MEPS 388:263-272 (2009)  -  DOI: https://doi.org/10.3354/meps08158

Intrinsic and extrinsic forcing in life histories: patterns of growth and stable isotopes in male Antarctic fur seal teeth

N. N. Hanson1,*, C. M. Wurster2, M. I. Bird2,4, K. Reid3,5, I. L. Boyd1

1Sea Mammal Research Unit, Gatty Marine Laboratory, University of St. Andrews, St. Andrews, Fife KY16 8LB, UK
2School of Geography and Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, UK
3British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK
4Present address: School of Earth and Environmental Science, James Cook University PO Box 6811, Cairns, Queensland 4870, Australia
5Present address: CCAMLR, PO Box 213, North Hobart, Tasmania 7002, Australia

ABSTRACT: Life-time records of the trophic sources of carbon, nitrogen and of growth rate can be generated from biogenic structures that show accretionary growth, including fish scales, whale baleen and the teeth of some animals. Records generated from individual teeth can also be combined to provide longer time series elucidating changes in environmental conditions encountered by a population. Both intrinsic (i.e. ontogenetic) and extrinsic (i.e. environmental) factors are important in modulating variation in growth and the apparent dietary sources of C and N. We used the canine teeth of a large marine predator, the male Antarctic fur seal Arctocephalus gazella from South Georgia, to investigate both intrinsic and extrinsic sources of variation. Substantial ontogenetic shifts occurred in both δ13C and δ15N values in individual teeth, indicating a change in the trophic sources of C and N as individual animals age. Over the 40 yr period from 1964 to 2005, and after statistical reduction of ontogenetic variation, we also detected long-term declines in δ13C and δ15N values, indicating that the population has become more dependent on energy from a lower trophic level. A concurrent decline in annular tooth growth may be a consequence of rapid population growth during this period. The time series of δ13C values was also inversely correlated with sea surface temperatures in the region, although isolating a causal relationship remains elusive. Our analyses suggest that both intrinsic and extrinsic sources of variation, and their interaction, must be considered from such time series data; failure to do so could result in a biased interpretation.


KEY WORDS: Annulus · Dietary reconstruction · Stable isotope · Time series analysis · Southern Ocean


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Cite this article as: Hanson NN, Wurster CM, Bird MI, Reid K, Boyd IL (2009) Intrinsic and extrinsic forcing in life histories: patterns of growth and stable isotopes in male Antarctic fur seal teeth. Mar Ecol Prog Ser 388:263-272. https://doi.org/10.3354/meps08158

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