A tale of two penguins

The Antarctic Peninsular is regarded as one of the fastest warming regions in the Southern Hemisphere.  It might seem small to you, but the increase in air temperature of around 2.8 degrees Celsius is resulting in some big changes.  According to the British Antarctic Survey some 25,000 square kilometres of ice has been lost from ten floating ice shelves, 87% of glacier termini have retreated, seasonal snow cover has decreased.  What exactly these sorts of changes mean for the inhabitants and seasonal visitors to the Peninsular is a question researchers are desperately trying to get a handle on.  The way each species reacts to this changing environment is likely to be very different, even among closely related species.

Take these two penguins for example.  The gentoo (Pygoscelis papua) on the left and the chinstrap (Pygoscelis antarctica) on the right have a lot of similarities.  Both live together in the Antarctic Peninsula.  During the chick rearing season, both have their foraging ranges constrained by the need to feed their offspring daily.  Both heavily consume Antarctic krill (Euphausia superba).  But since the Peninsula has really warmed up, there has been a huge difference in how the populations are faring.  With a stable and perhaps even growing population, the gentoo seems to be doing well.  The same can’t be said for the chinstrap which has undergone dramatic population declines.  The differing response to the changing Peninsula is likely to be down to what is known as niche differentiation – basically the idea that there has to be some differences between the penguins to allow both to coexist for so long.  These can be quite subtle differences, perhaps not at first obvious.  Differences like, aside from krill, what else are the penguins eating.  

Now watching exactly what penguins eat in the wild isn’t realistic.  They do, after all, hunt in the ocean.  Even though the size of their foraging ground is limited by how far they can go and return in a day, that’s still a pretty big area.  Then there is the technology needed to watch the penguins.  But as demonstrated in this recently published paper, led by Michael Polito from Louisiana State University, we don’t need to actually watch the penguins.  We can figure out what they are eating using something called stable isotopes.

Stable isotopes are pretty cool.  Every atomic element has a fixed number of protons, but the number of neutrons… well that can vary.  Let’s take carbon as an example.  Carbon typically has 6 protons and 6 neutrons.  But some carbon doesn’t want to be like all the others – it has extra neutrons.  Standard carbons are written δ12C.  With an extra neutron it becomes δ13C and with two extra neutrons δ14C.  Unlike their unstable/radioactive counterparts, these chemical isotopes don’t break down over time into other elements, which means they remain in the environment at a constant concentration – really handy if you want to take historical measurements of something, like what you ate last week, or last month, or even a few years ago.  For investigating diet, nitrogen isotopic values (δ15N) are commonly used to figure out at what trophic level an individual is feeding, and carbon isotopic values (δ13C) help trace trends in marine habitat use (inshore/benthic vs. offshore/pelagic).  Stable isotopes also come with an additional bonus.  Stomach content analysis typically underestimates the amount of soft-bodied prey, like squid that an individual eats (hard bodied prey remains in the stomach in an identifiable state longer).  And of course stomach content analysis is a snapshot – just what is in the animal’s stomach at that particular time.  Stable isotope analysis can help overcome these two issues, because if you analyse the stable isotopes in something that grows fairly consistently at a known rate for a good period of time, you can get an idea of what an individual has eaten over a long period of time.  Stable isotopes is a whole subject in itself.  If you want to read up a little more on stable isotopes, head over to this page from University of Wyoming.

So back to our penguins.  The researchers needed a source for their stable isotope analysis, which in this case came in the form of fledgling penguin feathers.  Feathers are really good for analysis because in them contains the stable isotopes of the food that they were being fed from their parents.  And because they grow over many months, they show diet over many months.  Yep – from the offspring we can get an idea of what the parents were eating.  The parents didn’t get away scot-free from direct investigations.  A number of birds from both populations had stomach content samples taken over a period of time using ‘water-offloading lavage technique’… basically having their stomachs flushed.  Probably not something that the penguins fancied having done to them.  As you can imagine the contents did provide a whole host of things for the researchers to look at, like fish otoliths (bony structures inside the ears of fish) to identify the species as far as possible, and the weight of krill.  So putting all these analysis together the team were able to come up with a good picture of the dietary differences between the two species.  It seems that the chinstraps are more specialist feeders, having a largely krill-based (low-trophic level) diet from pelagic and mesopelagic (aka twilight zone).  The gentoo on the other hand appeared to be more generalist, with high-trophic level species like fish – and primarily near shore and benthic species being a regular feature in their diet alongside krill.  And there is our niche differentiation.  These guys may share some similarities in food and foraging habitat, but they do not entirely overlap.

So how does this relate to climate change?  Well remember that loss of sea ice… that’s an essential young krill habitat and declining sea ice means declining krill.  Since the chinstraps are so heavily reliant on krill, this could be a major reason for their declining populations.  The gentoo on the other hand with their more flexible diet can cope with declining krill by feeding more on higher trophic level species.  Sure with the changing conditions both species will adapt somewhat to the changing prey availability, but having a narrower diet as with the chinstraps does make it more difficult, as demonstrated by the differences in the two penguin’s breeding populations on the Antarctic Peninsula.

The paper which was published in Marine Ecology Progress Series is open access.  You can access it here http://dx.doi.org/10.3354/meps11095

Image: The gentoo penguin on the left, and the chinstrap penguin on the right.  Image taken by Rachael Herman, Louisiana State University.