Ad libitum surveying of wildlife serves as a standard and universal method to count populations, identify and track individuals, and record survival . These surveys are often photographed to allow for later analyses, which results in databases of images filed away after serving their initial purpose . However, these images may contain additional valuable information, which can solve secondary objectives, even years after the original survey effort. In particular, obtaining morphometric data is fundamental to the study of ecology and evolution, allowing researchers to understand taxonomy, evolutionary divergence, mate choice, growth and development, and individual condition . Although morphological measurements from images are routine in the lab setting , few studies have attempted to extract morphometrics from photographs in situ in wild animals . Given the abundance of photographs available from ecological studies, optimizing survey efforts by extracting morphometric data could provide researchers with tangential information important to their study species and system.
Past studies have attempted to retrieve wildlife morphological information from images or still video by either 1) placing a scale in each image (e.g. [41, 42]), 2) photographing the subject at a known, fixed distance ([6, 20, 37]), 3) comparing the subject’s target morphology to known morphometrics (e.g. length of fish prey compared to bill length, ) or 4) by undergoing geometric morphometric analysis, which uses easily identifiable landmark points on the subject’s anatomy to create a 3-dimensional coordinate system to measure volume of morphology (e.g. beak shape in seven species of Darwin’s finches; ). Although 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. Mahendiran et al.  designed a methodology to measure morphology in situ by extracting the distance from the lens to the subject from each image’s metadata. Although this method provides researchers with a framework to obtain morphometrics from survey images in future studies, the compulsory metadata is often not available from old images (e.g. in our study, only 6 of 32,296 images had this metadata recorded), as the subject-distance is only recorded in specific cameras and lens systems (e.g. Canon EOS 5D Mark III; ). Therefore, 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.
African savanna elephants (Loxodonta africana) serve as an ideal species to develop a non-invasive, image-based morphometric methodology: as handling these animals is particularly invasive and expensive, involving anaesthetics . In addition, African savanna elephant’s IUCN ‘vulnerable’ status  and unsustainable overall decline of 8% each year  makes the African elephant a conservation priority. Large-tusked males are particularly vulnerable to ivory poaching and trophy hunting due to their attractive large size , yet the length or weight of most tusks and the individual’s age is primarily determined observationally and therefore subjective (Henley pers. comm. ). These large males are integral to the health of the population; they contribute disproportionately more to the population’s genetic and social structures [1, 8, 35]. Where hunting is legal, protocols mandate hunted males be of certain age (e.g. 20 to 35 years in the Associated Private Nature Reserves; ) to ensure the protection of males in their reproductive prime (35 to 55 years; ). Hunting quotas and the cost of trophy hunting large-tusked males, which provide funds for the conservation of both elephants and other species, are often dependent on the size of the tusks ; therefore, knowledge of an elephant individual’s tusk dimensions prior to licensed hunts could provide evidence for larger fees and identify unsustainable practices. Objectively identifying individuals by size and age is essential information for ecologists, reserve managers, conservation institutions, and hunters themselves, to understand the age structure of a population, which males to prioritize for protection, and to ensure that males are accurately aged if they are to be hunted.
Data on fitness-relevant physiological states can also potentially be extracted from photographs. For example, musth is a period of elevated androgens production (male sex hormones) associated with sexual activity and aggressive behaviour . In older elephants, a yearly musth cycle often forms, when males roam large distances away from their home ranges and are most attractive to females . The timing of this period is linked to individual fitness and condition and determines when the individual may be more exposed to human-wildlife conflict . Musth timing is determined by behavioural and visual cues in the field, including aggressive advances, urine dribbling, temporal gland swelling, and temporal gland secretions, which can be broken up into pre-, peak, and post-musth phases [16, 17]. However, studies have not yet focused on using visual cues to examine musth timing from photographs, which would provide researchers with a tool to understand individual condition and likelihood of mating success, as musth males outrank larger, more dominant males who are not in musth when competing for females .
Here, we test whether morphometric measurements obtained from a long-term photographic dataset of elephant can provide accurate age and relative tusk size indices by comparing the results with in situ measurements. Once established, we determine whether this method is free of random errors and how the results are influenced by the visibility of the focus individual’s body profile, photo quality, camera type and photographic settings. Lastly, we determine whether images can provide valuable information on the reproductive timing of males, using the criteria set by biologists to assess musth in the African elephant.