Aggregations (e.g. group basking) by snakes are usually limited to specific life cycle phases (e.g. mating) or are a consequence of drastic environmental changes (e.g. habitat destruction), high prey densities or highly limited resources within an environment (e.g. basking sites, wintering dens). Here, we report intra- and interspecific observations of four reptile species (primarily Natrix natrix and Vipera berus) sharing basking sites at the confluence of the rivers Dunajec and Poprad near the town of Stary Sącz in southern Poland. From a total of 84 records in the field between 2020–2022, there were 11 interactions from 24 July 2020 to 1 May 2022. Previous studies have indicated direct competition or interference in many species, which we did not observe. There is a noticeable lack of such observations of microhabitat sharing for basking between squamate species in scientific literature. Hence, the accumulation of such observations has the potential to reveal new insights into the behaviour and ecology of N. natrix and V. berus.

adder, aggregations, Anguis colchica, behavioural thermoregulation, Coronella austriaca, ectothermy, grass snake, group behaviour, herpetofauna of Poland, reptiles

Records of a group or paired basking in snakes are infrequent; squamates usually lead a solitary life and are not sociable (Doody et al. 2021), although there are cases where they form groups in hibernacula, refugia sites or at optimal foraging locations (Costanzo 1986; Shah et al. 2006; Mebert 2011; Clark et al. 2012; Meek 2014; Christopoulos et al. 2022). These records are most often transient aggregations limited to certain life cycle phases, environmental changes and even high prey densities (Mouton 2011; Edgehouse et al. 2014; Gardner et al. 2015). In squamates, two types of aggregations are recognised: ecological (when individuals are attracted because of limited habitat availability due to external factors or too concentrated availability of resources, for example, food, shelter, basking sites) (Vasconcelos et al. 2017) and social (when individuals are attracted to conspecifics to avoid predators by group defence, increased thermoregulation (including European lizards) or mating behaviour (Graves and Duvall 1995; Kearney et al. 2001; Chapple 2003; Gardner et al. 2015; Meek and Luiselli 2022b; Turner 2022). Noteworthy and rare are observations of intra- or interspecific interactions outside the specific context that requires aggregating behaviour, often correlated with resource use, for example, heat source, mating partner, hibernacula, shelter or biotic or abiotic structures for predation avoidance (Graves and Duvall 1995; Kearney et al. 2001; Ajtić et al. 2013; Turner 2022).

Within thermoregulatory behaviour, basking in the sun is one of the most typical of thermoregulation in reptiles (Seebacher and Franklin 2005). The animals expose the entire or at least a portion of their body to thermal radiation while being immobile (Bulté and Blouin-Demers 2010). Thus, habitat selection is a critical component of reptilian ectothermy. Various thermal qualities of microhabitats/substrates influence a reptile’s choice in selecting the most favourable basking sites in order to maintain preferred body temperatures (Meek and Luiselli 2022a). A poorly-understood type of behaviour is sharing basking microhabitats between individuals, in locations where there are many favourable sites and, thus, limited pressure from the lack of potential sources.

Our observations took place at the confluence of the Dunajec and Poprad rivers near the town of Stary Sącz in southern Poland (49°33'45"N, 20°38'11"E; Fig. 1), which is located south of the valley of Kotlina Sądecka. The observation area was 143 ha, including a wide variety of terrestrial and aquatic habitats suitable for the local herpetofauna. The landscape is dominated by deciduous trees (Betula sp., Cornus spp., Populus spp., Salix spp., Quercus robur) with a rich undergrowth (Arrhenatherum sp., Dactylis glomerata, Salvia spp., Urtica dioica) (Fig. 2). Open areas are characterised by a low percentage of shrub cover and an abundance of short herbaceous cover predominantly of Rubus spp. At one location (observation no. 7, Fig. 1), there were illegal landfills and trash dumping (anthropogenic refugia for snakes) covered by Parthenocissus sp. (Fig. 3).

Figure 1. 

Records of microhabitat sharing during basking observed between July 2020 and March 2022 near the Dunajec and Poprad rivers confluence, Stary Sącz, Poland. Map created by Aleksandra Kolanek using base maps from https://mapy.geoportal.gov.pl/.

Figure 2. 

Examples of the habitats for observations no. 4, 5, 6, 3, 10 (see Fig. 1).

Figure 3. 

A pile of trash (A) and stones surrounded by grasses and thickets (B). The location is a basking site and refugia for Vipera berus.

As reptile predators influence the social behaviour in lizards (Chapple 2003), we recorded tentative predators; we often observed Hooded Crows (Corvus cornix), Common Ravens (Corvus corax), Common Pheasants (Phasianus colchicus) and rarely the Western Marsh Harriers (Circus aeruginosus), Common Buzzards (Buteo buteo) and the Common Kestrel (Falco tinnunculus). We also observed Red Foxes (Vulpes vulpes), Northern White-breasted Hedgehogs (Erinaceus roumanicus) and feral cats (Felis catus). However, during the fieldwork, predators were never observed in the immediate vicinity of the described records.

The field surveys were performed from February to October each year. Interactions were observed in the period from 24 July 2020 to 1 May 2022 (Table 1). Fieldwork was carried out 4 to 6 times a month, approximately every 4 to 5 days, from 08:30 h to 16:00 h, by one or two researchers. We applied a Visual Encounter Survey (VES), repeated a minimum of two times per survey day (Bartman et al. 2016; Boback et al. 2020). Each animal observed was photographed if possible and only positively identified species were recorded. No animals were captured or handled. All records were made in an area seemingly providing an abundance of suitable habitats. Environmental variability (e.g. scarcity of suitable habitats) can exert substantial pressure in rapidly switching between social and solitary behaviour in squamates (Rabosky et al. 2012; Regnet et al. 2017; Vasconcelos et al. 2017). We defined grouped, or paired basking, as direct contact between individuals or a distance not greater than 50 cm between them.

Overall, we observed Vipera berus on 58 occasions; basking aggregations were recorded five times (in four instances with another V. berus and in one with N. natrix). For N. natrix, we observed communal basking on 7 out of 26 observations (four times with N. natrix and once each with V. berus, Coronella austriaca and Anguis colchica) (Table 1). In the case of adders, 8.62% share space for basking and 19.35% in the case of observed Grass Snakes.

First of our observations (and the only interaction from 2020) of N. natrix involving two adults basking next to each other (Fig. 4A). Subsequently, we observed snakes basking in three different locations regularly (Fig. 4B–D). Natrix spp. aggregates are frequently encountered during the breeding season, in limited areas and are rarely documented outside of this time period (Luiselli 1996; Meister et al. 2012; Ajtić et al. 2013).

Figure 4. 

Natrix natrix communal basking. A. Observation no. 1, the same specimens were seen several times (without photographic documentation) in August 2020; B. Observation no. 2 in June 2021; C. Observation no. 3 in September/October 2021; D. Observation no. 4, a single record from 19 October 2021. Photographs by Maksymilian Jarmoliński.

We also observed communal basking of an adult and juvenile N. natrix alongside an adult Eastern Slowworm, Anguis colchica Nordmann, 1840 (Squamata, Anguidae) (Fig. 5A, B) and another individual alongside an adult Smooth Snake Coronella austriaca Laurenti, 1768 (Squamata, Colubridae) (Fig. 5C). These were one-time observations, the snakes were no longer observed together during the survey at this site. Only solitary Smooth Snakes were regularly seen at this location (no. 6) in later observations. Coronella austriaca are known for feeding on snakes and legless lizards (Čeirāns and Nikolajeva 2017; Di Nicola et al. 2020; Kolanek et al. 2020), although we did not observe specific predation events.

Figure 5. 

A, B. Two Grass Snakes (Natrix natrix) basking with an Eastern Slowworm (Anguis colchica); C. Basking Natrix natrix alongside a Smooth Snake (Coronella austriaca). Photographs by Maksymilian Jarmoliński.

In 2020, we observed three gravid V. berus Linnaeus, 1758 (Squamata, Viperidae) sharing a basking site (Fig. 6A), a behaviour also observed in Crotalus horridus Linnaeus, 1758 (Squamata, Viperidae) (Clark et al. 2012). To the best of our knowledge, this behaviour has not previously been published in Polish populations of adders. Observation no. 10 was of a juvenile V. berus basking directly on an adult N. natrix (Fig. 6B). In this case, the direct contact between the two species was clearly visible (it was observed only on one occasion). In a separate observation (no photographic documentation was made), an adult V. berus was observed with a N. natrix at the same location (pers. obs.).

Figure 6. 

A. Three gravid adders (Vipera berus) share a site for thermoregulation; B. Juvenile V. berus basking on top of a Grass Snake (Natrix natrix). Two adult adders with a classic (C), melanistic (D) and mixed (E) pattern thermoregulating together. Photographs by Maksymilian Jarmoliński.

Observations 8, 9 and 11 of V. berus were of two adults with a classic pattern of thermoregulating and sharing a microhabitat (Fig. 6C), two melanistic adults basking together (Fig. 6D) and two snakes with a classic and a melanistic pattern after leaving the hibernaculum at the end of February 2022 (Fig. 6E). These few observations suggest that adders do not discriminate the body pattern of individuals with whom they bask. Of the 64 adders observed, 20 (31.75%) individuals were melanistic. Based on the hypothesis of thermal melanism in other species of snakes (N. natrix), the high rate of predation, as well as the benefits of climatic conditions, must be investigated (Bury et al. 2022). Additional observations might shed light on this interesting type of choice or the lack of it. Other factors need to be considered, such as age, sex, size and health of the animal, as well as possible effects from being gravid or digesting a meal.

Adders are typically solitary and social interactions are mostly observed between adult snakes and are limited to a short annual mating season (Nilson 1980; Otte et al. 2020). Adders hibernate individually or in small groups, infrequently basking together after emerging from the hibernaculum (Viitanen 1967; Phelps 2004; Dyugmedzhiev et al. 2019). Group basking and refugia sharing, apart from the above-mentioned situations, are only occasionally mentioned in scientific literature (Bauwens and Claus 2021).

Our observations indicate that there is more that needs to be discovered about inter- and intraspecific communal basking. Interspecific sharing of microhabitats/basking spots by squamate reptiles are rarely described in scientific literature and even less often in Polish language publications (Juszczyk 1974; Bauwens and Claus 2021). However, future studies should include additional records of this kind of behaviour that could help determine why there is no sharing of micro-habitats in other potentially-suitable localities. Supplementary data on body and microhabitat temperatures, substrate composition and type (Meek and Luiselli 2022b), habitat complexity and microhabitat characteristics, as well as local meteorological conditions, may be necessary to understand communal basking. More comprehensive research on specific populations sharing microhabitats opens up the possibility of studying in-depth different types of behaviour in the context of thermoregulation. Previous studies on reptiles show direct competition or interference in lizards and turtles (Cady and Joly 2003; Žagar et al. 2015; Hamilton 2021). However, so far there is a noticeable lack of such observations in snakes (Bauwens and Claus 2021) with the exception of garter snakes, Thamnophis spp. (Edgehouse et al. 2014). Here, we present data to aid in alleviating this lack of understanding of the relationship between animals in the context of habitat; this can significantly help in many cases, such as habitat management of protected species.

We thank Dawid Oruba for sharing his remarks and observations and Roger Meek, Apostolos Christopoulos, Yurii Kornilev, Mike Skinner and an anonymous reviewer for helpful suggestions that improved this manuscript. Many thanks to Aleksandra Kolanek for making the map, which exceeded our expectations. Special thanks to Paul Freed.