Birds who Walk on Water: Insights into the Antarctic Petrel
I have always loved the etymology and history of birds’ Latin names, often articulating beautifully the essence of an animal. The term petrel is actually inspired by St Peter, the Apostle who, encouraged by Jesus, walked on water – an apt allusion for our birds!
ANTSIE’s work has focused primarily around the snow petrel, Pagodrama nivea, meaning the ‘snow white sea-ice racer’. Indeed, their inextricable connection to the sea-ice and stomach oil deposits is what has made them so useful in studying the history and changes in Antarctic sea ice. However, less studied is the ‘sea dweller’ or ‘those whose home is the Antarctic sea’ – Thassaloica antarctica or the Antarctic Petrel. As their name suggests, these birds travel much further out to sea than their counterparts, often hundreds of kilometres, termed ice tolerant compared to ice dependent.
Over the years of Antarctic field seasons, there has been a slowly growing pile of data from the Antarctic petrels which has remained largely untouched. However, given their similarities and relatedness to our snow petrels, these birds provide an interesting comparison, whilst also of being of interest in their own right.
The pleasure of exploring and analysing these Antarctic petrel samples has fallen on myself. My name is Eve Sharples, and I recently graduated from the University of Glasgow with a BSc Zoology with Honours. I have a deep love of marine ecosystems and seabirds, having spent many seasons on remote islands monitoring species. Throughout my degree, I developed an academic interest in both Antarctic ecosystems and the use of genetic tools for conservation, primarily through my final year project studying the diets of sub-Antarctic bird predators through an impressive technique called DNA barcoding.
I came across ANTSIE whilst trawling through research opportunities, and immediately became fascinated by the amount of information we can glean from these Antarctic bids through molecular techniques, ranging from stable isotope analysis of the famous stomach oil deposits, to uncovering species population histories through their genetics (see below). I was delighted when offered a placement as a Genetics Research Intern, working with Dr Anna Rix (a post-doctoral researcher here at Durham). My aim is to find out a whole wealth of information about the Antarctic Petrels, using a variety of genetic techniques to generate results.
Sexual Dimorphism
Due to equal biparental care exhibited by seabirds, most species are monomorphic (more etymology – ‘mono’ meaning one and ‘morph’ meaning shape). However, there are some species, such as the snow and giant petrel, where the males are larger. There is little research into any dimorphism in Antarctic petrels, other than Lorentsen and Rov (1994), who suggest you can distinguish sexes using various morphometric measurements, including wing length. Most of my investigative tasks centred on these potential sexual differences – in body size, in foraging, and how the two might be related by the impact of flight capability. But first, crucially, we had to sex our samples!
Genetic Sexing
As Antarctic petrels, along with other seabird species, lack any visible differences between the sexes, genetic sexing is a useful tool. This utilises the chromosomal differences in birds between males (ZZ) and females (ZW) – confusingly the opposite way round to humans! If we extract bird DNA stored in feather tips, and amplify a specific gene called CHD, this difference is visible when we run the amplified DNA on a gel – heavier, longer pieces travel slowly, whilst lighter, shorter pieces run faster. The DNA pieces from Z and W chromosomes are different sizes, which shows up as two bands for females (ZW) and one band for males (ZZ). The process takes a little bit of tinkering and optimising, but we were able to sex all of our Antarctic petrels using this method.
Morphometrics and Flight Speed
Now we had the crucial data on sex, it was time to delve into a series of photos of wings, in order to extricate measurements on wing-length, area and ultimately a theoretical flight speed. This involved a lot of computer based work, editing and warping images of wings to record accurate measurements, as well as sussing out how to work a 2008 software programme that calculated theoretical flight speeds associated with the wing measurements.
Isotopic Differences
Stable isotope analysis has a wide range of applications, including investigating composition and location of foraging. I’m using the ratio of stable isotopes in carbon (13C/12C) and nitrogen (15N/14N). The nitrogen isotope ratio represents the trophic level of prey foraged, as 15N becomes enriched as you move up the food chain. The carbon isotope ratio is useful to detect foraging locations, as the ratio decreases as the individual moves towards the poles. I conducted stable isotope analysis on 30+ feathers, which were incredibly finicky to work with and weigh. It will be interesting to see if there is any correlation between morphometrics or flight speeds and the location and type of prey the Petrels are foraging, as well as any differences between the sexes.
Genomics and Population History
As mentioned, my real interest centres around genetics – both how we can use genes as a tool (such as the genetic sexing), but also what information is inherent in a species’ genetic code too, such as levels of inbreeding, subspecies and population history. My highlight of this experience has been sequencing different individual’s genomes from blood samples. As sequencing technologies return many pieces of the genome, a lot of computer power and coding is needed to then patch these pieces together, using related species’ complete genomes as a reference to scaffold onto. From this, you can learn how the species’ effective population size has changed over millions of years, through an analysis called PSMC. The Antarctic Petrel is monomorphic, meaning it is the only species within a whole genus, so it represents a unique part of the tree of life as well as having a distinct population history.
Conclusion
This internship has been an incredible opportunity to not only further my expertise in genetic techniques, but also learn and peak my interest in other areas such as stable isotopes and foraging behaviours. I’m looking forward to writing up my results as a paper, and seeing how my work on Antarctic petrels might inform the wider ANTSIE research on snow petrels.
References
Lorentsen, S.-H., & Røv, N. (1994). Sex determination of Antarctic Petrels Thalassoica antarctica by discriminant analysis of morphometric characters. Polar Biology, 14(2), 143–145. https://doi.org/10.1007/BF00234977