Supercooling tolerance in the Mexican lizard Barisia imbricata (Squamata: Anguidae)

Environmental temperature impacts the physiological processes of reptiles, determines their hours of activity per day, and may constrain their ability to meet critical ecological requirements. When environmental temperatures reach freezing, a few lizard species exhibit two mechanisms (supercooling and freezing tolerance) to survive freezing, and these two processes depend on cryoprotective molecules, such as glucose. Organisms produce high glucose concentrations to reach lower than normal crystallisation points, and this blood glucose concentration can double after freezing. The viviparous lizard Barisia imbricata lives along a wide elevational gradient (2100-4000 m) at tropical latitudes in temperate and subtropical climates. Populations at extremely high elevations experience environmental temperatures at or below 0 °C. We measured blood glucose concentrations in the lizard B. imbricata in different seasons and compared the values between seasons and between two populations occurring at the elevations of 2200 and 3700 m. In addition, we froze lizards from the two populations and measured their blood glucose concentrations before and after freezing. We did not observe any differences in blood glucose concentrations between different seasons or the two populations. In addition, all lizards survived freezing; their mean crystallisation point was -4.13 °C. Blood glucose concentration in the lizards increased after exposure to freezing temperatures during autumn and winter. Our results indicate that B. imbricata tolerates experimental freezing even in individuals not naturally exposed to subzero temperatures (i.e. populations at 2200 m). Elevated blood glucose concentrations (present year-round) may help B. imbricata individuals survive at low temperatures.