Why do blue whales grow fatter in the summer?

Blue whales don't actually grow *fatter* in the summer, they grow *larger* in the summer. This is because they feed heavily during the summer months when their primary food source, krill, is most abundant.

Here's why:

* Krill abundance: Krill, tiny crustaceans, thrive in the cold, nutrient-rich waters of the summer months. This makes the summer months the peak feeding season for blue whales.

* Energy storage: Blue whales eat massive amounts of krill during the summer, storing energy as blubber. This blubber serves as insulation in the colder months and provides energy for their long migrations.

* Migration: After summer, when krill become less abundant, blue whales migrate to warmer waters for breeding. Their larger size, built up from summer feeding, gives them the energy reserves needed for this long journey.

In summary: Blue whales don't get fatter in the summer, they simply become larger due to the abundant food supply and their need to build up energy reserves for the upcoming colder months and migration.