Wood Frog-ID

The Wood Frog -Lithobates sylvaticus or Rana sylvatica
The North-American Wood Frog can be found over a large range stretching from northern Georgia to Alaska. These rust-coloured frogs with a distinctive dark ‘eye mask’, range from 51 to 70 mm in length, with females being generally larger than males.

Thermoregulation
Temperature affects rate of respiration, digestion and most bodily processes and as ectotherms the wood frogs are dependent on their environment as the source of this heat. They must reach the optimum temperature for these reactions in order to produce maximum energy for movement and growth. Therefore they have a number of physiological and behavioural adaptations to help them regulate their body temperature, such as going into the shade or water to cool and basking in the sun to warm up. Although typically brown, individual frogs are capable of varying their colour, which is an important defence mechanism for times when they are exposed due to low body temperature and a lack of energy so that they cannot evade predators. It also allows them to increase absorption of heat from sunlight by turning a darker colour. They are remarkable in being able to survive north of the Arctic Circle; in fact they are thought to be one of the only ectotherms to do so.

Hibernation
The feature which contributes most to their tolerance of cold is their ability to freeze solid without causing any significant tissue damage and thaw again in spring. Like many ectotherms, wood frogs hibernate during winter. However they mostly inhabit areas that reach well below freezing temperature in winter and they cannot burrow into the ground for warmth, nor find a sufficiently insulated place to prevent freezing. Instead, these frogs bury themselves relatively shallow under snow or under leaf litter and freeze for the winter months.

In other animals, ice crystals forming in tissues forces water out of cells, causing them to collapse. Once the first ice crystals form in a Wood Frog, its skin freezes, special proteins in their blood, called nucleating proteins, cause the water in the blood to freeze first. This ice reduces the water potential of the blood and draws most of the water out of the frog's cells. At the same time the frog's liver starts releasing large amounts of glucose which balances out the difference in water potential and prevents freezing in organs and cells. All remaining circulating blood is directed to vital organs to distribute the glucose but cardiac function soon ceases. The frogs then enter a state of suspended animation without any brain activity but the large glucose stores and remaining water allow it to continue anaerobic respiration and some oxygen can reach tissues provided the skin is moist. Urea also gathers in tissues before freezing to prevent osmotic shrinkage and hinder the formation of ice crystals.

Research Opportunities
This adaptation of the Wood Frog has become the focus of research recently, as doctors hope they will one day be able to copy the technique to aid human organ transplants. Currently doctors only have hours to get a donated organ into a living patient before the organ becomes too damaged. However increasing temperatures pose a threat to this species as if they never freeze or hibernate, they would have to survive the winter months when food is scarce.