This week we are focusing on unique foraging strategies. Animals can be generalists, opportunists, or specialized in what they eat, and there are a lot of different ways marine mammals capture and consume their prey. Here, we pick a few interesting marine mammals that are rather unique in what the eat and/or how they go about capture it. So let’s dive into the deep with the sperm whale, filter out some krill with the crabeater seal (spoiler: they don’t eat crab!), and skim the ocean floor with the gray whale.
Sperm Whale (Physeter macrocephalus)
Sperm whales are the stuff of legend. The largest of the toothed whales, their huge heads filled with spermaceti oil were the covered prize of whalers during the 18 and 1900s, their massive presence inspiring classics such as Moby Dick. Their large heads, which make up 1/3 of their body size, inspired their Latin name, which literally means “unusually large head”. The spermaceti oil is a waxy substance that was used for lamp oil, lubricants, and candles. Whaling greatly reduced the global population of sperm whales, however now that whaling of sperm whales has been banned their populations are slowly recovering. They are still listed as Endangered under the Endangered Species Act (NOAA).
Found in all deep oceans around the world, sperm whales have one of the widest global distributions of any marine mammal. They spend more than 70% of their time foraging in these deep waters, diving to depths of 400-1200 meters with dives lasting between 30-50 minutes (45 minutes on average). Sperm whales are highly adapted to foraging in deep, cold waters, and from studies of stomach contents their prey consists of cephalopods (usually squid), sharks, skates and fish, with their preferred prey thought to be giant squid. Many of the scars on the heads of older sperm whales are thought to come from the suction cups of potential prey!
They have large conical teeth that are perfect for biting and grabbing prey, and like many predators have a large brain size – the largest brain of any animal on the planet, in fact! Sperm whales consume over 900 kg (~2,000lbs) of food every day, and while they are typically solitary hunters they have been known to work together to corral fish into bait-balls.
Sperm whales find their prey using echolocation, just like bats or beaked whales (see previous podcast!) that also forage in the dark or near-dark. In fact, sperm whale sonar is the loudest in the entire animal kingdom. Most of what we know about their foraging behavior comes from tags, sonar, and other technology that allows us to hear their clicks and model their movements (D-tags) at depth. Because they dive so deep, we can’t follow them to observe them hunting (nor could we see them do it even if we could dive that deep!).
Crabeater Seals (Lobodon carcinophaga)
The crabeater seal is found in the waters surrounding Antarctica, commonly seen hauled out on ice flows. This seal is the most abundant seal in the world, with surveys estimated at the very least at 7 million seals. However, with Antarctica being very remote and treacherous environment, some believe this is an underestimate leading to some guesses estimating to above 50-75 million individuals. This booming population is thought to be due to their diet – which is not crabs! Their Latin name Lobodon carcinophaga translates to “lobe toothed crab eater,” leading to their common name of “crabeater seal,” yet crab is hardly part of their diet. Instead, the diet of the misleading name focuses on krill, using specialized teeth to feast on the small crustaceans. With a vast abundance of available krill in the South Pole, the crabeater seal population is currently very healthy. It is hypothesized that due to humans killing a vast number of whales that eat krill, the seals have less competition for this food source and therefor growing in number.
As 90% of their diet being krill, the crabeater seal’s teeth are not serrated for shredding like other seals, rather they are shaped in a way that can filter out water while trapping krill in their mouth. These seals’ teeth are oriented in a way to interlock, with each tooth finely divided with multiple cusps or points, acting as a comb or a strainer to allow water to escape while trapping the krill to eat (similar to the function of the baleen in large whales). The seals typically will spend short 5-minute dives and swim through congregations of krill with their mouths open, catching them inside and then filtering out the water to leave only the krill behind.
Gray Whales (Eschrichtius robustus)
Most large baleen whales feed on schooling fish in the water column, gulping water and food in a big mouth full and then forcing the water out through their baleen where the food gets stuck – just like straining your macaroni at home! But gray whales found a slightly different niche. They actually feed on benthic (which means the sea floor) and epibenthic (“epi” means on top of, so above the sea floor) organisms. They are like larger whales in that they feed primarily on small invertebrates (animals without a backbone) such as amphipods and isopods (crusctaceans), polychaete worms, mollusks and others. But instead of gulping a mouth full of water, they suck up sediment (aka sand/mud) and food from the sea floor by rolling on their side (usually their right). They often do this in shallow water, so you can see their flippers in the air sometimes! They have 130-180 short, coarse baleen plates, and as they slowly swimming along they push the water and sediment out, and the food gets stuck. They have 2-5 grooves on their throat that can stretch to allow more food to be taken in, but they don’t expand as much as their rorqual whale cousins that have many more groves. You can look up videos of them feeding and see the plumes of mud pouring out the sides of their mouth. Would you like some mud with your dinner?
Some whales are breaking away from the norm as well by turning into the Salish Sea for a few months in the middle of their migration from winters in Baja California (where they give birth) to the Bering and Chukchi seas (where they feed) in the summers, one of the longest migrations of any animal. These North Puget Sound gray whales (aka “sounders”, Puget Sound Regulars or the Saratoga Grays) represent about a dozen whales that have been re-sighted year after year and come to eat ghost shrimp in the area before they reach their regular summer feeding grounds farther north. The coolest thing about this? Think about a whale up to 45 tons and 50 feet long digging in the mud – how big do you think those trenches will be? Well, you can actually see these feeding pits from space!! A local research group, Cascadia Research Collective has used images from Google Earth to look at when and where the North Puget Sound gray whales are feeding – check it out here: https://www.cascadiaresearch.org/projects/north-puget-sound-gray-whale-photo-id-and-feeding-study.
Another fun fact is that recent genetic work has shown that in the past, gray whales utilized a wider variety of food resources, and that some whales now are doing that as well – eating herring and krill like other baleen whales (https://insider.si.edu/2011/07/a-varied-diet-has-helped-gray-whales-survive-for-millions-of-years-study-reveals/). Being more of a generalist helps when things change and food disappears, which may have helped this species survive in the past, and continue on into the future. Lastly, their feeding habits can help bottom-feeding birds! Surf scoters in Puget Sound may associate with feeding gray whales. When the whales disturb the sediment, there is easier food to grab for the birds, kind of like egrets that hang around horses and cattle – as the grass gets pulled up, so do the bugs they like to eat – easier pickings! Now the only down side to this benthic feeding habit is that they may be more prone to ingesting trash, as it settles on the sea floor – just another reason to help keep the oceans clean and the trash out!