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.
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.