I've been working with Seth Newsome (Carnegie Institute
of Geophysics, Washington, DC) and Dan Monson (USGS, Anchorage) to see
what we can figure out about killer whale foraging ecology by looking at
their teeth. Killer whales, like many mammals, have teeth that grow
in layers (see photo below, as well as the sperm whale tooth on the "sperm
whale foraging ecology" page).
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of the tooth. The oldest, most recent years of growth are to the left, along the hollow pulp cavity. The tooth has been etched in formic acid to highlight the growth layers. (Photo by Mike Etnier) |
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| The same tooth, with every 5th growth layer group highlighted with dashed blue lines. The sample paths are highlighted in green, with every 5th sample path in red. (Photo by Mike Etnier) |
We can measure the concentrations of the stable isotopes 13C and 15N in individual growth layers (assumed to equate to one year of growth), and basically have a record of diet (albeit somewhat coarse in detail) throughout the entire lifetime of that individual. For killer whales we are testing the hypothesis that their diet has changed considerably during the latter half of the 20th Century (Springer et al. 2001). We have not analyzed enough teeth to test this yet. But along the way, we've also realized that we'll be getting data on the developmental, or ontogenetic shift in diet throughout the lifetime of an individual.
Like the shift from milk to solid food we've seen in sperm
whales (see "sperm whale foraging ecology"),
killer whales also show a clear shift in
15N
levels through the first several growth layer groups. We interpret
this as a gradual shift from a total reliance on milk to solid foods obtained
either through sharing or independent foraging.
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| Developmental, or ontogenetic, changes in 15N for two Southern Resident killer whales from Puget Sound (Newsome et al., in review). Data are anchored at the earliest (i.e., youngest) growth layer groups available. |
We're still working on lots of teeth from throughout the North Pacific that are curated in various collections. If you have any teeth you would be willing to let us section and sample, we would love to hear from you! Go to the "Contact Information" page if you have any teeth you're willing to share!