Hibernation is commonly observed in many species of animals, especially those inhabiting Northern regions where there are significant drops in temperature during the winter months. In Ontario, freshwater turtles are ectotherms (their internal temperatures mimic the external temperatures) which make them “unequipped” to directly face the ruthlessness of winter conditions. For this reason, they stow away in the winter to avoid the unfavourable conditions of the colder months. Scientists have found that some turtles overwinter close together in groups, and this practice is called communal hibernation (Edge et al., 2009; Litzgus et al., 1999; Brown & Brooks, 1994).

Communal hibernation is thought to have many benefits, but not in the way you’re used to hearing about. As ectotherms (where their temperature is determined by their environment), huddling together does not provide individuals with a rise in body heat like it does for endotherms such as humans and even penguins who may gather together for warmth (Duncan, 2016). Rather, the benefits of gathering together present themselves in less obvious ways. Some of these benefits include using group “signals” which would increases the chance of leaving hibernation sites at the same time (Litzgus et al., 1999), or as signifiers for a safe place to go when available hibernation sites become a limited resource (Greaves & Litzgus, 2007). Group hibernation may however, simply be a result of a group of individuals having overlapping home ranges (Edge et al., 2009). But there are some indications that there is more to this story: 

Interestingly, aggregations at overwintering sites are more common in Northern ranges. This observation indicates that overwintering areas may, in fact, be a limiting resource to turtle survival there and that may be why turtles overwinter in groups (Newton & Herman, 2009). 

Additionally, increased mating opportunities are thought to result in congregations of turtles. People may think that turtles mate in the early spring before nesting season begins. While this can be the case, turtles also perform courtship and mating at overwintering sites late into the fall before stowing away from the cold. This is possible for most turtle species (Pearse & Avise, 2001) because most females can physically store sperm in their bodies for future use (Environment Canada, 2015). In turtles, tubules in the oviduct that are specialized to store sperm are generally observed, indicating that this may be a common component of their reproductive behaviour (Pearse & Avise, 2001). Repeated paternity was commonly found in a study on Blanding’s turtles, displaying that females could use stored sperm over subsequent years from the same male to produce her eggs (Henning & Hinz, 2016).  Amazingly, there have been instances where female turtles in care (captive turtles) have been isolated from males, and yet, astonishingly, the females continue to produce offspring (Pearse & Avise, 2001). Therefore, hibernating where other turtles are found has advantages; the increased likelihood of finding a mate! Should a female be unable to find a mate in the spring, she can find a mate in the winter, and still successfully lay eggs during the nesting season using stored sperm (Carrière et al., 2009). 

Another benefit: males generally move great distances during the typical spring mating season to seek out females (Buchanan, 2017). However, if males mate at overwintering sites, they are less motivated to seek out mates and thus, able to spare energy during this active season, because, at times, female mates may be hard to find. Therefore, communal overwintering also allows a male turtle to save energy and yet increase success, by mating at the same site he overwinters (Carrière et al., 2009).

In a study investigating hibernation site selection of Blanding’s turtles, Christopher Edge and collaborators (2009) discovered multiple Blanding’s turtles copulating and hibernating together (≥ 5m) in Algonquin park. At these sites, groups of turtles ranged from two to seven individuals, with both sexes present in all scenarios. This could suggest that communal areas are promoted by the mating opportunities related to grouping together, or it may simply be because habitat destruction has limited the number of overwintering sites available for the population (Edge et al., 2009).

In a study conducted to investigate courtship and mating behaviours of Northern Map turtles, results suggested that communal hibernation was widely displayed in this population. In Lake Opinicon, 75% of studied males ventured near two popular turtle overwintering sites throughout the course of the study. The scientists were able to track the movement of these individuals using radio-telemetry. This tool uses radio signals and transmitters attached to animals (in the case of turtles it is typically attached at the rear-end of their shell) (Litzgus et al., 1999)) to observe their movement while off site. The data did not indicate what percent of males successfully completed overwintering at these sites. However, the results suggest that the majority of males in the area do travel close to communal sites to reap the benefits of group mating (Bulté et al., 2021).

Alternatively, in a study on Wood turtles conducted in Sudbury, Greaves and Litzgus (2007) did not find any evidence to support the idea that turtles overwinter in groups. This places some doubt on the general view that all turtles have a tendency to behave this way. The study was run using visual observation and tracker data. Although the general consensus was that turtles did not overwinter in groups, a pair was documented mating at the overwintering sites before separating for the winter. This supports the idea that turtles may use opportunistic fall mating interactions to increase their fitness (Greaves & Litzgus, 2007).

In Georgian Bay Ontario, a four-year hibernation study on Spotted turtles noted that communal hibernation was commonly found. The researchers again employed the use of radio-telemetry to track the locations of Spotted turtles over the years as well as visual observations to supplement this data. The primary purpose of this study was to assess the ecology and typical hibernation behaviours of Northern Spotted turtles. In the process the scientists also made other remarks. Of the eleven hibernation sites monitored, seven were found to be used for communal hibernation of turtles (≤ 9). This region is, however, considered to be the northern range of a Spotted turtles territory suggesting that this observation could be due to limited hibernation site availability, as previously discussed (Litzgus et al., 1999).

While hibernating in groups seems to have many upsides, this behaviour can nevertheless be dangerous for vulnerable subpopulations of freshwater turtles because grouping together can increase the risk of extirpation (a subpopulation being completely wiped out in an area). In cases where large groups of turtles come together, the increased activity can attract predators (Litzgus et al., 1999). Related is that during the winter, turtles are in a state of inactivity under the water because the low temperature causes metabolic depression and physiologically, metabolic depression limits an individual's ability to move (Newton & Herman, 2009; Edge et al., 2009). Therefore turtles cannot escape attacks by predators at this stage. These situations can be particularly detrimental to unstable subpopulations as it can result in a huge loss of reproductive adults, and potentially result in extirpation (Litzgus et al., 1999).

But also, habitat destruction or alteration can affect overwintering populations; The viability of typical overwintering sites can be changed and make that area unsuitable for overwintering (Litzgus et al., 1999). Examples include when wetlands and shorelands are “filled in '' or drained, or water levels are changed, most often, artificially. Typically, turtles select a pond or water body for overwintering, where there is the potential for a barrier to form between the turtle and the ambient air temperature. The barrier produces a comparatively warmer environment (Ross and Anderson, 1990). Some wetlands may be quite small with stable water levels, a thick warm substrate, also where a layer of ice may become the needed boundary between the water and the air. Without this stable underwater refuge, a turtle’s risk of desiccation and encountering freezing temperatures is significantly elevated (Markle et al., 2020). Unexpected changes in habitat conditions often result in indirect changes to the hydrological condition and water temperatures of the area (Bodie & Semlitsch, 2000) which then can result in many unintended deaths. Therefore, turtles grouping together for the winter, means that more individuals are vulnerable to negative events, and which may increase the chance of local extirpations. In other words, many adults can be killed off because of human alterations to habitats or during a particularly difficult winter (White, 2013). 

It is accepted that communal hibernation is exercised by freshwater turtles, however, researchers have not concluded that it is employed in every turtle species. Future studies may aim to determine whether this phenomena exists reliably in all turtle populations, and should also aim to assess the patterns of this behaviour based on the spatial ecology of areas. Doing so, will help to inform attuned policies and improved conservation measures. 

Generally, conservation of all hibernation areas (chiefly wetland habitats) is of the utmost importance because death of adult breeding turtles individually will destabilize local populations, but also where there may be communal groups, local extirpations of turtle populations is an immediate concern.  As habitat areas and their quality are compromised by human activity, and as temperatures become more inconsistent due to climate change, the threat to turtle populations is an increasing reality for Ontario freshwater turtles. Human development with subsequent habitat loss may also increase communal hibernation, and then in addition to the vulnerabilities from more destruction or climate change, increased threat from predation events is added to the mix. For all these reasons, continued efforts to preserve wetlands and surrounding areas are vital to the protection and longevity of freshwater turtles in Ontario.

Written by Andrea O'Halloran, edited by Leora Berman and Kiara Duval

Sources:

• Bodie, J. R., & Semlitsch, R. D. (2000). Spatial and temporal use of floodplain habitats by lentic and lotic species of aquatic turtles. Oecologia, 122(1), 138-146.
• Brown, G. P., & Brooks, R. J. (1994). Characteristics of and fidelity to hibernacula in a northern population of snapping turtles, Chelydra serpentina. Copeia, 1994(1), 222-226.
• Buchanan, S. W. (2017). The influence of altered habitat: landscape ecology of freshwater turtles in Rhode Island (Doctoral dissertation, University of Rhode Island).
• Bulté, G., Huneault, B., & Blouin‐Demers, G. (2021). Free‐ranging male northern map turtles use public information when interacting with potential mates. Ethology, 127(11), 995-1001.