You Are What You Eat

Today a neat new study was published demonstrating that it is possible to determine what a penguin was eating from its bones. The article appeared in Journal of Avian Biology, and was written by two of my favorite penguin collaborators, Dr. Daniel Thomas and Dr. Ewan Fordyce (recent winner of the Hutton Medal) and their colleague Keith Gordon.  They applied a method called Raman spectroscopy to measure the chemical composition of penguin bones from museums in New Zealand.  Raman is a cool technique. Basically, they blast the bones with a low-powered laser, and the way energy is scattered back gives a good estimate of what elements are prevalent in the bone. Raman spectroscopy is really useful because it does not damage the bone.  Other methods for determining chemical composition usually require a bit of bone to be vaporized, which many museums don’t want to see happen.

So what does this have to do with penguin diet?  Well, there is an old saying: You are what you eat!  It is true at the finest scales. Different species of penguins like to eat fish, squid, or krill.  These prey have different concentrations of elements in their bodies due to the different ways the metabolize food and pull oxygen from the water.  Krill are very rich in the element fluoride, which they tend to concentrate in their hard carapace.  You may be familiar with fluoride as a key ingredient in toothpaste!  It is actually toxic, which is why it helps fight the bacteria that cause cavities.  However, it is not dangerous at very low concentrations.  Our toothpaste has only a tiny, tiny fraction of fluoride, enough to zap a microbe but not enough to hurt us.  Fluoride in toothpaste also alters the mineralogy of our tooth enamel, making it harder and  less susceptible to dissolving when contacted by acids (like those in soda). Penguins digest their prey in such a way that most of the fluoride stays in the bones or carapaces and gets passed away harmlessly.  They do keep some though – think of it like shrimp flavored toothpaste for animals without teeth.

In the new study, the Raman data showed a big difference in the bones of krill-loving penguins compared to fish-loving penguins. Two species that eat large quantities of krill, the Emperor Penguin and the Adélie Penguin, showed higher spectra bands than the other four species included in the study (Humboldt, Little Blue, Yellow-eyed, and Fiordland Penguins), which eat almost no krill. This proves the method works, and it give penguin scientists a new tool to look at how feeding patterns have changed in penguins over time. Actually, there are other ways to reveal diet from long-passed penguins too, including isotopes in eggs and guano (penguin poop).

With all these techniques, it is possible to look at old museum specimens from one hundred years ago or more to see if the diet of penguins has changed along with the growth of fisheries or changes in climate.  Indeed, some of this work is already going on.  We will return to the topic later.

Effects of krill consumption on bone composition. The white and grey box plots show the amount of krill eaten relative to other food items, and the black circles show Raman values. Species that eat more krill show higher Raman values. From: Thomas et al. (2013).

Reference:  Thomas, D.T., R.E. Fordyce and K.C. Gordon. In Press 2013. Evidence for a krill-rich diet from non-destructive analyses of penguin bone. Journal of Avian Biology 44.

Penguins in the Food Chain

Penguins are part of the marine food chain.  Today, they dine on fish, squid and krill.  Different species have different preferences, and diets may also change between breeding and non-breeding months when some penguins change the average distance they travel from shore. One focus of penguin paleontology is figuring out what extinct species ate.  However, it is also interesting to know what organisms ate penguins.  Sharks and sea lions are two major penguin predators today, but the remains of a blubbery penguin get utilized by all sorts of small critters as well, once they hit the seafloor.  A paper on trace fossils in penguin bones recently looked at what markings on fossil can say about the fate of penguin remains back in the Miocene, about 20 million years ago.

A bite-marked fossil penguin bone.

Argentina is home to several penguin species today, and in the Miocene the most common type of penguin was Palaeospheniscus, an average sized penguin taxon with a fairly modern skeleton. One of the most common styles of markings I have seen on penguin bones is a series of slashes, which are interpreted by paleontologists as shark bite marks. Because bones with these types of markings always seem to show up isolated, it is possible that the shark caught the penguin, but also possible that only scavenged the remains.  More unusually, the team that studied the Argentine penguin fossils found bite marks that appear to have been made by mammals.  The cool part is that they may have been made by now-extinct marsupial predators, who either stumbled upon a dead penguin that washed ashore or caught a bird at a colony.

Smaller organisms occupy the “clean-up” sectors of the marine food chain, and it is apparent that a variety of invertebrates feasted on penguin bones once large predators had there fill, and even used some for homes. The markings that these animals make are formally classified by paleontologists as trace fossils, and can be given official scientific names just like skeletal fossils. An interesting trace fossil called Entobia represents the workings of a drilling sponge.  These simple creatures excavate holes in surfaces to live in, and sometimes do so with penguin bones littering the  seafloor.  Gnathichnus is another type of trace fossil, represented by an excavation believed to be made by drilling echinoderms (sea urchins) in their quest to eat algae or worms living on bones.  Both types of trace fossils were identified by the team.  Together, all these seemingly insignificant pits and scratches paint a vivid picture of life on the seafloor, and the community in which  these ancient penguins lived.

Reference:

Cione, A.L., Hospitaleche, C.A., Perez, L.M., Laza, J.H. & Cesar, I. 2010. Trace fossils on penguin bones from the Miocene of Chubut, southern Argentina. Alcheringa 34, 433–454.