Research Article |
Corresponding author: Alexandra E. Telea ( alexandra.telea@gmail.com ) Academic editor: Silke Schweiger
© 2022 Geanina Fănaru, Alexandra E. Telea, Iulian Gherghel, Raluca Melenciuc.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Fănaru G, Telea AE, Gherghel I, Melenciuc R (2022) Melanism in the grass snake Natrix natrix (Linnaeus, 1758) from the Danube Delta Biosphere Reserve, Romania. Herpetozoa 35: 257-263. https://doi.org/10.3897/herpetozoa.35.e85310
|
Animal colouration has a significant ecological role in defence, reproduction, and thermoregulation. In the case of melanism, it is a complex topic. Besides potential disadvantages such as higher risk of predation, melanistic ectotherms may have certain physiological advantages such as more efficient thermoregulation in colder climates and thus, reduced basking time. The common grass snake (Natrix natrix) is a widespread species throughout Europe and Asia. It exhibits a wide range of colour polymorphisms, from olive to dark grey, even albinistic and melanistic. Between 2016 and 2021, we conducted fieldwork in the Danube Delta Biosphere Reserve (DDBR) with the aim to document the geographic range of melanistic grass snakes. We categorised the melanistic expressions of N. natrix individuals as melanotic, completely melanistic, and partially melanistic. Melanistic snakes were encountered in all six localities visited, suggesting that the occurrence of melanistic grass snakes in the DDBR is geographically widespread. We observed both juveniles (n=2) and adults (n=11) with melanism, suggesting that individuals are born melanistic. However, the proportion of melanistic individuals in the general population of N. natrix from the DDBR is unknown. Only at Histria locality we studied the proportion of melanism in the grass snake population and 6.3% of the snakes caught were melanistic. Body size comparisons are not statistically significant because of the low sample size. The N. natrix melanistic morph’s geographical distribution in the DDBR is most likely due to an interaction of climate and habitats, which offer a thermal advantage in the face of predation pressure.
colour morphs, ectotherms, melanotic, polymorphism
Animal colouration provides ecological roles to organisms such as the capacity to warn predators, increase crypsis, signal potential mating partners, and is subject to strong selective pressures (
Melanism is generally not rare in certain snakes, especially in colder environments (
The grass snake N. natrix is a species known for its polymorphism, with a wide range of colours and morphs which have been reported within its range. Dorsal pattern can range from typical olive-green, brown or greyish to individuals with rows of black spots or bars, or double yellow stripes (
We still have a limited understanding of the distribution and frequency of the melanistic morph in N. natrix populations throughout its range. Reports of melanistic individuals of grass snakes have been made from Turkey (
The aim of our paper is to document the geographic range of melanistic individuals of grass snakes (N. natrix) from the DDBR. We also discuss possible advantages and disadvantages for the occurrence of melanism in the study region.
The Danube Delta is composed of a network of channels, lagoons, marshes, levees, and three main Danube river lobes (Chilia to the north, Sulina in the middle and Sfântul Gheorghe to the south) and the Razelm-Sinoe lagoon complex (
We conducted opportunistic and transect surveys of melanistic individuals in the areas accessible by foot, between 8:00 – 14:00h, during the snake’s active season (March-October), from 2016 to 2021 throughout the DDBR. Melanistic grass snakes were visually searched for and caught by hand when possible. Upon capture, we measured snake’s body mass (BM) using a spring / digital scale (0.1 g accuracy) and photographed alongside a graph paper, using DSLR cameras with standard lenses (Nikon D7100 and Olympus OM-D E-M1 MarkII). The snakes’ snout-vent length (SVL) was then measured in ImageJ (
Danube Delta Biosphere Reserve and the locations of the melanistic grass snakes Natrix natrix found during our study and in literature (map source: OpenStreetMap.org and geo-spatial.org).
Depending on the morphological expression of melanistic characteristics, we categorised the melanistic morphs of grass snakes following
For the statistical analysis between groups of melanistic and non-melanistic grass snakes from Histria locality we have applied a One-Way Anova test.
We found 13 melanistic individuals in close proximity to six localities: Letea, Maliuc, Sulina, Sfântu Gheorghe, Periboina, and Histria (Fig.
Characteristics of the melanistic individuals of grass snakes Natrix natrix found in DDBR between 2016 and 2021. Abbreviations as follows: Localities L-Letea, M-Maliuc, S-Sulina, SG-Sfantu Gheorghe, H-Histria, P-Periboina (Fig.
Locality | Date | Sex | Age | SVL (mm) | Weight (g) | Coordinates | Melanism | Figure |
---|---|---|---|---|---|---|---|---|
L | 14.05.16 | M | A | 500 | 42.9 | 45°17.16'N, 29°31.44'E | CM | |
L | 04.06.17 | – | J | 198 | 30 | 45°17.16'N, 29°31.44'E | CM | |
M | 16.08.17 | F | J | 185 | 18.8 | 45°12.00'N, 29°6.66'E | PM | |
S | 06.06.17 | – | A* | – | – | 45°9.36'N, 29°39.24'E | CM | |
SG | 17.06.17 | F | A | 870 | 284.8 | 44°53.76'N, 29°35.58'E | CM | |
SG | 22.07.17 | M | A | 882 | 191.4 | 44°53.76'N, 29°35.58'E | CM | |
SG | 22.07.17 | M | A | 590 | 95.9 | 44°53.76'N, 29°35.58'E | Ml | |
SG | 22.07.17 | M | A | 641 | 81.6 | 44°53.76'N, 29°35.58'E | PM | 3A |
P | 24.04.21 | M | A | – | – | 44°36.78'N, 28°55.80'E | CM | 3C |
P | 01.05.21 | F | A | 680 | – | 44°36.84'N, 28°55.86'E | CM | |
H | 13.10.18 | F | A | 625 | 76 | 44°32.82'N, 28°46.38'E | Ml | |
H | 17.10.20 | M | A | 630 | 82 | 44°32.82'N, 28°46.50'E | Ml | 2 |
H | 19.10.21 | F | A | 580 | 70 | 44°32.94'N, 28°45.72'E | Ml | 3B |
We encountered all three categories of melanism expression: 53.8% completely melanistic individuals (n=7), 15.4% partially melanistic (n=2), and 30.8% melanotic (n=4) (Table
Examples of encountered melanistic morphs of Natrix natrix. A. dorsal view of partially melanistic morph with double yellow stripes pattern, Sfântu Gheorghe (photo credit: I. Gherghel); B. dorsal view of melanotic morph, Histria (photo credit: E.A. Telea); C. partial lateral and ventral views of completely melanistic morph, Periboina (photo credit: F. Stănescu).
At Histria we caught a total of 48 adult grass snakes, of which only three snakes were melanistic (6.3%). The three melanistic grass snakes from Histria had an average SVL of 611.66 mm and BM of 76.00 g. The non-melanistic grass snakes had an average SVL of 609.71 mm, and BM of 76.26 g (Table
Melanistic grass snakes have been found in all six localities surveyed, four of these not mentioned before in literature (Fig.
We have encountered all three melanistic morph categories, as defined after
In the small sample size from Histria locality (Fig.
All surveys were done during the day, from morning to early afternoon. All melanistic grass snakes we found were either hidden in reed vegetation or in close proximity to an abundant reed vegetation habitat. This habitat choice might have affected the success of our detection and capture of melanistic grass snakes. However, such behaviour and habitat choice might not have been accidental. Hypothetically, it could have been advantageous to the melanistic grass snakes as they could maintain a thermal benefit in microhabitats with less exposure to the sun (e.g., high-density vegetation), while also avoiding predator (and researcher) detection. From an evolutionary point of view, in snakes, melanism is established when the thermoregulatory advantage outweighs the risks (
We are grateful to Dan Cogălniceanu for his helpful comments on versions of the manuscript and to our collaborators Florina Stănescu and Alexandra Șoimu for one of the observations presented here. We also thank the editor and the reviewers for their effort and time in helping us improve the manuscript; their commitment was of great value. This work was partially supported by a Romanian Ministry of Education and Research grant CNCS – UEFISCDI, project number PN-III-P1-1.1-TE-2019-1233, and project number PN-III-P1-1.1-PD-2021-0591 within PNCDI III. Research permits were provided by the Danube Delta Biosphere Reserve Administration (05/09.03.2016, 05/16.03.2017, 02/22.02.2018, 13/23.05.2019, 100/24.06.2020 and 13/28.01.2021). Access to the Histria Fortress Archaeological Complex was given by the National History and Archaeology Museum Constanța.