Abstract
BackgroundResearchers often document wildlife surveys using images. These images contain data that can be used to understand alterative research objectives, even years after they were originally captured. We have developed a method to measure age and morphology (body size measurements and tusk size) from survey image databases and future surveys, without the availability of a known subject distance or a scale in each image. African savanna elephants (Loxodonta africana) serve as an ideal model species to develop a non-invasive, image-based morphometric methodology: as handling these animals is particularly invasive and expensive, involving anaesthesia and because of their IUCN ‘vulnerable’ status. We compare in situ measurements, taken during collaring events, to tusk-to-body-size ratios, measured from the images.ResultsWe provide evidence that relative morphological measurements, musth timing, and age of male African savanna elephants can accurately be obtained from a survey image database of over 30,000 images, taken over an 18-year period. Of the 11 tusk to body size ratios calculated, we recommend the use of two in particular for future measurement in African elephants to determine size and age: 1) tusk length to tusk diameter and 2) tusk length to body height.ConclusionsWe present a practical, non-invasive measure to estimate morphometrics, including both age and tusk size from photographs, which has conservation applications to the protection of elephants and is relevant to a range of other taxa.
Highlights
Researchers often document wildlife surveys using images
Not all individuals photographed were measured in situ; 22 individuals were either measured or aged in situ out of a total of 406 individuals that were measurable from survey photographs
Without removing annotations from images where main anatomy was not visible, Welch two sample t-tests revealed that blurry images were significantly different from images in focus for one of the 7 ratio variables: tusk diameter to body length (t = − 2.88, d.f. = 166, p < 0.004; Additional file 2: Table S2)
Summary
Researchers often document wildlife surveys using images. These images contain data that can be used to understand alterative research objectives, even years after they were originally captured. Ad libitum surveying of wildlife serves as a standard and universal method to count populations, identify and track individuals, and record survival [43] These surveys are often photographed to allow for later analyses, which results in databases of images filed away after serving their initial purpose [12]. Black et al Frontiers in Zoology (2019) 16:14 a 3-dimensional coordinate system to measure volume of morphology (e.g. beak shape in seven species of Darwin’s finches; [13]) These methods are practical to study specific species in situ, each research group either had access to reference measurement, obtained by handling the animals, or photographed the subjects at a fixed, known distance; this is not feasible when attempting to understand morphometrics from identification survey images because these measures are not typically recorded. To measure morphology from survey image databases - without the availability of the subject distance or a scale in each image - we must develop and validate an alternative methodology
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