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
Because its convenient.
The biggest factor for carnivores when it comes to their choice of prey is how likely they are to be injured. Even a small injury can easily become infected and debilitating. A hunter which cannot hunt will starve.
A whale could probably *take a shark in a fight*, but why? The risk is simply not worth it when you can just be an animate water filter.
The reason is that it’s easy prey to catch, filter and eat.
It’s nutritious, ever-present, harmless and easy to do.
>It just seems logically, bigger animals would eat bigger preys to meet their high energy/caloric needs.
Only if you’re making assumptions. Bigger predators need bigger prey, which is harder to catch, kill and eat. Which takes more energy, presents more risk, causes you to have to expend more energy moving and hunting.
Cows are bigger than humans but they only eat grass. By your logic that shouldn’t happen?
Grass is easy to catch, plentiful, regenerates and there isn’t much competition for it. And it doesn’t fight back. Same for krill and plankton.
>So why do whales, the largest animal in the world, eat such small animals (zooplankton)? Such a weird contrast.
The biggest land animal alive today is the elephant. What do they eat? Vegetation.
What was the biggest land animal ever? A plant-eating dinosaur.
What’s the biggest water animal ever? A plant and plankton-eating whale.
It’s not a weird contrast, it’s the norm. You’re just assuming it shouldn’t be based on flawed thinking and missing knowledge.
Lots of the largest land animals are herbivores, though, so it doesn’t necessarily follow that animals higher in the food chain would always be the bigger ones. If you’re a predator, you need to be bigger than your prey (or at least big enough to fight it and not die) but you also need to be faster than your prey, which costs more energy the bigger you get.
If you’re an herbivore (or in the ocean, a filter feeder) you don’t really need to be fast to eat since your food is very slow. So either you need to invest in speed to outrun your predators, or sheer size, so your predators will have a hard time killing you.
Anything that eats krill is going to use 90% of that energy just keeping itself alive. It only uses 10% to contribute to growing its own body. If you eat the thing that eats the krill, you’re only getting 10% of the energy you could have gotten from eating the krill directly. This concept is called “feed conversion”.
If you’re able to eat small things directly, that’s an advantage because there are lots more of them. Whales can swim through water that’s full of krill, and they’re able to filter it out and eat it. It’s the most food they can get all in one place.
Almost every single giant animal eats plants. Plants that float into your mouth for you is even easier than what elephants eat
Krill form massive swarms so while individually they are tiny if you took a whale sizes mouthful of krill and seawater there would actually be a lot of krill in there by weight.
It’s like asking why cows (a large animal) feed on tiny blades of grass. Sometimes there is so much of a small thing it can easily support large animals.
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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