In situ measurements of animal morphological features: A non‐invasive method

Abstract

Measurements of morphological features are important for ecological studies, especially on free‐ranging wild animal species. Conventionally, specimens either dead or in captivity are used for morphometric studies, which is difficult in the case of wild species for several reasons. Capturing would be even futile when research questions are relating to issues such as prey size selection or estimation of intake rate under field conditions, where in situ morphometric measurements are inevitable. Remotely estimating morphometric features via, avian bill and head lengths are always laden with wide variances. Hence, we developed this technique for in situ measurements of animal morphology through digital photographs. This non‐invasive novel method for in situ estimation of individuals’ morphometric measurements uses digital photographs’ metadata. We extracted the metadata (field of view, focal length, subject‐distance, etc.,) in exchangeable image file format from digital images using freely available software, and the unknown sizes were estimated from each image. Cardboard (fish) models were, at first, used to estimate the accuracy of measurements comparing actual and estimated values, and the same protocol was repeated on museum specimens (bird). Finally, we checked the repeatability of this method under field conditions using free‐ranging Chickens (Gallus gallus domesticus). The results under different conditions were subjected to rigorous statistical testing. Upon comparing the actual with estimated values, obtained from three distinct types of objects (1. Cardboard model, 2. Museum specimens and 3. Live birds under field conditions), variations were found non‐significant affirming the assumption that actual value (Y) = estimated value (X) with inconsequential variance of the Y from X. Moreover, both the relative and absolute errors between the actual and estimated values were so low, indicating that this method yielded neither overestimation nor underestimation. The advantage of this non‐invasive method is that it could generate reasonably accurate morphometric measurements at a relatively low cost for routine morphometric works in field conditions. We envisage significant utility of this non‐invasive technique in different fields of science such as biology, morphology, ecology, evolution, wildlife, conservation science, agriculture, forensics and engineering design.

Rapid photogrammetry of morphological traits of free‐ranging moths

“…Addressing these issues usually requires expensive equipment and elaborate post-processing procedures. This had previously been mostly limited to large animals (mammals and some birds) with small population sizes and high conservation priority, in situations where they cannot be manipulated for measurement or even approached closely (Laws, 1953;Haley et al, 1991;Ratnaswamy & Winn, 1993;Bell et al, 1997;Durban & Parsons, 2006;Bergeron, 2007;Breuer et al, 2007;Berger, 2012;Kurita et al, 2012;Mahendiran et al, 2018). Such measurements have also been used to recognise individual animals (e.g.…”

A simple use of camera traps for photogrammetric estimation of wild animal traits

“…Another photogrammetry technique scales animals by providing a predetermined measurement unit in the photograph (Rothman et al, 2008;Webster, Dawson & Slooten, 2010). This technique is becoming more flexible due to the various types of units selected in different studies (Mahendiran et al, 2018). Researchers have used measuring poles close to the trait of interest (Ireland et al, 2006), conspecifics that have been physically measured (Willisch, Marreros & Neuhaus, 2013), or laser points with known distance (Durban & Parsons, 2006) as photo scales.…”

“…In addition to advanced digital cameras (e.g. Canon EOS 5D Mark III with lens EF24-105 mm f/4L IS USM; Mahendiran et al, 2018), most of the existing photogrammetric methods require specific equipment or technology, such as an ultra-telephoto lens, range finders or digiscoping techniques (i.e. the combination of a spotting scope and a digital camera; Willisch et al, 2013).…”

Mining morphometrics and age from past survey photographs

“…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 [11]. Although morphological measurements from images are routine in the lab setting [26], few studies have attempted to extract morphometrics from photographs in situ in wild animals [27]. 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.…”

“…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 [27] 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.…”