In 1847, the German biologist Christian Bergmann noted that mammals and birds living at higher latitudes (farther from the equator) are on average larger and bulkier than related species living at lower latitudes. Suggest an evolutionary hypothesis to explain this observation.

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1. Identify the pattern observed by Bergmann, which is that mammals and birds in colder, higher latitude regions tend to be larger and bulkier than those in warmer, lower latitude regions.
2. Consider the potential advantages of larger body size in colder climates. Larger animals have a lower surface area-to-volume ratio, which reduces heat loss and helps to conserve body heat in cold environments.
3. Formulate a hypothesis based on the principle of natural selection. Suggest that individuals with larger body sizes are more likely to survive and reproduce in colder climates due to their enhanced ability to retain heat.
4. Propose that over many generations, this selective pressure for larger size could lead to an evolutionary trend where species in colder regions evolve to be larger than their relatives in warmer regions.
5. Suggest further research or observations that could test this hypothesis, such as comparing the body size of the same species or closely related species across different climatic conditions and examining their survival and reproductive success.