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.

Relationship of the number of plexus arteries to body size and to environment. From Thomas and Fordyce (2012).

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.

 

Reference:

Thomas, D.B. and R.E. Forydce. In Press 2012. Biological Plasticity in Penguin Heat-Retention Structures. Anatomical Record.

Family Tree of the Living Penguins

A while back, we talked about the shape of the evolutionary tree of penguins.  There are lots of dead branches, lineages that separated from the main trunk and evolved in their own direction for millions of years, and then died off.  Most of my own interest lies in these ancient branches.  But the green part of the penguin tree is important too, and the branching pattern is far from settled.  Scientists are interested in knowing how living penguins are related to one another for many reasons, including understanding the timing of the penguin evolutionary radiation, determining which environment was preferred by the earliest “modern” penguins, figuring out how many times shifts in body size occurred, and more.

Today, we have the opportunity to meet 19 species of living penguins if we are willing to travel far and wide.  These are divided up into six genera.  Scientists classify groups of closely related species into genera, and together the genus and species name make up the formal binomial name of a species.  Thus, the Humboldt Penguin, a member of the genus Spheniscus, is officially known as Spheniscus humboldti.  Some penguin genera have only a single species, and others have many.

Network of penguin evolutionary relationships, from Ksepka and Ando (2011). The two arrows indicate the alternate hypotheses for where the root of the penguin tree belongs. Penguin artwork by Barbara Harmon.

Let’s take a quick tour of the extant penguin genera.  Spheniscus is the genus for “tuxedo penguins”.  Although all penguins look at least a bit like they are wearing tuxedos, Spheniscus penguins look finest of all because they have a black band of chest feathers that gives the appearance of a bow tie.  There are four living species, the South American Humboldt and Magellanic Penguins (Spheniscus humboldti and Spheniscus magellanicus), the  African Blackfooted or Jackass Penguin (Spheniscus demersus) and the Galapagos Penguin (Spheniscus mendiculus).  These penguins are all fish-eating specialists, and have strong jaw closing muscles and sharply hooked beaks.  In the details of their skeleton, they are more similar to primitive extinct penguins than are any of the other living species.  Spheniscus penguins inhabit some of the hottest environments that penguins survive in today, including coastal desert areas of South America and island directly on the Equator.

Eudyptula has only a single species, the Little Blue Penguin (Eudyptula minor). The Little Blue is by far the cutest of the penguin species (if you don’t believe me, see it being tickled here).  Little Blues received their common name from the fact that they are the smallest living penguins (standing a little over one foot high) and have a unique blue plumage.

Megadyptes also has only one species, the Yellow-eyed Penguin (Megadyptes antipodes).  Arguably the most visually striking species, Yellow-eyed Penguins are characterized by their eponymous bright yellow iris as well as a band of yellow feathers that spread like a bandit mask across their faces.  Yellow-eyed Penguins are one of the rarest species, with a only few thousand breeding pairs in the New Zealand region.

Eudyptes includes the eight species of Crested Penguins, which are perhaps the most noodley-looking penguins.  These species all have bright yellow head plumes, which give them a rakish or foppish appearance depending on the angle, posture and dryness of the bird’s head.  A popular aquarium species is the Macaroni Penguin (E. chrysolophus), so named for their fancy plumage’s resemblance to the 1700s Macaroni wig fashion.  Depending on whom you ask, there are between one and three species of Rockhopper Penguin (I feel the DNA and plumage evidence for three separate species is strong). The three candidates are the Eastern Rockhopper (E. filholi), Northern Rockhopper (E. moseleyi) and Southern Rochopper (E. chrysocome).  Two of the rarer species are the thick-billed Fiordland Penguin (E. pachyrhychus), a denizen of New Zealand, and the Snares Penguin (E. robustus) which breeds only on (you guessed it) Snares Island.  Rounding out the bunch are the Royal Penguin (E. schlegeli) which breeds far to the south in the Sub-Antarctic and the Erect-crested Penguin (E. sclateri), an endangered species that lives on several small Pacific islands.  You’re unlikely to see either of these in aquaria.

Pygoscelis is the genus that includes the stiff-tailed penguins (the Latin genus name refers to this trait).  There are three species, all of which favor cold environements.  The popular Adélie Penguin (Pygoscelis adelie) is one of the World’s most common penguins, with an estimate global population of around 5 million individuals.  The Gentoo Penguin (Pygoscelis papua) is a rather husky penguin with a bright carrot-colored bill.  The Chinstrap Penguin (Pygoscelis antarctica) gets its common name from the thin black band of feathers under its chin that make it look like it is wearing a tiny helmet.

Finally, the largest and most popular penguins belong to the genus Aptenodytes.  Emperor Penguins (Aptenodytes forsteri) and King Penguins (Aptenodytes patagonicus) are tall, graceful birds with long, thin bills and striking orange color patches on their necks and lower bills.  These species are unique in that they lay only one egg.  They are also unique in holding their chicks on top of their feet while brooding.  Given the incredibly harsh conditions they nest in, it would be impossible to raise two chicks this way.

Scientists have attempted to unravel the evolutionary relationships of the living penguin species using both their morphology (the shapes, colors, and arrangements of their bones, feathers, and muscles) and DNA evidence.  Both lines of evidence suggest that Spheniscus and Eudyptula form one closely related group, and that Megadyptes and Eudyptes  form another.  However, they disagree on which group of penguins is the most basal – that is which split off from the main trunk of the penguin tree earliest. DNA evidence suggests that Aptenodytes  was the first group to branch off, while skeletal evidence suggests that the Spheniscus+Eudyptula group was first. The diagram above shows where the two sources of data agree, with arrows indicating the alternate hypotheses of which branch is nearest the root of the tree.  While it might seem like an academic debate, the implications are huge.  If Aptenodytes is the most basal lineage, it would provide evidence that the common ancestor of living penguins was relatively large and that Spheniscus penguins reverted to a primitive skull configuration late in their evolutionary history.  But if Spheniscus is most basal, the skulls of modern penguins have probably changed in a major way only once, and a surprising  South American origin for penguins becomes quite plausible.  Because the two types of data (DNA and morphology) offer conflicting evidence, sequencing more genes and searching for more fossils from near-modern penguins will be important tasks for future studies.

Penguin Tree of Life

Today I would like to introduce the penguin evolutionary tree.  This tree serves as the framework to almost every avenue of research in fossil penguin evolution,  because we need to know where a species belongs on the tree before we can quantitatively study patterns of changes in features like bone density, flipper shape, geographic range, and body size.

Systematists construct these trees by searching for shared evolutionary novelties (synapomorphies) that support recent common ancestry between groups of species.  These features may be morphological – for example the shared novel feature of having  flippers (instead of wings, the primitive state for birds) indicates all penguins share a more recent common ancestor with one another than they do with any other species of bird.  At a finer scale, the King and Emperor penguins both have an opening in the mandible (lower jaw bone) that indicates these two species share a more recent common ancestor with each other than either does with any other penguin species.  Although morphological features are key to deciphering the evolutionary relationships of extinct species, molecular data (DNA) provide another line of evidence for grouping living species (and in some cases, extinct species too – we’ll explore that in a later post).

So with that brief introduction, here is the penguin tree:

Some of the names on the branches may be familiar from previous posts.  The pattern of the branches represents the evolutionary relationships of the species.  This is somewhat like a family tree, except that branches can only split (as two taxa split off from their common ancestor) but not reticulate  – i.e., they never join back together.

Rooting the Tree: It is intuitive to use tree terms to talk about the parts and shape of these evolutionary trees, and that is just what we do.   At the very base of the tree is the root.  This is the portion of the tree that represents the earliest part of the evolutionary history of a group.  We root trees so that the bottom of the tree represents the evolutionary split between the group pf interest and its relatives – in the case of penguins, we would root the tree to the point where penguins split off from the procellariform seabirds (albatrosses and petrels).

Here is what the tree would look like if we turned it on its side and “planted” it by turning it on its side.

Stem Taxa: Fossil species that are lower down the tree are called stem taxa.  Essentially, these species represent dead branches sticking off the stem or trunk of the tree.  They split off before the most recent common ancestor shared by the living species evolved.  Each branch from the stem part of the tree represents a lineage of penguins that evolved along their own path for some amount of time, and then died out, leaving no descendents.  Stem taxa are by definition extinct – each can be thought of as a short, leafless twig from a once growing, but now dead, part of the tree.

Crown Taxa: At the top of the tree are the crown taxa, or “modern” penguins. This is analogous to the crown of a tree – the leafy part at the top.  Each living species can be thought of as a green leaf sitting on the end of its own branch.  The crown clade includes all the living species of penguins plus all fossil relatives that share their most recent common area.  Thus, while stem taxa are always extinct, crown taxa may be living or extinct.  These fossil crown taxa can be envisioned as brown leaves on live branches.

The trees for different clades may have very different shapes. Those shapes depend both on the number of fossils and living species, and their pattern of evolutionary relationships.  Long, sparse branches characterize the trees for groups where deep evolutionary splits have left a handful of highly unique species  – like the monotremes  (the platypus, echidnas and their handful of fossil relatives).  Dense, leafy bushes represent clades where recent explosive radiations led to hundreds or thousands of closely species – groups like bats.

To provide a visual example, the penguin family tree is most similar to certain types of pine. There is a very long stem leading to the crown.  That is, there is a very long series of now completely extinct penguins leading from the root of the tree (where penguins first split off from related birds) to the modern radiation of the 19 living penguin species. The stem of the tree is “tall” because so many extinct species are represented. In fact, in penguins the fossil species outnumber the living species.

Building the evolutionary tree, and updating it as new fossils are found is a major endeavor.  How we collect the data that yields the shape of the tree will be the focus of a future post.

Waimanu, the first penguin

How long have penguins been around?  I suspect that most people would respond that they seem relatively young, in the grand scheme of things.  Penguins are so unique, and they seem particularly modern because of their constant presence in ads and movies.  There is also that constant mental association with icy environments that makes it hard to picture them along a steamy Paleocene coastline.

Waimanu is currently the oldest known penguin, and it is an ancient taxon indeed.  The rocks containing the Waimanu manneringi holotype skeleton are an astounding 61.6 million years old, far and away the oldest to produce penguin bones. A close relative, the smaller Waimanu tuatahi is found in rocks 58-60 million years old. To put this in perspective, these penguins lived just 4-5 million years after the mass extinction that killed off the dinosaurs (except for birds of course).

The first fossils of this taxon were collected almost 20 years ago by Al Mannering, in whose honor the first species is named.  Both come from the Waipara Greensand, a unit of sedimentary rocks laid down in nearshore waters during the Paleocene in present down North Canterbury.  During the Paleocene, this area of the South Island of New Zealand was submerged, and penguins, plankton and shellfish often became entombed in the dark sandy sediments upon death. Millions of years later, these rocks and their trove of fossils were exposed as tectonic forces lifted the ancient seafloor up to the sun and the Waipara River cut away the overlying layers.

These early penguins inherited a world in which a reset button had been firmly pressed. It was warm, rather homogenous in temperature across most of the latitudinal gradient, and most importantly, nearly every major niche was hung generously with “help wanted” signs.  For much of the Mesozoic, dinosaurs dominated terrestrial ecosystems and large marine reptiles occupied the aquatic tetrapod predator niche.  Mosasaurs, plesiosaurs and pliosaurs swam the seas worldwide.  But at the end of the Cretaceous, an asteroid impact wiped out all of these groups.  Even sharks were decimated, though of course some survived to re-supply our oceans and imaginations with toothed nightmares.

This extinction spelled opportunity for many groups.  Mammals radiated into the void left by dinosaurs, and some dinosaurs got a new opportunity.  The volant (flying) ancestors of penguins had a window in which the seas were free of largely free of competitors and low on predators.  This was a perfect time to drop flight altogether.  By 60million years ago, Waimanu manneringi and Waimanu tuatahi, two closely related species, had reached this critical stage in penguin evolution.

A reconstruction of Waimanu tuatahi from Slack et al. (2006).

Waimanu is both amazingly penguin-like and amazingly primitive.  Waimanu manneringi was a healthy size, about halfway between a King Penguin and an Emperor Penguin in standing height, while Waimanu tuatahi was a bit smaller, about 2 1/2 feet (80cm) tall. Waimanu manneringi is only known from a single hindlimb and pelvis, while specimens of Waimanu tuatahi is much more complete – multiple specimens together combine to give us almost the entire skeleton.  From head to toe, the skeleton of Waimanu combines primitive and derived characters.  The skull exhibits the long, narrow beak seen in other early fossil penguins rather than a stubbier modern penguin beak.  The flipper is much shorter than the wing of a flighted bird, but significantly longer relative to the body than in living penguins (indicating it would have a lower wing load).  The bones are also more flattened than flighted birds but less flattened than living penguins, which have highly compressed bones to form a more knife-like wing profile.  In the hindlimb, Waimanu is very close to modern penguins.  The shape of the limb bones indicate an upright posture like modern penguins employ, and the feet are short and stubby.  So Waimanu walked like a penguin on land, swam like a less-efficient penguin in the water, and probably ate the same basic foods (perhaps a little fish heavy).  There is a lot more to say about these fascinating species, but I will await some upcoming work by the Waimanu team to cover that story.

In closing, I should point out that the title of this post is actually a bit inaccurate. Waimanu manneringi is in fact the oldest penguin we know of. But, it is highly unlikely it was actually the first penguin.  The rock record is incomplete, and there is a roughly 10 million year gap between Waimanu tuatahi and the next oldest penguin fossil, showing we are missing big pieces of penguin history – probably on both sides of the 60 million year mark.

The closest relatives of penguins that are alive today are the Procellariiformes, the group that includes albatrosses and petrels.  These birds are commonly called tubenoses because their nostrils take the form of short tubes instead of flat openings. Most likely, the penguin lineage and the tubenose lineage split off from one another and started on their own evolutionary paths deeper in time, perhaps even during the Cretaceous Period. At this deep split, the birds heading off along the evolutionary trajectories to modern penguins and modern petrel probably looked a lot more like a petrel than a penguin – certainly volant (capable of flight) and probably with a similar ecology to some modern tubenose birds.  Whether we would call the bird on the penguin side of the split a “penguin” is debatable – it would probably be very hard for us to recognize a fossil penguin in the rock record until, like Waimanu, they evolved  a flightless lifestyle.  So, pending the discovery of a mind-bending fossil of a flying penguin, we’ll let Waimanu revel in its place in the sun.

There is a lot more about Waimanu here.

References: Slack, K.E., C.M. Jones, T. Ando, G.L. Harrison, R.E. Fordyce, U. Arnason, and D. Penny. 2006. Early Penguin Fossils, Plus Mitochondrial Genomes, Calibrate Avian Evolution. Molecular Biology and Evolution 23: 1144-1155.