Under Pressure, Orcas Adopt Killer Survival Strategy
Bio major wins Class of 21 Award for work on killer whales.
In the wine-dark waters of the San Juan Islands, a band of killer whales is fighting for survival.
Loss of habitat, human meddling, and intense competition for chinook salmon, its main source of food, have put severe pressure on the whales. This band, known as the Southern Residents, is now smaller than any other group of resident killer whales, which live in communities scattered along the cold coastal waters of the North Pacific.
But the Southern Residents aren’t giving up without a struggle, according to bio major Michael Weiss ’16. On the contrary, they have responded by rewiring their complex social structure to hunt for chinook more aggressively by working together.
“This is a new dimension of killer whale society that people hadn’t known about before,” says Michael, who won the prestigious Class of ’21 Award for his senior thesis on the social organization of the Southern Residents. “Outside humans, we don’t often see groups of animals co-operating like this.”
Scientists believe that the Southern Residents have roamed this area, known as the Salish Sea, for several hundreds of thousands of years, maintaining a genetically distinct population. In the late sixties and early seventies, however, the band was , whose agents captured (and sometimes accidentally killed) as many as 58 whales. In addition, dams and agriculture have depleted runs of chinook salmon, their primary source of food, pushing the band perilously close to extinction.
That the whales have held out so long may be due to their intricate social organization, one of the most sophisticated in the animal world. Their fundamental social unit is the matriline, which revolves around a grandmother and her offspring; it is the cetacean equivalent of the nuclear family. Members of the matriline spend most of their lives in each other’s company, communicating through a complex system of squeaks, clicks, and groans that include at least 25 discrete calls. Together they travel, forage, sleep, and—crucially—hunt.
Matrilines roam their territory in larger kinship groups known as pods. Each pod communicates through a distinct acoustical “dialect” which help members identify each other as belonging to the same pod and reinforces their clannish tendencies. The three pods that comprise the Southern Residents, known as J, K, and L, that sound radically different.
Until now, scientists believed that almost all the whales’ social interaction, with the exception of mating, took place within their pod. But Michael has discovered that the Southern Residents are breaking down this barrier and cooperating across pods in order to overcome the logistical challenges posed by their scanty population.
When they’re hunting for salmon, matrilines cooperate in a remarkable division of labor. The grandmothers lead the group to territory where fish are likely to be found—they are responsible for locating the prey. But it is the adult males who are responsible for diving deep into the water and flushing the fish to the surface, so that the whole family may feast. Matrilines that include both grandmothers and adult males can therefore assemble the most efficient hunting parties. Unfortunately, several matrilines among the Southern Residents have lost one or more of these key members, which limits their ability to hunt.
Michael spent most of last summer recording the whales with a hydrophone (an underwater microphone) on board a 17-foot speedboat. Once ashore, he analyzed the calls on a spectrogram and then applied a statistical concept known as social network analysis to search for patterns. After intensive study, he realized that understaffed matrilines were co-operating with each other to fill the gaps in their hunting parties. A matriline that lacks an adult male will “borrow” one from another matriline, even if it means crossing pods.
“It’s like sharing pieces of a jigsaw puzzle so that both groups can succeed,” says Michael. “They fill each other’s missing social roles.”
Many animals cooperate while hunting, but almost always in family groups. This degree of cooperation outside the family, known as mutualism, is virtually unknown in the animal kingdom—except, of course, among humans.
“I don’t think killer whales are the only animals that have this multidimensional social structure,” Michael says. “But they’re just the first one we’ve found.”
Michael’s professors say his thesis breaks new ground in our understanding of altruism and cooperation among animals. “Michael’s thesis should be of great interest to marine mammal researchers around the world who study the complex social interactions of these animals,” says his advisor, Prof. Suzy Renn [bio]. “They are also likely to be useful for conservation efforts for these whales and similar populations. Beyond the research world of marine mammals, the tendency to form non-kinship bonds while maintaining close association and information transfer with kin is a cornerstone of complex societies such as humans, and is extremely rare in other species.”
“The type and quality of research that resulted from his dedication is something one would expected from a seasoned field biologist with a Master’s or PhD,” says Prof. Derek Applewhite [bio]. “Michael drew heavily from statistics, computational biology, and animal behavior, successfully integrating these fields into an outstanding senior thesis.”
In fact, Michael worked with a new type of statistical technique, the exponential random graph model, which had never been used to study animal networks before. Fortunately, Prof. Andrew Bray [math] happens to be a former oceanographer who used his insight into the underwater world to figure out how best to apply this statistical tool.
The Class of ’21 Award is bestowed upon a couple of seniors whose theses demonstrate "creative work of notable character, involving an unusual degree of initiative and spontaneity." Religion major also won the award.
Michael’s fascination with killer whales began with a trip to SeaWorld in Florida when he was six years old. He spent a couple summers as a tour guide on whalewatching boats in the San Juans and last year co-founded the non-profit with Monika Wieland ’07, who also wrote her thesis on killer whales, and who now works as a scientist and photographer.
After graduation, Michael will return to the San Juans to obtain more data on the Southern Residents. “They’re like this alien intelligence that we know nothing about,” he says. “They’re huge. They’re smart. Realizing how much we don’t know is what inspired me.”
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