I've been working with Seth Newsome (Carnegie Institute
of Geophysics, Washington, DC) to see what we can figure out about sperm
whale foraging ecology by looking at their teeth (see also "Other Publications"
at the bottom of the page). Sperm whales, like many mammals, have
teeth that grow in layers (see photo below, as well as the orca tooth on
the "killer whale foraging ecology" page).
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| Longitudinal section of a sperm whale tooth. The
earliest years of growth are to the left, at the tip
of the tooth. The oldest, most recent years of growth are to the right, along the hollow pulp cavity. 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. In certain special circumstances, we can do even more. Back in 1979 there was a mass stranding of sperm whales in Florence, OR. John Rozdilsky, working for the Burke Museum, and Dale Rice, working for what would later become the National Marine Mammal Laboratory, headed up efforts to recover tooth samples from every single one of the 41 animals that stranded that day, and they also got complete skulls from 5 individuals. Those samples have proven to be an invaluable source of data, first for age and growth studies (Rice et al. 1986, full citation below), then population genetics (see series of papers on the SW Fisheries Science Center website), and now stable isotopes.
There are two main ways we are analyzing the isotope data
from these growth lines. First, we can examine how diet changes throughout
the lifetime of individuals from the time they are born to the time they
died. Nobody really knows how sperm whales transition from a diet
of milk to one of fish and squid. Although isotope data cannot identify
exactly what species are being consumed, there are clear chemical differences
between mother's milk and solid foods (see also "killer
whale foraging ecology"). If we are able to measure the isotopic
composition of the first year of growth (not always possible if, for instance,
the tip of the tooth is worn away), we can "anchor" the data and see that
transition:
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| Developmental, or ontogenetic, changes in 15N for two sperm whales from the 1979 Florence, OR mass stranding (Newsome et al., unpublished data). Data are anchored at the earliest (i.e., youngest) growth layer groups available. |
In these two individuals, we cannot clearly identify the point at which milk is no longer consumed. But we're hopeful that when we have all of the teeth analyzed we will be able to get that worked out.
Because the date of death is the same for all of these
individuals, we can also compile a dietary record for the whole pod linked
to calendar year, with the end-point "buoyed" at 1979 (I'm experimenting
with this terminology, as a contrast to "anchoring" the data at growth
year 1). This allows us to evaluate what environmental conditions
the pod as a whole were experiencing. But it is important to keep
in mind that sperm whales tend to be matrilineal, which means females and
their associated calves make up the bulk of the pod. Sub-adult and
adult males tend to be solitary for much of their lifetime, and only join
the matrilineal groups later in life.
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| Developmental, or ontogenetic, changes in 15N for five sperm whales from the 1979 Florence, OR mass stranding (Newsome et al., unpublished data). Data are buoyed at 1979 (at the right) and project back through time to the left. |
The plot above shows a group of animals at the top of the plot (all females) that all appear to have been doing pretty much the same thing. Part of the gradual decline in 15N values is related to the shift from milk to independent foraging (see above plot), and some of it is probably related to changes in the environment immediately around the pod. Then there is the adult male on the bottom of the plot who looks as if he spent the last 3 years of his life doing the same (or at least similar) stuff as the females. But before that, he was off doing his own thing somewhere else in the North Pacific.
We're working on getting these data written up and submitted. I'll update the "publications" page when we get this stuff out!
Rice, Dale W., Allen A. Wolman, Bruce R. Mate, James T. Harvey. 1986. A mass stranding of sperm whales in Oregon: sex and age composition of the school. Marine Mammal Science 2(1): 64-69.
Other publications on sperm whale foraging and isotopes:
Marcoux, Marianne, Hal Whitehead, and Luke Rendell.
2007. Sperm whale feeding variation by location, year, social group
and clan: evidence from
stable isotopes. Marine Ecology Progress Series
333:309-314.
Sónia Mendes, Jason Newton, Robert J. Reid, Alain F. Zuur, and Graham J. Pierce. 2007. Stable carbon and nitrogen isotope ratio proWling of sperm whale teeth reveals ontogenetic movements and trophic ecology. Oecologia 151:605-615.
Sónia Mendes, Jason Newton, Robert J. Reid, Alexandros Frantzis, and Graham J. Pierce. 2007. Stable isotope profiles in sperm whale teeth: variations between areas and sexes. J. Mar. Biol. Ass. U.K.87:621–627.