Very small animals are rarely found in polar regions. Which of the following is not a reason for this? 

Compared larger animals, smaller animals have greater basal metabolic rate per unit of body weight.

Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and humming birds. Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. This is the main reason why very small animals are rarely found in polar regions.