Application of a parallel laser apparatus to measure forearms and flanges of wild Bornean orangutans (Pongo pygmaeus wurmbii)

Abstract

We constructed a parallel laser photogrammetry apparatus constructed from commercially available parts, and measured forearm lengths and flange widths of 16 wild Bornean orangutans. Our objectives were to validate our method and apparatus, discuss issues encountered, and construct preliminary growth curves.For adult males, we also compared flange width to forearm length as a way to investigate the relationship between body size and flange development. We used a camera cage around a DSLR camera, on top of which we attached two parallel green lasers. We estimated error with repeatability, accuracy, and interobserver reliability measures, and measured forearm lengths in three different ways to see which was most consistent. The longest forearm measure was the most repeatable (CV = 1.64%), and was similar to flange repeatability (3.50%). Accuracy measurements of a known object were high (error = 0.25%), and Interobserver discrepancy low (3.74%). Laser spacing increased with distance to the subject, but we corrected for this using calibration photos after each session. We transparently discuss the issues we encountered with the aim that this accessible method can help expand the use of laser photogrammetry. Preliminary measurements show that male flange widths and forearm length do not reliably increase in tandem, and that female growth plateaus at around the age at first birth (15 years old). We conclude with suggested improvements to the apparatus and method to ensure the lasers remain parallel.

Measuring mantled howler monkey (Alouatta palliata) testes via parallel laser photogrammetry: Expanding the use of noninvasive methods

“…For future studies, we suggest that the researchers determine a maximum error value for interobserver reliability before using the measurements for subsequent analyses; this value will be context-dependent and take into consideration the study species and the number of observers. Nevertheless, the interobserver error rates among the rest of the sample population (x̄= 4.08%) remained relatively low and consistent with previous studies: 3.62% across anatomy in mantled howler monkeys Barrickman et al (2015) and 4.74% across anatomy in Bornean orangutans (Brown et al, 2022). Both of these studies also show that error rate is higher for smaller anatomy compared to larger anatomy.…”

“…The between‐photo repeatability error (x̄ = 5.5%) in this study is comparable to that of other studies: average error ranged from 5.4% (glabella to rump) to 18.0% (distal hindlimb length) in mantled howler monkeys (Barrickman et al, 2015), 1.6%–5.1% (forearm length) and 3.5% (flange width) in Bornean orangutans (Brown et al, 2022), and 3.7% (forearm length) in Assamese macaques (Anzà et al, 2022). This relatively low error suggests that even if the same individuals were photographed from varying angles, distances, and light conditions, similar estimates of testes size were produced.…”

“…Within-photo repeatability error (x̄= 4.7%) was higher than the few previous studies that reported such data: 0.28%-0.98% for gelada shoulder-rump length (Lu et al, 2016) and 0.28%-0.74% and 0.62% for Bornean orangutan forearm length and flange width (Brown et al, 2022), respectively. We hypothesized that this could have been because of poor photo quality, but our analysis of the relationship between photo quality score and within-photo error did not support this hypothesis.…”

“…Unlike other forms of photogrammetry, PLP does not require an object of known size to be in the frame or other information for scaling. Furthermore, it has been used for a wide variety of taxa, including ungulates (Bergeron, 2007; Kumar et al, 2012; Weisgerber et al, 2015), fishes and marine mammals (Durban & Parsons, 2006; Leurs et al, 2015; O'Connell & Leurs, 2016; Rizzo et al, 2017; Rogers et al, 2017; Rohner et al, 2011; Webster et al, 2010), and both terrestrial and arboreal primates (Anzà et al, 2022; Barrickman et al, 2015; Brown et al, 2022; Chappell et al, 2015; Galbany et al, 2016, 2017; Lu et al, 2016; Richardson et al, 2022; Rothman et al, 2008; Sandel et al, 2022; Schuppli et al, 2016; Wright et al, 2019, 2020).…”

“…Most studies using this method have focused on larger body parts such as limb, trunk, fin, or total length as proxies for overall body size to make predictions about body growth. However, some researchers have attempted to measure smaller body parts such as the body length of darters (Rizzo et al, 2017) and the flanges of male Bornean orangutans (Brown et al, 2022). In the present study we demonstrate that PLP can measure testes, and to our knowledge, we are the first to do so for any primate.…”