Why are there fewer venomous animals in colder climates?

This week's question comes to us from Paul Colbourne, in St.John's, Nfld. He asks:

I'm just wondering why there aren't as many venomous creatures in North America, particularly Canada, than there are in places like Australia, South America or Central America?

Bethany Nordstrom graduated with a Masters in biology from Dalhousie University in Halifax where she focused on the predator-prey dynamics of leatherback sea turtles and jellyfish. Nordstrom says most venomous animals are ectotherms or cold-blooded animals such as snakes, spiders and jellyfish. Since they're unable to regulate their own temperature, fewer are found in cooler climates. 

Freeze avoidance strategy  

It is also thought that producing venom or poison is quite energy intensive, particularly for ectotherms living in colder climates. Instead, many of these animals have evolved some sort of freeze-avoidance or freeze-tolerance strategies. The wood frog is one Canadian example. The wood frog converts liver glycogen to glucose, which acts as an antifreeze), and it is possible they can't energetically afford to produce and use venom, Nordstrom said.

Canada is home to four species of venomous snakes: the Northern Pacific rattlesnake, Massasauga rattlesnake, Desert Nightsnake and the Prairie rattlesnake. The range of these snakes is restricted to southern Canada, and they would avoid the cold seasons by hibernating below the frost line.

Nordstrom added it's also important to consider evolutionary history and history in general. Earth is not static, and species and groups of animals move over time given changes in climate, geography and other factors.