flukeprintphotography: “Why won’t they eat so…

flukeprintphotography:

“Why won’t they eat something else?”

The southern resident killer whales are in the news again, and sadly, not for a good reason. J17, also known as Princess Angeline, was photographed with a peanut head, a reduction of fat around the skull that is typically an indication of poor food intake or illness. The southern residents feed mainly on chinook salmon and there has been a large decline in salmon populations in their habitat. This has caught the attention of many people and the question “Why don’t they just eat something else?” has been posted on social media very often. And it’s a very good question! 

To understand why the southern residents are so picky, we need to start by looking at animal biology as a whole. There are two types of animals: generalists and specialists. Generalists thrive in a variety of environments and can feed on an assortment of different things depending on what is available. This strategy works well in environments that are constantly changing or where food choices are limited. Specialists, as their name implies, are adapted to live in certain habitats or feed on particular types of food. This strategy is beneficial when there is little change in the environment and when there is an abundance of multiple types of food, as specialization can reduce competition with other species. Most species do not fit neatly into either category; there is typically some overlap and it is more appropriate to view it as a continuum. The southern residents, however, are as close to true specialists as you can come. They will only eat fish and the species of fish they will eat can be counted on two hands. Chinook salmon are the most important and can make up 90% of their diet alone, depending on the season.

Many killer whales ecotypes and populations around the world exhibit prey specialization, though those in the North Pacific seem to be the most stringent in their food preferences. The southern residents’ cousins, transient killer whales, feed on marine mammals, such as seals, sea lions, porpoises, dolphins, and large whales, with the occasional seabird or cephalopod. These prey choices are largely influenced by culture. Culture is extremely prevalent in killer whales and it is likely that the choosiness in the southern residents is driven by their cultural preferences. Young southern residents are probably taught that fish are food and seals and sea lions are not. In fact, cultural preferences are so strong and influential it is actually having an impact on their physiology. For example, the teeth of resident killer whales are adapted to catching fish, not mammals. Their skulls and jaws are smaller transient killer whales and their ability to process protein likely differs from their mammal-eating cousins [1].

Culture can rapidly speed up the process of specialization. Though there are a variety of killer whale ecotypes that display radically different morphologies and prey preferences, all ecotypes share a common ancestor 250,000 years ago. In terms of evolution, that is an incredibly short amount of time. Cultural specialization, however, presents dilemma. Recent modeling studies with killer whales show culturally-driven specialization is beneficial at first as it increases the fitness of the individuals and is generally adaptive. However, over the long-term, it becomes maladaptive and can lead to extinction [2]. Could a specialized killer whale ever switch to a different source of prey?

Possibly. 

During the heyday of killer whale captures in the 60s and 70s, nobody knew that there were specialized types of killer whales that fed on different prey items. A family of mammal-eating transients, known today as the T2s, were captured in 1970 in British Columbia and were held in a sea pen, awaiting transport to the various aquariums and parks that had purchased them. Their captors, not knowing any better, only offered them fish to eat. The whales, viewing only mammals as a source of food, refused to eat the fish and began two months of self-inflicted starvation. After 75 days, an adult female died. The other two whales, now severely emaciated, were given injections of medication to boost their appetite. The male tentatively took a large salmon two days after the injections and split it in half to share with the female. The pair then began to eat large amounts of salmon and gained their weight and their health back. Though they were intended to be sent to marine parks, the whales escaped back into the wild. The sea pen was loosened and sank by an unknown member of the public and the whales managed to free themselves. They then returned to a wholly transient diet of seals, sea lions, and porpoises [3]. There are other instances of captured transients being “taught” how to eat fish by their tankmates, such as how transients Chimo and Nootka were given fish by resident male Haida at SeaLand of the Pacific. More recently, there have been similar reports involving the captured transients in Russia. Narnia was the first whale captured in the modern Russian captures and taught newly captured transients how to eat fish [4]:  

“Narnia was directly involved in the feeding of
males from the group of captured in 2013 killer
whales: immediately taking the leading position
in the formed group, at the first feeding she
actively started to offer all of them fish in turns,
persisting in pushing fish into animals’ mouth,
and periodically demonstrating to new-comers
how she used fish as feed. Nord started to eat fish,
sharing it with Narnia (parting fish by halves)
right on the second day. Orpheus began stable
eating of fish by the end of day 10 after placement
into the cage.”

We know that some transients, when faced with no other option, can switch their diet in captivity. Could southern residents do the same in their natural habitat? Because these whales are social learners, it would require an intrepid individual to branch out and feed on a marine mammal, subsequently teaching pod members how to consume a new prey item and lead them away from extinction. Southern residents will occasionally chase down and kill porpoises, but never consume them. Perhaps if one whale had the curiosity to take a bite, we could see a revolution within the southern resident killer whale population. However, it seems more likely that the cultural boundaries are too strong and prevent the whales from branching out. 

What is most important to the southern residents right now is to boost Chinook salmon populations through habitat restoration, dam removal, and hatchery production. We cannot count on the southern residents changing their deeply rooted cultural practices in time to save themselves. 

References below.

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