Do Ontario Freshwater Turtles Show Site Fidelity to Overwintering Sites?

Turtles will soon be coming out of hibernation, which may prompt you to wonder where they spend all that time! It turns out, turtles return to the same site to hibernate each year, displaying something scientists call "site fidelity".

Simply put, site fidelity is the behavioural inclination to return to a site that was previously used. Animals, from birds to reptiles, employ this strategy in an attempt to restrict energy expenditure and to limit the threat of mortality (Switzer, 1993). Ontario turtles are known to exhibit site fidelity in several ways, including returning to hibernation sites, nest sites, and travel routes.

 

A hibernation site is defined as an area an individual selects to live in for the winter, where they will enter an inactive state called torpor (Palumbo et al., 2021). This area does not need to possess the same characteristics that summer habitats do because the animal does not need the same amount of resources, such as oxygen, when in torpor (Brown & Brooks, 1994). Hibernation site fidelity is the act of returning to the same area year after year to spend the winter (Refsnider et al., 2012). These sites are vital for survival when overwintering in areas where the temperature drops below freezing during the winter (Cadi et al., 2008). The win-stay/lose-switch rule denotes that a turtle will stay loyal to the same overwintering site if last season was deemed successful. However, if the site was not conducive to successful overwintering the previous year, the individual tends to change sites (Switzer, 1993).

Site selection is believed to be based on evaluation of all accessible sites for sufficient oxygen availability, protection from predators, defense against freezing temperatures (Brown & Brooks, 1994), and the consistency of site conditions (Switzer, 1993). The quality of an overwintering site is thought to be determined based on a combination of these factors and is complete when the individual selects the best available habitat (ibid.).

Consideration of factors determining hibernation sites are species-specific. For example, sufficient oxygen availability is not required for all species of turtles: some species, such as Painted Turtles, can withstand anoxic conditions for extended periods of time. Others however, require a constant supply of oxygen to maintain limited extrapulmonary (outside the lungs) respiration (breathing) function for survival. During overwintering, turtles enter a state of inactivity, meaning that minimal respiration is needed to meet survival needs. As a result, metabolism is drastically reduced. This decreased rate of activity and metabolism means turtles cannot flee from attackers, putting them at an increased risk of predation (Brown & Brooks, 1994) – therefore, hibernating in  an area with high availability of hiding places (such as stumps or fallen branches) is advantageous. To avoid freezing, individuals will typically occupy areas below the surface of the water, and/or areas with flowing water where ice formation would be unlikely (Smith & Cherry, 2016). Site fidelity is overall believed to occur when it reduces the probability of adult mortality and enhances fitness, in accordance with the win-stay/lose-switch rule (Switzer, 1993).

Site fidelity is beneficial to survival as it increases familiarity with an area. By selecting the same overwintering area each year, an individual is able to gain an understanding of the environment and  decrease the cost of movement between sites. However, if the site conditions change unexpectedly due to human activity or natural disaster, this could create problems for a turtle and ultimately may force them to switch sites. Unfortunately, there is a lot of uncertainty when it comes to abandoning a typical overwintering site. The turtle loses the familiarity of the old site and must now invest additional energy in discovering a new habitat through movement. The turtle further suffers a decreased fitness due to increased potential for predation (ibid.). 

Mating opportunities are also thought to influence site selection. Some scientists believe that individuals will return to overwintering habitats with other turtles to increase mating opportunities. This essentially “kills two birds with one stone” when referring to energy use (Newton & Herman, 2009). Late season matings are believed to be advantageous because female turtles are able to store sperm in the fall for use during egg development in the spring. Interestingly, this interaction creates an opportunity for sperm competition (the process whereby the sperm of multiple males compete to try to inseminate a female) (Bulté et al., 2021) and the potential for multiple paternity (more than one father) in a single clutch of eggs (Refsnider, 2009). This is advantageous to the population as it will likely increase genetic diversity within the population. Overall this could feasibly give turtles an edge when adapting to and surviving in the ever changing environmental conditions associated with climate change.

In a hibernation site fidelity study conducted on a population of Snapping Turtles, Brown and Brooks (1994) found that 13 of the 18 studied individuals displayed fidelity to previous overwintering sites. The researchers believe that some of the turtles that did not show site fidelity may have experienced a change in their home range, making the return to the expected overwintering site extremely difficult. Additionally, the studied population displayed congregation and fidelity to sites, suggesting that site selection is not random. In a study on Northern Spotted Turtles, scientists observed many individuals displaying site fidelity for two winters in a row. However, their findings suggest that the remaining turtles either lacked fidelity entirely, or showed some disparity in fidelity between the years of this study (Litzgus et al., 1999).

From these experiments, scientists agree that hibernation site fidelity can be observed in nature. However, the results do not indicate that all turtles reliably display this behaviour. The findings, thus far, demonstrate that individuals may forgo this behaviour to avoid mortality in non ideal conditions, or simply do not use this kind of fidelity. Therefore, there is still some work to be done to confirm whether or not this phenomena occurs consistently in nature. Should this phenomena be confirmed, it would be beneficial to identify critical turtle overwintering sites and implement protective measures to conserve the integrity of these areas. Alterations to the landscape may affect the hydrology of the area and have devastating implications to population viability during winter conditions (Litzgus et al., 1999).