4 Adaptation of living organisms to natural changes in temperature

A. Adaptation to freezing

• The wood frog lives in cold areas in the north (Alaska, Canada), where winter temperatures fall below zero. In winter the frog's body partly freezes: breathing stops, the heart stops beating, and it remains in a deep state of hibernation. Before freezing, the frog stores large amounts of glucose in its vital organs (heart, liver, brain). Glucose acts as an antifreeze: it prevents the formation of ice crystals inside the cells (which would burst them) and protects them from damage (the ice forms in the spaces between cells instead). In spring, the ice melts, the heart starts beating and vital functions restart.
• The ice fish lives in the freezing waters of Antarctica where water temperature is below zero (about −1.9 °C), lethal for most marine animals. It secretes special proteins in its blood called antifreeze proteins, which prevent the formation of ice crystals in the blood and tissues. It is one of the rare vertebrates whose blood contains no haemoglobin; it absorbs oxygen directly from the water — oxygen dissolves well in cold water and its blood plasma carries it dissolved. (Its large heart and wide blood vessels pump a large volume of the pale blood.)

B. Adaptation to high temperatures — desert lizards

• Desert lizards live in extremely hot environments where temperatures may be too high for other creatures.
• They show many adaptations: they can tolerate a high body temperature; they are active in the cooler morning and evening and shelter in burrows or shade at midday; their scaly skin reduces water loss.
• The thorny devil lizard has tiny channels on the surface of its skin that collect moisture from the air or from damp sand by capillary action and direct it to its mouth, helping it to stay hydrated in the dry desert.

(Other examples: the camel stores fat in the hump and tolerates a body-temperature range of 34–41 °C; fennec foxes have large ears to lose heat.)