Dietz Rune

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Last Name
Dietz
First Name
Rune
ORCID
0000-0001-9652-317X

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  • Article
    Upside-down swimming behaviour of free-ranging narwhals
    (BioMed Central, 2007-11-19) Dietz, Rune ; Shapiro, Ari D. ; Bakhtiari, Mehdi ; Orr, Jack ; Tyack, Peter L. ; Richard, Pierre ; Eskesen, Ida Gronborg ; Marshall, Greg
    Free-ranging narwhals (Monodon monoceros) were instrumented in Admiralty Inlet, Canada with both satellite tags to study migration and stock separation and short-term, high-resolution digital archival tags to explore diving and feeding behaviour. Three narwhals were equipped with an underwater camera pod (Crittercam), another individual was equipped with a digital archival tag (DTAG), and a fifth with both units during August 2003 and 2004. Crittercam footage indicated that of the combined 286 minutes of recordings, 12% of the time was spent along the bottom. When the bottom was visible in the camera footage, the narwhals were oriented upside-down 80% of the time (range: 61 100%). The DTAG data (14.6 hours of recordings) revealed that during time spent below the surface, the two tagged narwhals were supine an average of 13% (range: 9–18%) of the time. Roughly 70% of this time spent in a supine posture occurred during the descent. Possible reasons for this upside-down swimming behaviour are discussed. No preference for a clockwise or counter-clockwise direction of roll was observed, discounting the possibility that rolling movements contribute to the asymmetric left-handed helical turns of the tusk.
  • Article
    Low hunting costs in an expensive marine mammal predator
    (American Association for the Advancement of Science, 2024-05-15) Rojano-Donate, Laia ; Teilmann, Jonas ; Wisniewska, Danuta M. ; Jensen, Frants H. ; Siebert, Ursula ; McDonald, Birgitte I. ; Elmegaard, Siri L. ; Sveegaard, Signe ; Dietz, Rune ; Johnson, Mark ; Madsen, Peter T.
    Many large terrestrial mammalian predators use energy-intensive, high-risk, high-gain strategies to pursue large, high-quality prey. However, similar-sized marine mammal predators with even higher field metabolic rates (FMRs) consistently target prey three to six orders of magnitude smaller than themselves. Here, we address the question of how these active and expensive marine mammal predators can gain sufficient energy from consistently targeting small prey during breath-hold dives. Using harbor porpoises as model organisms, we show that hunting small aquatic prey is energetically cheap (<20% increase in FMR) for these marine predators, but it requires them to spend a large proportion (>60%) of time foraging. We conclude that this grazing foraging strategy on small prey is viable for marine mammal predators despite their high FMR because they can hunt near continuously at low marginal expense. Consequently, cessation of foraging due to human disturbance comes at a high cost, as porpoises must maintain their high thermoregulation costs with a reduced energy intake.