Why are there no snakes in New Zealand?
First off let’s clarify one point: there are snakes in New Zealand, and in Hawaii. Sea snakes are quite common on both archipeligo. There are however no land snakes. When New Zealand broke away from Australia and South America both snakes and mammals were already widespread groups of animals, so New Zealand almost certainly started its life with numerous species of snakes and mammals. However none exist there today. Why? Because over the eons New Zealand has repeatedly been reduced to just a few scattered islands when sea levels rose during warm periods, and then covered in glaciers during the ice ages. All of that disturbance made it impossible for snakes or mammals to survive. Snakes simply don’t handle cold conditions too well. Mammals OTOH have such high energy demands that they have difficulty surviving on small, cold islands. So New Zeland and Ireland both lack snakes for the same reason: snakes can’t handle ice age glaciers.
