Written by Martin van Aswegen
We are pleased to announce the following publication in Scientific Reports, entitled:
Morphological differences between coastal bottlenose dolphin (Tursiops aduncus) populations identified using non-invasive stereo-laser photogrammetry. doi:10.1038/s41598-019-48419-3
· If we are to protect long-lived, slow-reproducing animals like marine mammals effectively, a comprehensive understanding of population-specific demographics, life-history traits and behavioural ecology is essential. While age and body length are key parameters in marine mammal biology, accurately estimating the age and length of wild specimens is challenging.
· Aim and Methods: We used non-invasive stereo-laser photogrammetry to measure the body length of bottlenose dolphins. Two laser pointers were mounted to a DSLR camera and calibrated to be exactly 10 cm apart. These lasers activated simultaneously when the photographer pressed the shutter button, resulting in two green dots projected onto the dolphin. The size of the dolphin within the image was measured using a 10 cm scale i.e. the distance between the two green laser dots (Fig. 1).
· We combined our laser-derived length measurements with long-term demographic data to compare length-at-age growth curves of two well-studied populations of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in southwestern (SW: Bunbury and Mandurah) and Shark Bay (SB), mid-western Australia (Fig. 2).
· Results: Marked differences in growth were detected between SW and SB regions, with dolphins in the SW in excess of 35 cm longer than dolphins in SB. In fact, the average length of a three-year-old dolphin in SW was equivalent to the length of a 12-year-old dolphin in SB, despite being members of the same species. This difference in body length was not caused by regional variation in birth size, but by a distinct difference in growth across all life stages (i.e. neonates, calves, juveniles and adults; Fig. 3).
· SW calves less than two weeks old averaged 107.6 cm in length and were estimated to increase in length by 48.3 cm (44.8 %) by the end of their first year. SB calf growth was modest in comparison, with calves only growing 17.3 cm (16.8 %) by the end of their first year.
· While calves in SW were larger than SB calves, they were also younger when they became fully independent from their mothers.
· SW females were both younger and larger the first time they give birth, relative to SB females.
Why the difference in size?
· A difference in sea surface temperature between temperate SW and sub-tropical SB is likely the dominant factor affecting dolphin size. The inverse relationship between body size and sea surface temperature is characteristic of Bergmann’s rule, which describes a trade-off between surface area and volume. The surface area of a warm-blooded animal represents its ability to reduce heat, while its volume serves as a measure of its heat generation capability. A reduced surface-area-to-volume ratio is accordingly considered a selective advantage, enabling large-bodied animals residing in cooler, temperate environments (such as the southwest of Western Australia) to regulate body heat more efficiently.
Take away messages:
· Our ability to quantify differences in growth over a relatively small geographical distance demonstrates the value of using this technique to investigate body size at various ages and life history stages (birth, independence, first reproduction and physical maturity).
· Our results suggest calf growth may be a valuable proxy of maternal investment and condition, with future studies recommended to incorporate laser photogrammetry into long-term monitoring efforts. This will provide an opportunity to explore the influence of maternal investment on calf size and any associated fitness consequences.
· Stereo-laser photogrammetry, therefore, provides a valuable opportunity to collect morphometric data on wild cetacean populations in an accurate, non-invasive manner, which can ultimately inform conservation management strategies.
The full publication can be downloaded free at: www.nature.com/articles/s41598-019-48419-3.
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Full citation details:
van Aswegen, M., Christiansen, F., Symons, J., Mann, J., Nicholson, K., Sprogis, K., & Bejder, L. (2019). Morphological differences between coastal bottlenose dolphin (Tursiops aduncus) populations identified using non-invasive stereo-laser photogrammetry. Scientific Reports, 9(1), 12235. doi:10.1038/s41598-019-48419-3
We thank C. Karniski, V. Senigaglia, D. Chabanne and the numerous research assistants who assisted with data collection in Bunbury, Mandurah and Shark Bay. We thank N. Stephens for kindly providing bottlenose dolphin measurement data collected during necropsy examinations. We thank A. Hordyk for his input on the growth curve analyses used in this study. We also thank J. Tierney and L. Spilsbury (Bunbury Dolphin Discovery Centre), S. and H. Kirby (Mandurah Dolphin Rescue Group) and Mandurah Cruises for contributing valuable long-term life history data. We thank Tim Barton and Barbara Cheney for providing technical support and advice relating to use of the laser equipment. Tim Barton (Barnacle Electronics) designed, manufactured and supplied the laser equipment and Barbara Cheney provided advice on laser safety, calibration, sampling, and measurement protocols. We also thank the Western Australian Department of Biodiversity, Conservation and Attractions and Monkey Mia Dolphin Resort.
Long-term life history data were collected across several projects dependent on different funding sources. Data obtained through the SWMRP were made possible by: Bemax Cable Sands, BHP Billiton Worsley Alumina Ltd, the Bunbury Dolphin Discovery Centre, Bunbury Port Authority, City of Bunbury, Cristal Global, the Western Australian Department of Biodiversity, Conservation and Attractions, Iluka, Millard Marine, Naturaliste Charters, Newmont Boddington Gold, South West Development Commission, Southern Ports Authority and WA Plantation Resources. Data obtained through the MDRP were made possible by: the City of Mandurah, J. & B. Perry, Mandurah Cruises, Mandurah Dolphin Rescue Group, Murdoch University, Peel Development Commission. Long-term data obtained through the SBDP were made possible by funding grants to J.M: NSF Awards: #0847922, 0820722, 9753044, 0316800, 0918308, 0941487,1559380; ONR: 10230702, Georgetown University, with special thanks to Monkey Mia Dolphin Resort and Royal Automobile Club of Australia.
Permits and Ethics:
This study was approved by the Western Australian Department of Biodiversity, Conservation and Attractions (SF010738, CE005422), with all fieldwork conducted in accordance to standards set by the Murdoch University Ethics Committee (R2649/14).