I always get a little bit antsy when I see morning news reports about a new study appearing in Nature, Science, or some other journal, not having access to the actual paper until sometime later in the day. This past week, however, the mass-media slipped up a bit in this regard; they announced a new paper in the Proceedings of the National Academy of Sciences about two huge, extinct penguins on Tuesday, but the paper itself has only just now become available.
Art by Kristin Lamm. From right to left; Icadyptes salasi, Spheniscus humbolti (extant Humbolt Penguin), and Perudyptes devriesi
The reason this particular paper is so significant (other than 5-foot-tall penguins being utterly cool in and of itself) is that the prevailing hypothesis for penguin evolution was that they originated in cooler, southern latitudes and eventually made their way up to warmer areas like Peru, the Galapagos, etc. after temperatures during the Cenozoic began to cool. In fact, the Eocene (the time period in which the two new penguin species pictured above lived) experienced a temperature spike that threw various systems out of whack and contributed to the extinction of many groups of animals, including mammals. At the onset of the Eocene, there was a sharp thermal spike known as the Paleocene-Eocene Thermal Maximum, following by an overall rise in temperature before cooling down, and looking at the overall trend the world was much warmer during the time of the giant penguins than it is now, reversing the notion that penguins could only advance as far north towards the equator as cold temperatures would allow.
Analysis of the two penguins (there are also species yet to be identified from the same site, so expect more from Peru in the future) shows that they are not the ancestors of living penguins, belonging to the Order Sphenisciformes but outside the Family Spheniscidae, the authors citing the radiation that produced living groups taking places approximately eight million years ago during the Miocene. While much of the mass-media has focused on the “giant” size of Icadyptes, it should be noted that Icadyptes is only one of many known large, extinct penguin varieties, the largest yet known being Anthropornis nordenskjoeldi from Seymour Island and New Zealand, other examples being Pachydyptes ponderosus and Palaeeudyptes klekowskii.
Icadyptes is notable amongst its nearly equally-sized relatives for another reason; we wouldn’t expect to find such a large penguin in such a warm climate. While a foraminiferan indicative of warm-water has been found in association with the giant Pachydyptes, it was generally assumed that penguins followed “Bergmann’s Rule.” Bergmann’s Rule essentially states that among warm-blooded animals, we should expect body mass to increase with increasing latitude (hence, colder climates). This rule seems to work not only across species, but within species as well, working on the inherent plasticity of the animals to vary their size (i.e. wide-ranging animals are usually larger in the far north or south than close to the equator). Icadyptes gives “the flipper” to Bergmann’s Rule, being a warm-blooded bird in a hot environment of equal size to contemporaries from cooler latitudes, but there are exceptions to every rule.
The general reason why we observe Bergmann’s Rule is because of body heat; if you live in the cold you want to retain it, which large body size allows you to do, and in hot climates you don’t want to overheat, so it behooves you to be small and thus give off plenty of heat. Bob Bakker provides perhaps the best illustration of this in The Dinosaur Heresies; if you have to spend the night in a zoo and want to stay warm, should you snuggled up to one elephant (and remember, elephants are large and do live in hot climates, also “in violation” of the Bergmann’s Rule) or cover yourself with 100 bunny rabbits? The rabbits, although not even coming close to the mass of the elephant, radiate more heat, and while I think 100 is a bit excessive (I don’t want to toast in my sleep), you’ll be far warmer under their mass of fur than up against the wrinkly skin of an elephant. It is therefore apparent that something else is at work with Icadyptes; either there were patterns of upwelling that kept things cool enough for them (as in the case with Galapagos penguins today), or they were somehow otherwise adapted to keep their large body size without overheating in the equatorial region.
Overall, I am probably more excited about this find than I was about Gigantoraptor, and I actually enjoyed reading the PNAS paper (you need to be up on your anatomy to get the most out of the species descriptions, but otherwise it was easily accessible), and it seems to be a rare joy to find a paper that is both important and able to keep me engaged. I can’t wait to see what else has come out of the strata in Peru, and hopefully there can be a greater collaboration between paleoclimatologists, paleontologists, ecologists, and others to find out why there were giant penguins in the very last place we would expect to find them.