Non-invasive Body Composition Analysis in Geckos

The study was conducted at Midreshet Ben-Gurion in the northern Negev desert, Israel (30°51′8.27″N 34°47′0.24″E) from summer 2003 until autumn 2004. The study site was a complex of guest rooms surrounded by a two-meter high wall over an area 13x150m. A dense population of the Israeli fan-toed gecko Ptyodactylus guttatus inhabited the premises [30 –32 (link)]. The Israeli fan-toed gecko is a medium-sized, insectivorous, rupicolous, scansorial lizard [6 , 45 ] Zlotkin et al. 2003 [50 (link)], Sion et al. 2020 [33 (link)] in the family Phyllodactylidae [7 (link)] common in mesic and arid parts of the Middle East (Israel, Egypt, Saudi Arabia, Oman, Palestine, Jordan, and Syria). It often inhabits cliffs or masonry walls where it can easily be observed from a distance ([48 , 47 ], 2016 [6 , 15 (link), 30 –32 (link), 46 ]).
Fifty-five geckos were hand captured, measured (morphometrics) and scanned (DXA) and released at the site of capture. Of these, 30 gecko’s scan data were included in the comparison, since their body mass was above the lowest possible accurate reading with minimal body mass (> 4.8 g) as indicated in the results (Table 1). The snout vent length (SVL) of these 30 geckos was 60.6–91.7 mm and their body mass 4.99–22.5 g. We used these 30 geckos to compare the real wet mass (as measured by a scale) and the wet mass measure by DXA (see below). From each captured gecko, we recorded the mass using Ohaus digital scale to 0.1 g precision, snout-vent length (SVL), using digital calipers, and the width at the base of the tail. Six additional individuals were captured and euthanized for the calibration necessary for this study (two males, three females and one too small to be sexed without a probe) The smallest gecko (55.6 mm) with body mass 4.2 g was excluded to improve accuracy from 55 to 8.5% error. The snout vent length (SVL) of these geckos was 61.5–91.7 mm and their body mass 4.8–11.5 g. We killed only six geckos in order to minimize destructive sampling as much as possible.
Table 1

Sex, snout-vent length (SVL), live wet body mass, DXA mass reading and fat mass chemical extraction of six Israeli fan-toed geckos Ptyodactylus guttatus used to validate the application of dual-energy X-ray absorptiometry (DXA) to non-invasively calculate body fat indices in small lizards

Sex	SVL (mm)	Live Wet Mass (g)	DXA Mass Reading (g)	Fat Extraction (g)	DXA Fat Reading (g)
Male	61.51	4.8	5.7	0.26	1.5
Male	87.45	11	12	0.37	1.65
Female	80.74	10.9	11.9	1.60	2.4
Female	65.97	11.5	13	2.09	2.8
Female	67.14	6.5	7.6	0.93	2
Unknowna	55.64	4.2	5.2	0.43	0.95

^a Indicates the lizard that was removed from the validation experiment (see text for details)