You are what you eat

Beneath the beauty of the animal kingdom there is a fierce world, in which you are either hunting or you are dinner. Life is a constant struggle for survival, and finding food is essential to existence. That is why one of the most important questions in the study of wild animals is: "What do they eat?."

A wandering albatross wandering in search of a meal.
(Will Rayment, University of Otago)

In the ocean, the quest for food often requires going far or deep. Some animals travel hundreds of miles over the ocean just to find a meal. Such is the case of the wandering albatross, a magnificent seabird that we often see off the coast of Kaikoura. Satellite tracking of wandering albatross has revealed that they can cover up to 15,000 km from their nest on a single foraging trip, flying day and night at speeds of up to 80 km per hour (50 mph).

Almost everything about sperm whales is superlative: they are the largest of the odontocetes (toothed whale), the deepest diving cetacean, and their echolocation clicks are the loudest underwater sound produced by any animal. Their biology and physiology are fine products of thousands of years of evolution which allow them to go as deep as 2,500 meters (1.5 miles) and as long as two hours to locate their prey.

To date, there has been just one analysis of the diet of sperm whales in New Zealand. The study revealed that sperm whales eat squid and a relatively high proportion of bottom-living fish. This is intriguingly distinct from sperm whale populations elsewhere, which eat almost exclusively squid. Understanding the diet of these animals allows scientists to identify their role in the ecosystem and provides a better understanding of their distribution and habitat use. In turn, this information is very valuable for conservation management.

But how do we find out what these animals eat if they spend most of their time at great depths underwater and don’t feed at the surface? The answer can be provided by the analysis of stable isotope signatures from whale skin. Studying stable isotopes is a non-invasive method that can provide information on diet as well as physiology and ecotoxicology.

Collecting sperm whale skin from a DTAG.
(Wayne Perryman, SWFSC, NOAA)

Isotopes are variants of a particular chemical element, and stable isotopes are those variants that remain stable over time. The stable isotope composition (say, for carbon and nitrogen) in an animal is largely determined by the isotopic composition of the food, water, and gas that the animal takes in. These elements are eventually used for making new soft tissue (like skin) and biological minerals. Through the analysis of the composition of stable isotopes, researchers can track down the primary source of the animals’ diet by matching those isotope signatures of the skin to the ones of potential prey species.

Getting our hands on samples of whale skin is a tale of persistence and patience. Every time a sperm whale dives, there’s a chance that small strips of sloughed skin will remain at the surface. We only have to keep our eyes open for those skin fragments in the slick left by a fluking whale, have a net at hand, scoop it up, and put it in the freezer. We also often find small pieces of skin trapped in the suction cups of DTAGs. Later on, stable isotope analysis can be carried out back at the laboratory.

During the expedition we have so far collected nine skin samples. They are happily frozen and waiting to be analysed—ahopefully to reveal a little more about the diet of sperm whales and what makes Kaikoura such a productive foraging ground.  

Further reading on stable isotope analysis: 
Newsome, S.D, Clementz, M.T., and Koch, P.L., "Using stable isotope biogeochemistry to study marine mammal ecology," Marine Mammal Science 26 (2010): 509-572.