin blood vessel system run onto the flipper, they split into multiple parallel branches called a plexus.  Each branch is closely aligned with at least two veins.  The heat from the blood in the arteries warms the returning blood in the veins, raising the blood temperature before it returns to the heart.  At the same time, the blood heading towards the flipper in the arteries is cooled, resulting in the flipper temperature dropping.  This can lead to an impressive difference of up to 30 degrees Celsius (86 degrees Fahrenheit) between the core temperature and wingtip temperature of penguins.  The vital organs remain toasty, while the flippers dip towards freezing.

Schematic illustration of the rete in a Little Blue penguin, modified with permission from Thomas and Fordyce (2008). Arteries are shown in red and veins in blue, superimposed on a photo of the bones of the flipper. At the purple rectangle, each artery is associated with at least two smaller veins, forming the rete.

 

 

Recently Dr. Daniel Thomas and Dr. R. Ewan Fordyce studied this system in Little Blue Penguins (don’t worry, no penguins were hurt – only dead specimens found on beaches were dissected) and compared the number of arteries in the rete from different living species. They found that penguins from colder areas like Antarctica have more arteries than those from warmer environments.  This would make sense in that a more sophisticated rete may be necessary in more extreme conditions.   Alternatively, the number of vessels may correlate to size, as the species that lives in the most extreme environment, the Emperor Penguin, is also the largest.

This is another interesting example of just how specialized penguins are for life in extreme marine environments.   The smallest species “cheat” to get in under the normal limits of viable body size for marine endotherms.  While their very specialized feathers and flipper-like wings are easily visible, some of the cool characteristics that make penguins “work” lie beneath the surface.

References:

Thomas, D.B. and R.E. Fordyce.  2007. The heterothermic loophole exploited by penguins. Australian Journal of Zoology 55: 317-321.