[MARMAM] New publication on sexually dimorphic measurements in Delphinus spp.

Sam Leander sgm.leander at gmail.com
Fri Apr 2 09:02:57 PDT 2021

Dear members of the MARMAM community,

My co-authors and I are pleased to announce the publication of the
following paper on sexually dimorphic measurements in *Delphinus *spp. in
waters off California:

Leander SGM, Durban JW, Danil K, Fearnbach H, Joyce TW, Ballance LT.
dimorphic measurements from stranded and bycaught specimens contribute to
the characterization of group composition in free-ranging common
dolphins (*Delphinus
*spp.) from aerial images. Mar Mam Sci. 2021;1–7.

Here we use data from stranded and fisheries bycaught common dolphins to
develop sexually diagnostic measurements of *D. d. delphis* and *D. d.
bairdii* that can be applied to aerial images of free ranging *Delphinus* spp.
to describe demographic composition. Based on criteria designed to ensure
accurate measurability in aerial images, three morphometrics were selected
for further analysis: standard total body length (hereafter “total
length”), an “anterior” measurement of length between the snout to anterior
insertion of the dorsal fin, and a “posterior” measurement of length
between the anterior insertion of the dorsal fin and the fluke notch. All
three measurements exhibited interspecific differences, but they varied in
the extent of sexual dimorphism within species. For both species, posterior
showed the greatest degree of sexual dimorphism, with 58.7% and 37.3% of
adult *D. d. bairdii* and* delphis* males, respectively, were larger than
95% of adult females. Total length showed the next greatest degree of
sexual dimorphism and anterior showed the lowest. We then applied these
measurements to an aerial image taken at an altitude of 58m (190ft). We
were able to successfully measure five flat individuals using a segmented
line and determine, using the posterior measurement, that two of the five
measured were likely males. Our results will facilitate characterization of
group composition in aerially photographed *Delphinus* spp. schools off
Southern California (photogrammetric analysis currently underway).
Additionally, these methods are broadly applicable to free-ranging cetacean
populations around the world, providing a non-invasive monitoring tool that
can be integrated into management and conservations plans of vulnerable

*Sam Leander *(*she/her)*
Associate Scientist
*Southall Environmental Associates, Inc.*
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