You Are What You Eat: Insights Into The Sperm Whale Habitat, Based On Stomach Contents
You are what you eat. And if you like to eat beaked squid and cephalopods, then chances are that you may be one of two very rare sperm whale species foraging off the southeast US coast. In a research spanning over 13 years, marine ecologist Michelle Staudinger and her team from University of North Carolina Wilmington (UNC) analyzed the stomach contents of dwarf and pygmy sperm whales to understand more about their feeding ecologies. Their research appears in the April issue of Marine Mammal Science, according to a press release Friday.
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This research is the largest of its type conducted to date in the US. Understanding feeding ecologies of rare species is an important step towards their conservation as climate change, pollution, and human activities like fishing alters their natural environment and food sources.
"Understanding what resources support populations of these incredibly rare animals is important to conservation," Staudinger, adjunct assistant professor in environmental conservation at the University of Massachusetts Amherst, says of the pygmy and dwarf sperm whales she studied. "If there are changes in the environment or their prey, we can now hope to better anticipate the potential impacts. There had been quite a knowledge gap about these animals, but this work gives us an idea of their ecological niche and requirements in the current environment."
Dwarf and pygmy sperm whales belong to the Kogia genus and along with the sperm whale make the superfamily Physeteroidea. The researchers specifically wanted to know the difference between feeding ecologies of dwarf and pygmy sperm whales that till recently were considered to be the same species. For their research, the team analyzed stomach contents collected by the marine mammal stranding network from 22 pygmy and nine dwarf sperm whales found dead on the mid-Atlantic coast between 1998 and 2011.These whales feed exclusively on beaked squid and cephalopods and are able to digest all their prey except for the hard beaks that are made of chitin.
Staudinger explains, "All deceased stranded marine mammals are necropsied, and scientists save and evaluate the stomach contents. So the stranding network had a stockpile of stomachs collected over 13 years from two of the most commonly stranded whales along the southeast and mid-Atlantic coast." Staudinger also has a special talent. She says, "Here I have to confess that I have a kind of unusual ability I learned in earlier research: I can identify cephalopod species by their beaks, a characteristic similar to birds. So when I heard about this study, I jumped at the chance to study these whales."
For the cephalopod species that she couldn't identify, Staudinger consulted the archives in the Smithsonian Institution's collection, where there are references to specimens collected by whalers and fishermen dating back to the 1800s. Analysis of the cephalopod beak remains from the whale guts reveals that pygmy sperm whales have a more diverse diet than their dwarf sperm whale cousins. Pygmy whales also prefer larger prey than dwarfs, but this has not been proven statistically.
The team also performed a second analysis to characterize whale foraging ecology. In a first study of its kind they evaluated ratios of carbon and nitrogen isotopes in whale muscle samples, an indicator providing information on which habitats the whales were feeding in. Sea-water can be divided into different pelagic eco-zones where water conditions change according to depth and analyzing the isotope ratio gave the scientists an idea of whether the whales were feeding in mesopelagic or bathypelagic zones. They also determined if the whales were feeding on prey higher up in the food chain such as fish or small crustaceans. Staudinger says, "As far as we know this the first time the isotopic signatures have been published for dwarf sperm whales."
The results did not show much difference in the foraging parameters of the two whales, even though they are genetically distinct. "We found the ecologic niche of the two species is very similar in U.S. Atlantic waters, which is consistent with other global studies," Staudinger summarizes. "The pygmy sperm whale consumes a greater diversity and size of prey, which means they may be diving deeper than dwarf sperm whales to feed, this makes sense because pygmy sperm whales grow to larger sizes than dwarf sperm whales, however, this could also be an artifact of small sample sizes."
This is important information for conservationists, since it means the whales are not encroaching on each other's resources and there likely is enough food to support both their populations in the region. If whale habitats do show food shortages in the future, pygmy whales may have an edge over sperm whales, since they are more diverse in their feeding habits and areas. Staudinger next plans to research on deep-sea squid ecology and cephalopod biodiversity on the Bear Seamount in the North Atlantic with the Smithsonian Institution, and the National Marine Fisheries Service National Systematics Laboratory.
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