It just seems logically, bigger animals would eat bigger preys to meet their high energy/caloric needs. So why do whales, the largest animal in the world, eat such small animals (zooplankton)? Such a weird contrast. What is the biological reason for this?
In: Biology
There’s an inverse relationship between net metabolic rate and size, which means that caloric needs per unit of size/mass decrease as an animal gets bigger. In other words, animals don’t need to eat as much as you think as they get bigger.
Pound for pound, it takes more energy to maintain homeostasis and living functions for a smaller animal than a big one. Heat is lost much more quickly because of the higher surface area to volume ratio. Think of how quickly a small drop of hot water will cool down compared to how long it takes for a big mug of coffee to cool down.
Applying this to animals, you can see why a hummingbird constantly needs to feed on energy dense foods like nectar (or else they’ll starve and die) whereas huge animals like whales can subsist on plankton. Whales definitely need to consume literal tons of plankton as well, but the amount of food they need in relation to their body size is nowhere near as close as how much a hummingbird needs.
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