Anguis fragilis, (Linnaeus, 1758) (Linnaeus, 1758)

RESULTS AND DISCUSSION View in CoL

Amelanistic slow worm

On 15 th September 2021 a neonate amelanistic slow worm was found by the first author under a piece of chipboard at Møvik Fortress in Kristiansand   GoogleMaps , southern Norway ( 58°5'51.95845" N 7°57'48.54466" E). It measured 10.2 cm in total length and weighted 00.55 grams, which is the normal size of newborn Norwegian slow worms (pers. obs.). The eyes were red, some reddish pigmentation appeared in the skin, and the dorsal stripe and the dark sides were visible. Larger blood vessels and inner organs were visible through the ventral side ( Figure 1 a, b, c View Figure 1 ). Based on the definitions of albinism and amelanism by Borteiro et al. (2021), this is an amelanistic slow worm, and the only whitish (meaning albino/amelanistic/leucistic/hypomelanistic) slow worm from Norway to our knowledge. Albino or amelanistic slow worms are reported from Great Britain ( Knight 1966) and France ( Robert et al. 1965), and four leucistic slow worms with dark eyes are reported from Great Britain ( Jablonski & Purkart 2018, Harkness & Allain 2020). Allain et al. (2023) found that albinism was the most reported chromatic anomaly in the herpetofauna of the British Isles, although several of these reports may not concern true albino individuals, but also amelanistic, leucistic or hypomelanistic due to inconsistency in term use.

The light colours of amelanistic individuals imply they are running a higher risk of predation. Lack of melanins in the skin also make them vulnerable to UV radiation.

Melanistic slow worm

On   GoogleMaps 23 rd June 2024 an adult female melanistic slow worm ( Figure 2 View Figure 2 ) was found by the second author at Svestad, Nesodden near Oslo ( 59°46'41.0648" N 10°35'42.57518" E). While melanistic adders Vipera berus and grass snakes Natrix natrix are common in Norway (pers. obs.) this is the first truly melanistic slow worm recorded in Norway. Melanistic slow worms are reported from several locations in Sweden (Tommie Lundstedt, Daniel Iglesias, Lin Johanna Rylander, pers. comm.), from The Netherlands ( Struijk 2007), England ( Gleed-Owen 2012) and the French Pyrenees (Eduardo Fernández, pers. comm.). Based on these few reports, we cannot see a clear pattern of increasing prevalence of melanism among slow worms towards the northern pole, as is the case for the adder and grass snake ( Jansen et al. 2024, Fritz & Ihlow 2022). Melanism is generally regarded as a thermoregulatory advantage for ectotherms in cold environments. Exposed to solar radiation, black animals are shown to heat up faster than normal coloured ones (Forsmann 1995). However, an increased risk of predation is also seen ( Andrén & Nilson 1981). The slow worm is a secretive species spending most of its life hidden in vegetation or under objects. The advantage and disadvantage of melanism may be reduced under such conditions. In the common lizard Zootoca vivipara melanism is rare but widespread within the species’ distributional range. A higher prevalence in cold environments is not seen ( San-Jose 2008, Recknagel et al. 2018). Nahrung & Allen (2005) suggest that melanism could be controlled by neutral alleles in some species giving neither advantage nor disadvantage to survival or reproduction. As a result, such alleles are maintained at a constant low level in the populations, a situation possibly relevant for both the slow worm and the common lizard in Norway.

Adders and grass snakes normally become melanistic with age because of accumulation of melanin. The situation regarding ontogenetic development of melanism in A. fragilis is not known. Melanism is possibly more frequent than widely supposed since recognition needs that the observer is familiar with the species’ normal colour range ( Bechtel 1995; Borteiro et al. 2021), and potentially black slow worms may not be recognized as slow worms and therefore are underreported.

Erythristic slow worms

On 17 th August 2024 a young erythristic slow worm ( Figure 3 View Figure 3 ) was found in a potato field by Karin Helen Skipø on the island Hidra in Flekkefjord   GoogleMaps , southern Norway ( 58°12'58.04127" N 6°34'42.49018" E). The colour was bright red and there was no doubt that this specimen was substantially more red than normal colouration and we define this specimen as erythristic. Two medium sized reddish specimens ( Figure 4) were found on 29 th July 2007 by the first author on the mainland of Flekkefjord, at Svege   GoogleMaps farm ( 58°17'22.68419" N 6°38'44.59693" E). Another reddish juvenile slow worm ( Figure 5) was found by Tarald Reinholt Aas the 18 th of August 2024 in Homme, Lindesnes   GoogleMaps ( 58° 03'42.4" N 7°17'53.9" E), southern Norway. These individuals we classify as having a tendency towards erythrism. The juvenile from Homme was more bright towards orange than the two older, medium sized individuals from Svege farm. We find slightly reddish slow worms in Norway from time to time, but since erythrism is a variable degree of red pigmentation, it is questionable when we may label them as erythristic. This is the case with individual Figure 6 in Table 1 View Table 1 , a slow worm from Oslo with only slightly more red colour than the normal warm brown colour. We do not define this individual as erythristic, but it has a weak tendency towards erythrism.

Erythristic slow worms are reported from Sweden ( Iversen 1999) and England ( Allain et al. 2023) and from these few published cases it appears that erythristic slow worm are rare in Europe. However, the lack of a quantitative assessment tool to tell if a reddish slow worm is erythristic or within the normal colour regime might be the cause of this, as observers might be unsecure about the erythristic status.

As in melanism, an ontogenetic variation in the amount of red pigmentation might be seen. The fact that most highly red specimens in our study are young animals indicate an ontogenetic reduction in red pigments or rather an increase in covering melanins. The conspicuousness of erythristic slow worms may increase the risk of predation, contributing to the rareness of this colour aberration, despite the fact that the slow worms spend most of their time under vegetation or objects. In early spring the individuals may be found heating up in the sun openly (own observations), and also at this time of the year the vegetation cover is less. We may imagine that a bright red colour may be more susceptible for predation at this time of the year.

Test of software

We tried to establish an objective method for categorising a specimen as erythristic, using the CorelDRAW Graphics Suite software, reading the red, green and blue intensity in the photographs. The RGB values as well as the Hex codes are presented in Table 1 View Table 1 . We have included also normal coloured slow worms in this table to demonstrate how the RGB values differs between normal grey or brown individuals, to reddish, black or white aberrations. As expected the RGB values of the whitish amelantistic individual ( Figure 1 View Figure 1 ) were all over 200 towards the maximum of 255 that is white. The melanistic slow worm ( Figure 2 View Figure 2 ) had all three RGB values at 3, which means that this individual is truly very black. For an object to be perceived as red, the R value should be significantly higher than the G and B values. The slow worms in Figures 3 View Figure 3 , 4 and 5 are more red than normal based on our eye perception of the photos, and according to Table 1 View Table 1 the R values are 2–4 times higher than the G and B values for these three individuals. The reddish female in Figure 6 also has R values mainly as the double of G and B, but all the values are lower than the slow worms in Figure 3 View Figure 3 and 5. The values for the slow worms in Figures 4 and 6 are quite similar, showing R values the double of G and B values. However, the latter is also true for the deep brown female in Figure 8, which also has R values about the double of G and B. The pale males in Figures 9 and 10 have quite similar and high R, G, and B values as expected, since these resemble whitish colour and we expect the three colour values to approach the maximum of 255 as white would be. For the mating couple in Figure 6, the paler male has higher values of all three colours than the more brown female, as expected.

The shiny skin of the slow worm is highly reflective. Body parts in different angels reflect light differently causing specifically B values to vary significantly over the body. Setting limits for RGB values to categorise erythrism is therefore difficult. We consider the CorelDRAW Graphic Suite software a supportive tool in categorising erythristic animals but still leaving the assessment with a subjective element.

The inspection of a total of 1487 photos of A.fragilis on iNaturalist and Artsobservasjoner did not reveal any photos of albino/white, melanistic or erythristic individuals, and this supports our impression that these colour aberrations in A. fragilis are very rare in Norway.