Evolutionary Convergence in Mammalian Bio-sonar
Source: reprinted from materials provided by the Acoustical Society of America
Though they evolved separately over millions of years in different worlds of darkness, bats and toothed whales use surprisingly similar acoustic behavior to locate, track, and capture prey using echolocation, the biological equivalent of sonar. Now a team of Danish researchers has shown that the acoustic behavior of these two types of animals while hunting is eerily similar. The findings were made possible by a new type of whale tag that allows scientists, for the first time, to track whales’ foraging behavior in the wild (Figure 1). The researchers present their results at the Acoustics 2012 meeting in Hong Kong, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.
Figure 1. A dolphin in the Mediterranean (123rf stock photos)
Bats and toothed whales (which include dolphins and porpoises) had many opportunities to evolve echolocation techniques that differ from each other, since their nearest common ancestor was incapable of echolocation. Nevertheless – as scientists have known for years – bats and toothed whales rely on the same range of ultrasonic frequencies, between 15 to 200 kilohertz, to hunt their prey. (For comparison, the human hearing range is between 20 hertz to 20 kilohertz.) This overlap in frequencies is surprising because sound travels about five times faster in water than in air, giving toothed whales an order of magnitude more time than bats to make a choice about whether to intercept a potential meal.
Now, thanks to new technology that records what a whale hears as well as how it moves in the wild, Peter Teglberg Madsen of Aarhus University in Denmark and Annemarie Surlykke of the University of Southern Denmark have uncovered more similarities in the animals’ acoustic tactics.
Bats increase the number of calls per second (what researchers call a “buzz rate”) while in pursuit of prey (Figure 2). Whales were thought to maintain a steady rate of calls or clicks no matter how far they were from a target. But the new research shows that wild whales also increase their rate of calls or clicks during a kill – and that whales’ buzz rates are nearly identical to that of bats, at about 500 calls or clicks per second.
Figure 2. A rhinolophid bat from the Southern Caucasus (123rf Stock Photo)
“On a purely physical basis, you would predict that whales and bats would operate at different [echolocation] rates and frequencies,” Madsen says. “But instead, they operate at the same rates and frequencies.” The similarities support the idea that the acoustic behavior of bats and whales may be defined by the auditory processing limitations of the mammalian brain.
Until now, Madsen continues, “it was not known how [a whale] would coordinate its acoustic behavior” in the wild to intercept its prey.
To track whales’ hunting behavior in the wild, researchers relied on a new device called the DTAG, which was developed by electrical engineer Mark Johnson at the Woods Hole Oceanographic Institution in Woods Hole, Mass. The DTAG attaches to a whale’s skin via suction cup and records ultrasonic frequencies (allowing scientists to analyze what a whale hears) as well as inertia and pressure readings (which allow scientists to reconstruct a whale’s movements in the water in three dimensions).
By making it possible for scientists to track whales’ foraging behavior in more detail, the new tags will also help conservationists to assess environmental impacts on whales’ behavior, Madsen says.
Though they evolved separately over millions of years in different worlds of darkness, bats and toothed whales use surprisingly similar acoustic behavior to locate, track, and capture prey using echolocation, the biological equivalent of sonar. Now a team of Danish researchers has shown that the acoustic behavior of these two types of animals while hunting is eerily similar. The findings were made possible by a new type of whale tag that allows scientists, for the first time, to track whales’ foraging behavior in the wild (Figure 1). The researchers present their results at the Acoustics 2012 meeting in Hong Kong, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.
Figure 1. A dolphin in the Mediterranean (123rf stock photos)
Bats and toothed whales (which include dolphins and porpoises) had many opportunities to evolve echolocation techniques that differ from each other, since their nearest common ancestor was incapable of echolocation. Nevertheless – as scientists have known for years – bats and toothed whales rely on the same range of ultrasonic frequencies, between 15 to 200 kilohertz, to hunt their prey. (For comparison, the human hearing range is between 20 hertz to 20 kilohertz.) This overlap in frequencies is surprising because sound travels about five times faster in water than in air, giving toothed whales an order of magnitude more time than bats to make a choice about whether to intercept a potential meal.
Now, thanks to new technology that records what a whale hears as well as how it moves in the wild, Peter Teglberg Madsen of Aarhus University in Denmark and Annemarie Surlykke of the University of Southern Denmark have uncovered more similarities in the animals’ acoustic tactics.
Bats increase the number of calls per second (what researchers call a “buzz rate”) while in pursuit of prey (Figure 2). Whales were thought to maintain a steady rate of calls or clicks no matter how far they were from a target. But the new research shows that wild whales also increase their rate of calls or clicks during a kill – and that whales’ buzz rates are nearly identical to that of bats, at about 500 calls or clicks per second.
Figure 2. A rhinolophid bat from the Southern Caucasus (123rf Stock Photo)
“On a purely physical basis, you would predict that whales and bats would operate at different [echolocation] rates and frequencies,” Madsen says. “But instead, they operate at the same rates and frequencies.” The similarities support the idea that the acoustic behavior of bats and whales may be defined by the auditory processing limitations of the mammalian brain.
Until now, Madsen continues, “it was not known how [a whale] would coordinate its acoustic behavior” in the wild to intercept its prey.
To track whales’ hunting behavior in the wild, researchers relied on a new device called the DTAG, which was developed by electrical engineer Mark Johnson at the Woods Hole Oceanographic Institution in Woods Hole, Mass. The DTAG attaches to a whale’s skin via suction cup and records ultrasonic frequencies (allowing scientists to analyze what a whale hears) as well as inertia and pressure readings (which allow scientists to reconstruct a whale’s movements in the water in three dimensions).
By making it possible for scientists to track whales’ foraging behavior in more detail, the new tags will also help conservationists to assess environmental impacts on whales’ behavior, Madsen says.
Warm Spring Weather Favors Female Newborns in Bats
Source: from materials provided by University of Calgary.
There must be something in the warm breeze. A study on big brown bats (Figure 1) suggests that bats produce twice as many female babies as male ones in years when spring comes early.
Figure 1. A hibernating big brown bat (Eptesicus fuscus). (From USFWS headquarters)
The earlier in the spring the births occur, the more likely the females are to survive and then reproduce a year later, as one-year olds, compared to later-born pups, according to Robert Barclay’s research published in PLoS ONE (Figure 2).
Figure 2. Seasonal variation in Eptesicus fuscus offspring sex ratio with birth date at three colonies in Medicine Hat, Alberta, Canada from 1990 to 2004. (From Barclay 2012)
“The early-born females are able to reproduce as one year olds, whereas male pups can't,” explains Barclay, professor in the Department of Biological Sciences.
“Thus, natural selection has favored internal mechanisms that result in a skewed sex ratio because mothers that produce a daughter leave more offspring in the next generation than mothers who produce a son.”
The length of the growing season has an impact on the ratio of female to male offspring and the time available for female pups to reach sexual maturity, the study found. This suggests that not only does sex-ratio vary seasonally and among years, but it also likely varies geographically due to differences in season length.
Barclay analyzed long-term data on the variation in offspring sex-ratio of the big brown bat, Eptesicus fuscus, a common North-American species that consumes insects.
“In this species, more eggs are fertilized than eventually result in babies, so there is some mechanism by which a female embryo is preferentially kept and male embryos are resorbed early in pregnancy,” says Barclay. But, he adds, the biochemistry behind the skewed sex ratio is unknown.
“Some other mammals and some birds have the ability to adjust the sex ratio of their offspring,” says Barclay. “Even human-baby ratios vary—there is a study showing that billionaires produce more sons than daughters, for example.”
This is the first long-term study on sex ratios in bats, says Barclay and it “suggests some pretty interesting physiology.”
References
Barclay, R. (2012). Variable Variation: Annual and Seasonal Changes in Offspring Sex Ratio in a Bat PLoS ONE, 7 (5) DOI: 10.1371/journal.pone.0036344
Read More...
There must be something in the warm breeze. A study on big brown bats (Figure 1) suggests that bats produce twice as many female babies as male ones in years when spring comes early.
Figure 1. A hibernating big brown bat (Eptesicus fuscus). (From USFWS headquarters)
The earlier in the spring the births occur, the more likely the females are to survive and then reproduce a year later, as one-year olds, compared to later-born pups, according to Robert Barclay’s research published in PLoS ONE (Figure 2).
Figure 2. Seasonal variation in Eptesicus fuscus offspring sex ratio with birth date at three colonies in Medicine Hat, Alberta, Canada from 1990 to 2004. (From Barclay 2012)
“The early-born females are able to reproduce as one year olds, whereas male pups can't,” explains Barclay, professor in the Department of Biological Sciences.
“Thus, natural selection has favored internal mechanisms that result in a skewed sex ratio because mothers that produce a daughter leave more offspring in the next generation than mothers who produce a son.”
The length of the growing season has an impact on the ratio of female to male offspring and the time available for female pups to reach sexual maturity, the study found. This suggests that not only does sex-ratio vary seasonally and among years, but it also likely varies geographically due to differences in season length.
Barclay analyzed long-term data on the variation in offspring sex-ratio of the big brown bat, Eptesicus fuscus, a common North-American species that consumes insects.
“In this species, more eggs are fertilized than eventually result in babies, so there is some mechanism by which a female embryo is preferentially kept and male embryos are resorbed early in pregnancy,” says Barclay. But, he adds, the biochemistry behind the skewed sex ratio is unknown.
“Some other mammals and some birds have the ability to adjust the sex ratio of their offspring,” says Barclay. “Even human-baby ratios vary—there is a study showing that billionaires produce more sons than daughters, for example.”
This is the first long-term study on sex ratios in bats, says Barclay and it “suggests some pretty interesting physiology.”
References
Barclay, R. (2012). Variable Variation: Annual and Seasonal Changes in Offspring Sex Ratio in a Bat PLoS ONE, 7 (5) DOI: 10.1371/journal.pone.0036344
Read More...
Face to Face With Primate Facial Diversity
Why do some primates have boldly colored faces while other species exhibit only a monotone color with little pattern? Facial color patterns likely serve several functions in primates, including intraspecific communication, species recognition, and possibly ecological or physiological roles as well (Figure 1). One hypothesis is that facial color patterns are used primarily for species recognition, with more subtle color variations used to assess individual identity.
Figure 1. Maximum-likelihood diagram of facial color complexity in Neotropical primates. Higher facial color complexity is indicated by reds and oranges and higher numbers. Primate species illustrated include: (1) Cacajao calvus, (2) Callicebus hoffmansi, (3) Ateles belzebuth, (4) Alouatta caraya, (5) Aotus trivirgatus, (6) Cebus nigritus, (7) Saimiri boliviensis, (8) Leontopithecus rosalia, (9) Callithrix kuhli, (10) Saguinus martinsi and (11) Saguinus imperator. (Illustrations by Stephen Nash. Figure from Santana et al., 2012)
According to the behavioral drive model, social behaviors drive the evolution of increasingly complex facial colors and fur patterns. An alternative hypothesis, the metachromism hypothesis, provides a non-adaptive explanation for primate color patterns. This hypothesis posits that primate lineages exhibit predictable sequences of color changes over time beginning with the ancestral agouti condition and progressively evolving a more uniform black or red color and ending with an unpigmented bleached color.
Sharlene Santana and her colleagues at the University of California, Los Angeles, set out to test these hypotheses using New World primates. They predicted that species living in smaller groups and in sympatry with more congener species would evolve more complex facial color patterns. In addition, they tested the metachromism hypothesis using a phylogenetic approach to trace color patterns through Neotropical primate lineages. They quantified facial color patterns using photos of adult males from a wide array of Neotropical primate species (Figure 2).
Figure 2. Primate faces (here a white-faced capuchin monkey, Cebus capucinus) were subdivided into 14 areas (b) to record hair and skin color, and hair length. These 14 areas were grouped into 5 more general regions that varied across species. (From Santana et al., 2012)
The results reveal that primate facial patterns do function in communication and species recognition. Primate species living in smaller groups and in regions with a higher number of congener species (species within the same genus) have evolved more complex patterns of facial color. There was no support for the metachromism hypothesis. In fact, ecological factors, and geographical patterns also shaped facial diversity in Neotropical primates (Figure 3). For example, primate species closer to the equator tended to have darker crowns and darker eye masks. Species living in the far western Neotropics tended to have darker noses and mouths, but lighter eye masks.
Figure 3. Geographical trends in primate facial traits. Facial parts become darker (regions highlighted in black) or hair becomes longer (region highlighted in grey) in the directions indicated by the arrows. (From Santana et al., 2012)
Perhaps darker facial regions in more tropical habitats serve to make individuals more cryptic or protect against the powerful UV radiation in these regions. The underlying causes for these patterns is likely complex and multifaceted. Nevertheless, these results “demonstrate the interaction of behavioral and ecological factors in shaping one of the most outstanding facial diversities of any mammalian lineage.”
References
Santana, S., Lynch Alfaro, J., & Alfaro, M. (2012). Adaptive evolution of facial colour patterns in Neotropical primates Proceedings of the Royal Society B: Biological Sciences, 279 (1736), 2204-2211 DOI: 10.1098/rspb.2011.2326
Figure 1. Maximum-likelihood diagram of facial color complexity in Neotropical primates. Higher facial color complexity is indicated by reds and oranges and higher numbers. Primate species illustrated include: (1) Cacajao calvus, (2) Callicebus hoffmansi, (3) Ateles belzebuth, (4) Alouatta caraya, (5) Aotus trivirgatus, (6) Cebus nigritus, (7) Saimiri boliviensis, (8) Leontopithecus rosalia, (9) Callithrix kuhli, (10) Saguinus martinsi and (11) Saguinus imperator. (Illustrations by Stephen Nash. Figure from Santana et al., 2012)
According to the behavioral drive model, social behaviors drive the evolution of increasingly complex facial colors and fur patterns. An alternative hypothesis, the metachromism hypothesis, provides a non-adaptive explanation for primate color patterns. This hypothesis posits that primate lineages exhibit predictable sequences of color changes over time beginning with the ancestral agouti condition and progressively evolving a more uniform black or red color and ending with an unpigmented bleached color.
Sharlene Santana and her colleagues at the University of California, Los Angeles, set out to test these hypotheses using New World primates. They predicted that species living in smaller groups and in sympatry with more congener species would evolve more complex facial color patterns. In addition, they tested the metachromism hypothesis using a phylogenetic approach to trace color patterns through Neotropical primate lineages. They quantified facial color patterns using photos of adult males from a wide array of Neotropical primate species (Figure 2).
Figure 2. Primate faces (here a white-faced capuchin monkey, Cebus capucinus) were subdivided into 14 areas (b) to record hair and skin color, and hair length. These 14 areas were grouped into 5 more general regions that varied across species. (From Santana et al., 2012)
The results reveal that primate facial patterns do function in communication and species recognition. Primate species living in smaller groups and in regions with a higher number of congener species (species within the same genus) have evolved more complex patterns of facial color. There was no support for the metachromism hypothesis. In fact, ecological factors, and geographical patterns also shaped facial diversity in Neotropical primates (Figure 3). For example, primate species closer to the equator tended to have darker crowns and darker eye masks. Species living in the far western Neotropics tended to have darker noses and mouths, but lighter eye masks.
Figure 3. Geographical trends in primate facial traits. Facial parts become darker (regions highlighted in black) or hair becomes longer (region highlighted in grey) in the directions indicated by the arrows. (From Santana et al., 2012)
Perhaps darker facial regions in more tropical habitats serve to make individuals more cryptic or protect against the powerful UV radiation in these regions. The underlying causes for these patterns is likely complex and multifaceted. Nevertheless, these results “demonstrate the interaction of behavioral and ecological factors in shaping one of the most outstanding facial diversities of any mammalian lineage.”
References
Santana, S., Lynch Alfaro, J., & Alfaro, M. (2012). Adaptive evolution of facial colour patterns in Neotropical primates Proceedings of the Royal Society B: Biological Sciences, 279 (1736), 2204-2211 DOI: 10.1098/rspb.2011.2326
Social Evolution in Mole-rats
Naked mole-rats (Heterocephalus glaber) have received a lot of scientific attention because they are the only mammals with a eusocial mating system. Like honey bees, naked mole-rats have colonies with a single breeding “queen,” a few breeding males, and numerous non-breeding “workers” who forage and maintain the complex burrow system. It is not surprising then, that these unusual mammals have received the lion’s share of attention from scientists.
As fascinating as naked mole-rats are, they are but one of 22 species of mole-rats. Interestingly, mole-rats exhibit a very wide range of social behavior, from the eusocial naked mole-rat to species that are completely solitary. This variation in social structure makes them an ideal group for studying the factors that influence the evolution of social behavior in mammals.
Five scientists from the Czech Republic, Germany, and Malawi (Lovy et al., 2012) studied two mole-rat species that live in different ecological habitats in the same region of Malawi. The silvery mole-rat (Heliophobius argenteocinereus, Figure 1) is a solitary species that lives in high altitude grassland habitats, whereas Whyte’s mole-rat (Fukomys whytei) is social and lives in drier, lower-altitude woodlands. The authors sought to tease out what ecological factors drive the evolution of social behavior.
Figure 1. A slivery mole-rat (Heliophobius sp). (from Chris Faulkes)
Because mole-rats are fossorial, soil quality and food availability are likely to play important roles in shaping their social systems (Figure 2). Whyte’s mole-rats in Malawi live in harsh habitats where soils are harder and food is relatively scarce. In contrast, silvery mole-rats inhabit cooler grasslands where soils are easier to burrow through and where food biomass was four times greater than in the woodlands.
Figure 2. A principal component analysis showing the relationship between food availability and soil paramters for Heliophobius (NR, North Rumphi; NRa, North Rumphi alluvium; and FL, Fort Lister.) and for Fukomys (J, Jalawe). Open symbols represent burrow systems numbered from lowest to highest altitude. Solid symbols represent locality centroids. (From Lovy et al., 2012)
Although both solitary and social species of mole-rat coexist in the Nyika Plateau, Malawi, there is niche differentiation between silvery and Whyte’s mole-rats. What factors are responsible for separating the niches of these two species? The authors suggest that the solitary species could not survive in the poor, hard soils of the drier woodlands where patchy underground tubers are in short supply.
In addition, Heliophobius mole-rats, living in afromontane grasslands are subject to colder temperature and consequently have thicker fur and tolerate low temperatures better. Thus, it may be that Fukomys are ill-prepared to compete with Heliophobius at the cooler, higher-altitude grassland sites. In sum, neither food availability, nor soil density alone explain the observed niche differentiation and social structure in these mole-rats. Rather, it is likely to be a combination of thermoregulatory and competitive abilities, perhaps shaped by ecological factors, that explains the evolution of different social systems.
References
Lövy, M., Šklíba, J., Burda, H., Chitaukali, W., & Šumbera, R. (2012). Ecological characteristics in habitats of two African mole-rat species with different social systems in an area of sympatry: implications for the mole-rat social evolution Journal of Zoology, 286 (2), 145-153 DOI: 10.1111/j.1469-7998.2011.00860.x
As fascinating as naked mole-rats are, they are but one of 22 species of mole-rats. Interestingly, mole-rats exhibit a very wide range of social behavior, from the eusocial naked mole-rat to species that are completely solitary. This variation in social structure makes them an ideal group for studying the factors that influence the evolution of social behavior in mammals.
Five scientists from the Czech Republic, Germany, and Malawi (Lovy et al., 2012) studied two mole-rat species that live in different ecological habitats in the same region of Malawi. The silvery mole-rat (Heliophobius argenteocinereus, Figure 1) is a solitary species that lives in high altitude grassland habitats, whereas Whyte’s mole-rat (Fukomys whytei) is social and lives in drier, lower-altitude woodlands. The authors sought to tease out what ecological factors drive the evolution of social behavior.
Figure 1. A slivery mole-rat (Heliophobius sp). (from Chris Faulkes)
Because mole-rats are fossorial, soil quality and food availability are likely to play important roles in shaping their social systems (Figure 2). Whyte’s mole-rats in Malawi live in harsh habitats where soils are harder and food is relatively scarce. In contrast, silvery mole-rats inhabit cooler grasslands where soils are easier to burrow through and where food biomass was four times greater than in the woodlands.
Figure 2. A principal component analysis showing the relationship between food availability and soil paramters for Heliophobius (NR, North Rumphi; NRa, North Rumphi alluvium; and FL, Fort Lister.) and for Fukomys (J, Jalawe). Open symbols represent burrow systems numbered from lowest to highest altitude. Solid symbols represent locality centroids. (From Lovy et al., 2012)
Although both solitary and social species of mole-rat coexist in the Nyika Plateau, Malawi, there is niche differentiation between silvery and Whyte’s mole-rats. What factors are responsible for separating the niches of these two species? The authors suggest that the solitary species could not survive in the poor, hard soils of the drier woodlands where patchy underground tubers are in short supply.
In addition, Heliophobius mole-rats, living in afromontane grasslands are subject to colder temperature and consequently have thicker fur and tolerate low temperatures better. Thus, it may be that Fukomys are ill-prepared to compete with Heliophobius at the cooler, higher-altitude grassland sites. In sum, neither food availability, nor soil density alone explain the observed niche differentiation and social structure in these mole-rats. Rather, it is likely to be a combination of thermoregulatory and competitive abilities, perhaps shaped by ecological factors, that explains the evolution of different social systems.
References
Lövy, M., Šklíba, J., Burda, H., Chitaukali, W., & Šumbera, R. (2012). Ecological characteristics in habitats of two African mole-rat species with different social systems in an area of sympatry: implications for the mole-rat social evolution Journal of Zoology, 286 (2), 145-153 DOI: 10.1111/j.1469-7998.2011.00860.x
Meet and Greet in the Dolphin World
By contributing writer Sarah Buckleitner
Many people believe that an important difference between humans and other animals is language--that what has brought us from fields and forests to our comfortable homes is our ability to communicate effectively with one another. And so the discovery that humans are not alone in their power of conversation is one that affects our definitions of our selves, and one opens many research opportunities for scientists.
Research into the calls made by bottlenose dolphins (Tursiops truncatus) at sea indicates that as dolphin groups encounter one another they exchange unique whistles; researchers claim that the whistles carried information about their identity and alliances with other individuals. In captivity, dolphins and parrots can learn to use signals to convey information about their surroundings, but there isn't much research about whether or not they use these techniques in the wild.
Figure 1. A pair of bottlenose dolphins (Tursiops truncatus). (From Ed Clayton/Flickr)
Every bottlenose dolphin has a unique whistle that they develop at a young age and practice while alone. After a call has been established, males and females differ in their use of signature whistles: females' calls tend to stay stable for about a decade, while males' whistles change to reflect alliances with other individuals. They also have been known to copy the unique calls of their companions--similar to a human calling a friend's name to attract his attention.
By using passive acoustic localization while following pods in Saint Andrews Bay, Scotland the researchers found that unique whistle exchanges mainly occurred when groups first encountered each other (Figure 2). They ensured that the whistle exchanges were unique by running a sequence analysis, and also noted that none of the calls were repeated.
Figure 2. Spectrograms of repeated whistle sequences (Letters indicate different whistle types). (From Quick and Janik 2012)
Data was collected over the course of six months, and scientists used focal boats to follow pods of dolphins during good weather. Individuals were identified with photos of their dorsal fins, and groups were defined by the distance between dolphins (less than ten meters apart equaled a group). Then using both visual data indicating the position of each dolphin and auditory data, they determined which individual had used which call and when.
By analyzing this data, researchers affirmed that dolphins exchange signature whistles when meeting at sea, and believe that these calls are meant to convey information about their identity (Figure 3). They also found that only one dolphin from each group uses its signature whistle before joining with another group, which could have various explanations:
Figure 3. A histogram of all whistle exchanges and joining events for dolphin groups. (From Quick and Janik 2012)
While this study shed new light on the way dolphins use sound to communicate, it also opened up many more questions about how these organisms use signature whistles to interact, and what that means in terms of how humans define themselves.
References
Quick, N., & Janik, V. (2012). Bottlenose dolphins exchange signature whistles when meeting at sea Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.2537
Many people believe that an important difference between humans and other animals is language--that what has brought us from fields and forests to our comfortable homes is our ability to communicate effectively with one another. And so the discovery that humans are not alone in their power of conversation is one that affects our definitions of our selves, and one opens many research opportunities for scientists.
Research into the calls made by bottlenose dolphins (Tursiops truncatus) at sea indicates that as dolphin groups encounter one another they exchange unique whistles; researchers claim that the whistles carried information about their identity and alliances with other individuals. In captivity, dolphins and parrots can learn to use signals to convey information about their surroundings, but there isn't much research about whether or not they use these techniques in the wild.
Figure 1. A pair of bottlenose dolphins (Tursiops truncatus). (From Ed Clayton/Flickr)
Every bottlenose dolphin has a unique whistle that they develop at a young age and practice while alone. After a call has been established, males and females differ in their use of signature whistles: females' calls tend to stay stable for about a decade, while males' whistles change to reflect alliances with other individuals. They also have been known to copy the unique calls of their companions--similar to a human calling a friend's name to attract his attention.
By using passive acoustic localization while following pods in Saint Andrews Bay, Scotland the researchers found that unique whistle exchanges mainly occurred when groups first encountered each other (Figure 2). They ensured that the whistle exchanges were unique by running a sequence analysis, and also noted that none of the calls were repeated.
Figure 2. Spectrograms of repeated whistle sequences (Letters indicate different whistle types). (From Quick and Janik 2012)
Data was collected over the course of six months, and scientists used focal boats to follow pods of dolphins during good weather. Individuals were identified with photos of their dorsal fins, and groups were defined by the distance between dolphins (less than ten meters apart equaled a group). Then using both visual data indicating the position of each dolphin and auditory data, they determined which individual had used which call and when.
By analyzing this data, researchers affirmed that dolphins exchange signature whistles when meeting at sea, and believe that these calls are meant to convey information about their identity (Figure 3). They also found that only one dolphin from each group uses its signature whistle before joining with another group, which could have various explanations:
Figure 3. A histogram of all whistle exchanges and joining events for dolphin groups. (From Quick and Janik 2012)
While this study shed new light on the way dolphins use sound to communicate, it also opened up many more questions about how these organisms use signature whistles to interact, and what that means in terms of how humans define themselves.
References
Quick, N., & Janik, V. (2012). Bottlenose dolphins exchange signature whistles when meeting at sea Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.2537