Natrix astreptophora astreptophora ( López Seoane, 1884 )

Natrix astreptophora astreptophora ( López Seoane, 1884) View in CoL – phenotypic variation

According to our analysis, Natrix astreptophora astreptophora from the Iberian Peninsula and France ( Fig. 2 View FIGURE 2 )displays considerable variation in coloration and pattern. Three different non-melanistic phenotypes can be distinguished. Their occurrence is not mutually exclusive, but two phenotypes are geographically restricted. Kindler et al. (2018) had included samples from the entire European distribution range in their phylogeographic study, and since no obvious genetic differentiation was revealed, these distinct phenotypes merely represent local variation without taxonomic relevance.

Regular phenotype (n=490)

The most widespread phenotype occurs across the whole European range, and snakes representing this phenotype are always characterized by an orange to reddish iris. Hatchlings and small juveniles have a highly variable primary color, generally beige, grey, or greenish. Their dorsal body pattern consists of small black spots.Almost all hatchlings and small juveniles have a yellow collar, usually with black markings in the nuchal region; rarely the collar is whitish instead. Only 19 of the 47 studied hatchlings and small juveniles from Andalusia had a bicolored pileus. After the first year of life, the black color of the pileus, if present, fades. Later, the light collar increasingly fades and the primary body color becomes green or greenish; rarely adults may even show a turquoise body coloration. Typically, only aged females completely lose the black nuchal markings and the dark dorsal spots on the body, resembling then the pale northern phenotype (see below). In some populations (Puertollano, Girona, Alicante), also young adults lack dark dorsal markings, but differ from northern grass snakes in their primary coloration (beige instead of chestnut in northern conspecifics, see below). In Murcia, the Comunidad Valenciana, Catalonia, and France, adult specimens can show contrasting black nuchal and dorsal markings, similar to the western phenotype (see below), although the dorsal spots are typically smaller and the green color is more vivid in the regular phenotype. Other snakes in these areas resemble Moroccan grass snakes, but with more dorsal black markings and without the strong black color in the pileus in juveniles and young adults, typical of the Moroccan specimens.

The largest known N. astreptophora belongs to the regular phenotype. It was found in Toledo, Spain, and had a SVL of 112 cm and an estimated TL of 142 cm ( García-Antón et al. 2017). For Andalusia, specimens with similar sizes are known (J.P. González de la Vega pers. comm.).

Northern phenotype (n=264)

López Seoane (1884) described Tropidonotus natrix var. astreptophorus from Galicia ( Spain) based on the lack of a distinct light collar in adults and the plain chestnut brown body color, without any markings. This plain coloration develops during aging, when the light collar and the dark body markings increasingly fade. Such pale-colored adults occur across the whole European distribution range, but in northern and northwestern Iberia, all aged snakes are pale colored, i.e., in Galicia, Asturias, Cantabria, the Basque Country, the northern parts of Castilla León (north of Zamora, León, and Burgos), and the north of Portugal ( Fig. 3 View FIGURE 3 ). In other regions, pale-colored adults co-occur with patterned adults that retained at least some traces of the collar and body spots. Most pale individuals represent fully adult and aged snakes. An adult syntype of T. n. var. astreptophorus in the collection of the Muséum national d’Histoire naturelle, Paris (MNHN-RA-1889.582, Galicia, Spain), is a largely uniform pale specimen which has left only a few small dark spots on its back (see fig. 11 in Fritz & Schmidtler 2020).

Hatchlings and small juveniles in the north are more variable patterned than elsewhere: They possess a brownred-grey primary color with many small dark markings, which can occur sometimes as wide bars or as a tessellate pattern. Also, plain-colored hatchlings and small juveniles, without any dark markings, or very dark individuals are frequent. Out of nine hatchlings from Asturias, seven were very dark both in the pileus and primary body color and had dorsally many dark spots. A hatchling syntype of T. n. var. astreptophorus in the collection of the Muséum national d’Histoire naturelle, Paris (MNHN-RA-1889.581, Cuntis, Pontevedra, Spain) shows many dark spots and a closed collar (see fig. 10 in Fritz & Schmidtler 2020) .

Approximately 80% of the hatchlings in these northern populations (n=25) had a reddish iris, while 20% had a yellow iris, resembling most populations of N. helvetica , or a maroon iris, suggestive of incipient melanism. This could be the reason why López Seoane (1884) did not mention a reddish iris in his astreptophora description – if he did not only refer to preserved material.

The distribution range of the northern phenotype is characterized by an oceanic climate, which is colder than the more southern and Mediterranean regions of Iberia, with cold night temperatures, frequent rainfall, and cloudy weather, even in summer ( McKnight & Darrel 2001). We suppose that these climatic conditions explain why we also recorded nearly all melanistic snakes in this region (see below the section on the melanistic phenotype). It seems likely that the plain colored and melanistic N. astreptophora reflect an adaptation to the relatively cool and humid oceanic climate. This adaptation seems to be so effective that N. astreptophora is one of the two most common snake species in Galicia, besides Coronella austriaca (see López Seoane 1884; Asensi Cabirta 2024). Likewise, N. astreptophora was recently identified as the second most frequent snake species in a Citizen Science Project (#Sugebizi of the Aranzadi Society, https://www.aranzadi.eus/sugebizi-proyecto-ciudadano) in the Basque Country, Spain.

Western phenotype (n=120)

Along the Atlantic coast of the Iberian Peninsula, from approximately the region of Sines-Botinhas (south of Lisboa, Portugal) to the north of La Coruña ( Galicia, Spain), occur N. astreptophora that typically retain an intensely spotted body pattern in the adult stage, with many big dark dorsal markings. Each marking covers 3–6 scales. The dark elements can sometimes have a circular form or be bar-like and arranged like in N. helvetica . Pronounced large black nuchal markings are present, and the body has an orange-brown to dark green primary color; the iris is frequently yellow or maroon. Snakes of the western phenotype may resemble the coloration of N. maura ( González de la Vega et al. 2021) . Hatchlings and small juveniles in the distribution range of the western phenotype may possess a rare and unique dorsal pattern with many big round dark markings.

Notably, we recorded the majority of grass snakes of the western phenotype along or near the coast ( La Coruña, Pontevedra, Porto, Lisboa; n=41). However, other snakes of this phenotype were also found inland ( Santiago de Compostela , Braga, Coimbra, Peneda-Gerês, Serra de la Estrela; n=30). Representatives of the other two phenotypes also occurred together with the western phenotype, in particular in western Galicia .

Finally, it should be noted that there are adult specimens of the three European phenotypes in which the dorsal head coloration differs from that of the body. In the regular phenotype, a bluish head coloration may occur. In the northern phenotype, the head can be greenish, and in the western phenotype, grey. From northwestern Navarra, close to the Basque Country border, two adults of the regular phenotype had retained a yellow collar and had a complete black pileus. In close neighborhood, N. helvetica occurs and we cannot exclude that these unusually colored snakes are hybrids, as genetically confirmed for the Eastern Pyrenees ( Asztalos et al. 2020). Figure 4 View FIGURE 4 shows the typical body coloration of the three non-melanistic phenotypes from Europe compared to a Moroccan and a Tunisian grass snake .

Melanistic phenotype (n=46)

Completely black snakes are rare in Natrix astreptophora . Many melanistic individuals, even with jet black body and head, retain some lighter colored supralabials. Also, the collar and gular area are often lighter ( Fig. 5 View FIGURE 5 ). We never recorded black individuals with many small light speckles (the so-called ‘ picturata morphotype’) in N. astreptophora . Such snakes are known both from N. helvetica and N. natrix ( Fritz & Schmidtler 2020; Bruni et al. 2022; Fritz & Ihlow 2022; Jablonski et al. 2023). The only known grass snake of the picturata morphotype from Spain, figured in Fernández Guiberteau et al. (2015), originates from the Val d’Aran, where N. helvetica occurs ( Amat et al. 2024).

Melanistic N. astreptophora View in CoL are unevenly distributed across the Iberian Peninsula. From Central Spain, there is only one record known (El Tiemblo, Ávila; J.C. Campos pers. comm.). The remaining 45 out of 46 melanistic snakes (98%) were from the northern and northwestern parts of the peninsula, regions where the northern and western phenotypes of N. a. astreptophora View in CoL also occur ( Fig. 3 View FIGURE 3 ). The abundance of melanistic grass snakes has already been mentioned by previous studies for these regions ( Albadalejo 2008; Fernández Guiberteau et al. 2015). Melanistic individuals of other reptile species ( Vipera seoanei View in CoL , V. latastei View in CoL , Coronella austriaca View in CoL , Chalcides striatus View in CoL ) have also been reported from there ( Martínez-Freiría et al. 2012; Benito et al. 2022). Several studies suggested that a dark or melanistic coloration in snakes and other reptiles is generally favorable, and thus under positive selection, in environments with reduced insolation, many cloudy days, or lower temperatures (e.g., Gibson & Falls 1979; Andrén & Nilson 1981; Meijide & Pérez-Melero 1994; García Antón et al. 2023). This also explains the frequent occurrence of dark or melanistic grass snakes in Scandinavia, the northern distribution range in Russia ( N. natrix View in CoL ; Fritz & Ihlow 2022), or at high altitudes in the Alps ( N. helvetica View in CoL ; Neumann et al. 2024).