**Background:** Self-recognition—the ability to recognise cues originating from oneself—has been studied primarily via the mirror self-recognition (MSR) test in humans, great apes, elephants, dolphins, and cleaner fish. However, many species rely more on chemical than visual cues. In lizards, chemical cues from skin, femoral glands, and cloacal glands deposited on faeces are important for individual recognition, territoriality, and social interactions. Tokay geckos (Gekko gecko) are a promising model because they are territorial, scat-piling, and social. This study aimed to determine whether tokay geckos discriminate self from other using skin and faecal chemicals, whether they show behaviour indicative of comparison between self and other, and whether faecal chemicals serve a similar communicative function to skin chemicals.
**Methods:** Twenty-two captive-bred adult tokay geckos (10 males, 12 females; male SVL range 11.35–15.02 cm, female SVL range 11.29–13.72 cm) were tested in their home enclosures under red light (718 nm, not detectable by gecko photoreceptors). Each gecko was presented with four stimuli on cotton swabs: (1) tap water (within-treatment control), (2) peppermint essential oil (between-treatment novelty control), (3) their own odour (skin or scat), and (4) odour from a same-sex unfamiliar individual from a separate room. Skin chemicals were collected by rubbing a dry cotton swab 10 times over the gecko's back and sides; scat chemicals were collected by rubbing a dry swab on a fresh (<2 days old) scat until a stain appeared. Each animal was tested six times (three for skin, three for scat) with randomised stimulus order. Trials lasted up to 120 seconds without a bite or tongue-flick, or 60 seconds after the first bite or tongue-flick. Videos were analysed blind. Behaviours scored included swab-directed tongue-flicks, ground-directed tongue-flicks, bites, gular pumping, deep breaths, and turns. Inter-observer reliability was high (swab-directed TFs: rs=0.706, p<2.2×10⁻¹⁶; ground-directed TFs: rs=0.871, p<2.2×10⁻¹⁶). Data were analysed using generalised linear mixed zero-inflation negative binomial models and linear mixed effects models.
**Key Results:** Geckos showed no difference in response to water across treatments (peppermint-skin: estimate=−26.43, SE=27.79, p=0.609; peppermint-scat: estimate=−30.77, SE=23.92, p=0.405; skin-scat: estimate=−4.34, SE=8.18, p=0.857). Lizards tongue-flicked significantly more to scat chemicals than peppermint oil (estimate=−18.92, SE=5.98, p=0.005) and to skin chemicals than peppermint oil (estimate=−17.03, SE=7.14, p=0.047), with no difference between skin and scat (estimate=−1.88, SE=2.99, p=0.804). Males tongue-flicked significantly less than females (estimate=−47.915, CI: −82.139 to −13.961, p=0.006); males produced only three tongue-flicks total. In females, there was very strong evidence of more ground-directed than swab-directed tongue-flicks when unfamiliar odour was presented (estimate=−33.970, SE=4.950, p=0.0001). Females produced significantly more swab-directed tongue-flicks to water than to own odour (estimate=19.2, SE=5.76, p=0.003) and to unfamiliar odour (estimate=20.7, SE=6.01, p=0.002). Ground-directed tongue-flicks were significantly higher towards unfamiliar odour than own odour (estimate=−30.1, SE=5.28, p=0.0001) and towards water than own odour (estimate=17.2, SE=4.64, p=0.0007). No significant effects were found for gular pumping or deep breaths. Size difference between test subject and stimulus donor did not affect tongue-flick rate (estimate=−0.034, p=0.976).
**Clinical Implications:** This study provides evidence that tokay geckos can chemically discriminate self from other using both skin and faecal cues, and that they engage in behaviour suggestive of comparison between self and other (ground-directed tongue-flicks in a home environment saturated with self-odour). The similar response patterns to skin and scat chemicals suggest a social communicative function for scat piling. However, the authors note that alternative explanations such as habituation or familiarity cannot be ruled out without additional controls (e.g., testing in a clean environment, familiarisation with conspecific odour, testing response to modified self-odour). The study is not a clinical investigation and has no direct clinical implications; it contributes to the comparative cognitive and behavioural biology of reptiles.