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Narwhal - Monodon monoceros
Narwhal - Monodon monoceros

[Image: photo.jpg]

Scientific classification 
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Monodontidae
Genus: Monodon
Species: Monodon monoceros

The Narwhal (Monodon monoceros) is an Arctic species of cetacean. It is a creature rarely found south of latitude 70°N. It is one of two species of white whale in the Monodontidae family (the other is the Beluga whale). It is possibly also related to the Irrawaddy dolphin. Narwhal means "corpse whale".

The Porpoise was one of the many species originally described by Linnaeus in his Systema Naturae. The English name narwhal is derived from the Dutch name narwal, which in turn comes from the Danish narhval. This is based on the Old Norse word nár, meaning "corpse". This is a reference to the animal's color. The narwhal is also commonly known as the Moon Whale. In some parts of the world, the Narwhal is colloquially referred to as a "reamfish". In Inuit language the narwhal is named qilalugaq

The most conspicuous characteristic of male narwhal is their single extraordinarily long tusk, an incisor that projects from the left side of the upper jaw and forms a left-handed helix. The tusk can be up to three metres (nearly 10 ft) long (compared with a body length of 7–8 m [23–26 ft]) and weigh up to 10 kilograms (22 lbs). About one in 500 males has two tusks, which occurs when the right tooth, normally small, also grows out. Although rare, a female narwhal may also produce a tusk. There is a single recorded case of a female with two tusks.

The purpose of the tusk has been the subject of much debate. Early scientific theories suggested that the tusk was used to pierce the ice covering the narwhal's Arctic Sea habitat. Others suggested the tusk was used in echolocation. More recently, scientists believed the tusk is primarily used for showmanship and for dominance: males with larger tusks are more likely to successfully attract a mate. This hypothesis was suggested by the activity of "tusking", in which two males rub their tusks together.

[Image: photo.jpg]
Female narwhal with calf

However, recent work by a research team led by Martin Nweeia suggests that the tusk may in fact be a sensory organ. Electron micrographs of tusks revealed millions of tiny, deep tubules extending from the tusk's surface, apparently connecting to the narwhal's nervous system. While such tubules are present in the teeth of many species, they do not typically extend to the surface of healthy teeth. The exact sensory purpose of the tusk remains unknown, but scientists now hypothesize that it may detect temperature, salinity, pressure, and/or particulate makeup of the water in which the narwhal swims. Unlike the tusks of elephants, narwhal tusks do not regrow if they break off. If damaged, however, the tusks can repair themselves to a certain extent.

Male narwhals weigh up to 1,600 kg (3,500 lb), the female around 1,000 kg (2,200 lb). Most of the body is pale with brown speckles in color, though the neck, head and edges of the flippers and fluke are nearly black. Older animals are usually more brightly colored than younger animals.

Behaviour methods and usual diet
Narwhals are quick, active mammals which feed mainly on species of cod that reside under ice-enclosed seas.

In some areas their diet seems to have adapted to include different squid, shrimp, and various fish, such as schooling pelagic fish, halibut, and redfish. Canadian Researcher William Sommers has found that when food is scarce, narwhals will even eat baby seals. Narwhals normally congregate in groups of about five to ten. Sometimes several of these groups might come together, particularly in summer when they congregate on the same coast.

[Image: photo.jpg]
Group of narwhals swimming at surface

At times, male narwhals rub their tusks together in an activity called "tusking". Recent findings of a marine mammal researcher at the Smithsonian Institution showed that the tusk also play a role in the animal's sensory perception, with as many as 10 million tiny nerve endings reaching the surface of a tusk (which is a modified tooth). This suggests that the tusking may simply be a way of clearing encrustations from the sensory tubules, analogous to brushing teeth.

[Image: photo.jpg]
Male narwhals showing tusks above water 

Narwhals prefer to stay near the surface. During a typical deep dive the animal will descend as fast as 2 m/s for eight to ten minutes, reaching a depth of at least 1,500 m (5,000 ft), spend perhaps a couple of minutes at depth before returning to the surface.

[Image: monodon_monocerus_sm.jpg]

Population and distribution
The narwhal is found predominantly in the Atlantic and Russian areas of the Arctic. Individuals are commonly recorded in the northern part of Hudson Bay, Hudson Strait, Baffin Bay; off the east coast of Greenland; and in a strip running east from the northern end of Greenland round to eastern Russia (170°E). Land in this strip includes Svalbard, Franz Joseph Land, and Severnaya Zemlya. The northernmost sightings of narwhal have occurred north of Franz Joseph Land, at about 85°N.

The world population is currently estimated to be around 50,000 individuals. Most estimates of population have concentrated on the fjords and inlets of Northern Canada and western Greenland. Aerial surveys suggest a population of around 20,000 individuals. When submerged animals are also taken into account, the true figure may be in excess of 25,000.

Narwhals are a migratory species. In summer months they move closer to coasts. As the winter freeze begins, they move away from shore, and reside in densely-packed ice, surviving in leads and small holes in the ice. As spring comes these leads open up into channels and the narwhals return to the coastal bays.

Predation and conservation
The main predators of the narwhal are polar bears and Orcas. Inuit people are allowed to hunt this whale species legally. The northern climate provides little nutrition in the form of vitamins which can only be obtained through the consumption of seal, whale, and walrus. The livers of these animals are often eaten immediately following the killing by the hunting party in an ancient ceremony of respect for the animal. In Greenland, traditional hunting methods in whaling are used (such as harpooning), but high-speed boats and hunting rifles are frequently used in Northern Canada. PETA and other animal rights groups have long protested the killing of narwhals. Narwhals usually travel in pods of 10-100.

[Image: photo.jpg]
Inuit hunters with caught narwhal

A study published in April 2008, in the peer-reviewed journal Ecological Applications found the Narwhal to be the most potentially vulnerable to climate change when a risk analysis of other Arctic Marine Mammals was conducted. The study quantified the vulnerabilities of 11 year-round arctic sea mammals.
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  • Claudiu Constantin Nicolaescu
Narwhal's Trademark Tusk Acts Like a Sensor, Scientist Says
The Arctic whale's tusk is actually a tooth that can grow more than nine feet long; it has baffled people for centuries.

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A male narwhal surfaces in the Atlantic Ocean near Baffin Island in Canada.

Jane J. Lee
National Geographic

The arctic whale known as the narwhal is famous for the spiral tusk protruding from its head, but scientists have long battled over the horn's purpose.

On Tuesday, scientists published a study that advances a bold theory about how the whale uses its tusk. They say the horn, which is actually a tooth, is a sensory organ.

The scientists speculate that the tusk, usually found only the males, can pick up differences in the whale's environment, like the salt content of seawater, helping the marine mammals to navigate their frigid homes or perhaps find food.

But the theory is highly controversial; many marine mammal experts reject the idea that the tusk plays a central role in a narwal's ability to sense its environment, insisting that the tooth is most likely a lure to attract mates. 

"There's just zero evidence" for the possibility that a male narwhal's tusk plays a large role in whether the animal can sense things like changes in salinity or where to find food, says Kristin Laidre, a marine mammal biologist at the University of Washington in Seattle.

Martin Nweeia of the Harvard School of Dental Medicine, lead author of the new study published in The Anatomical Record, agrees that the tusk is like the brilliant feathers of a peacock—used to attract females in the mood to mate. But he says that doesn't necessarily preclude other uses.

"It's very typical for a sensory organ to have multiple functions," he says.

Tickling the Ivories

Nweeia, a practicing dentist, argues that the narwhal's tusk seems especially suited to sense the environment when one considers its anatomy. There are channels scattered throughout the tusk's external layer that allow seawater to enter the tooth.

That layer connects to another underneath—called dentin—that also contains small tubes. Those tubules run to the innermost part of the narwhal's tusk, the pulp, which is full of blood vessels and nerves. The nerves run from the base of the narwhal's tusk directly to the brain.

In other mammalian teeth—like ours, for example—there is no direct connection from outside stimuli like cold water to the brain unless there's something wrong with the tooth, Nweeia explains. (Anyone who's had a root canal can attest to the sensitivity of exposed nerves.)

But in the narwhal, this constant sensitivity to the outside world seems to be normal, he says.

Nweeia's study of live narwhals suggests their tusks can sense changes in salinity, or the salt concentration of water. Different salinities correlated with changes in a whale's heart rate.

But the University of Washington's Laidre notes that researchers collected those heart rate measurements shortly after the whales were captured in nets and brought into shallow water.

A heart rate obtained shortly after an invasive capture reflects the animal's stress, she says, not necessarily its reaction to a change in salinity.

The biologist doesn't disagree with the study's description of the tusk's anatomy. "The narwhal tusk is a tooth, and teeth are sensitive," she says.

But she says that the study authors' conclusions take things too far.

"In mammals, females are critical to helping populations grow," she says. "So there's no way that females wouldn't have a sensory organ that would help them survive or give them sort of an advantage in terms of finding food." (Watch a video to learn more about the narwhal.)

An Elusive Charge

Male narwhals are normally the only ones to sport the long tusks, which can reach up to nine feet long, though females sometimes develop small ones.

From the peacock's tail to deer antlers, it's not uncommon for male animals to advertise their availability. Charles Darwin is probably the best known scientist to suggest that use for the narwal's tusk.

But narwhals are elusive and difficult to study, Laidre says, and we may never know for sure how the whale uses its tusk.

Sensory ability in the narwhal tooth organ system

Martin T. Nweeia, Frederick C. Eichmiller, Peter V. Hauschka, Gretchen A. Donahue, Jack R. Orr, Steven H. Ferguson, Cortney A. Watt, James G. Mead, Charles W. Potter, Rune Dietz, Anthony A. Giuseppett, Sandie R. Black, Alexander J. Trachtenberg andWinston P. Kuo
Article first published online: 18 MAR 2014

The Anatomical Record Volume 297, Issue 4, page C1, April 2014

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high-salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence. Anat Rec, 297:599–617, 2014. © 2014 Wiley Periodicals, Inc.
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  • Claudiu Constantin Nicolaescu
Heart monitors on wild narwhals reveal alarming responses to stress
As sea ice melts, new findings add to concerns about the effects of ocean noise and increased human activity on deep-diving Arctic whales

Date: December 7, 2017
Source: University of California - Santa Cruz
Stress from human disturbances could cause behavioral responses in narwhals that are inconsistent with their physiological capacities, researchers say. They found that narwhals released after entanglement in nets and outfitted with heart monitors performed a series of deep dives, swimming hard to escape, while their heart rates dropped to unexpectedly low levels of three to four beats per minute.

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After their release, narwhals made a series of deep dives, swimming hard to escape, while their heart rates dropped to shockingly low levels (3 to 4 beats per minute). This put them in danger of not getting enough oxygen to the brain and other critical organs.
Credit: Photo by Terrie M. Williams

Narwhals released after entanglement in nets and outfitted with heart monitors performed a series of deep dives, swimming hard to escape, while their heart rates dropped to unexpectedly low levels of three to four beats per minute. This combination of hard exercise and low heart rate while not breathing under water is costly and could make it difficult for the deep-diving whales to get enough oxygen to the brain and other critical organs, according to a new study.

"How do you run away while holding your breath? These are deep-diving marine mammals, but we were not seeing normal dives during the escape period. I have to wonder how narwhals protect their brains and maintain oxygenation in this situation," said Terrie Williams, a professor of ecology and evolutionary biology at UC Santa Cruz who has studied exercise physiology in a wide range of marine and terrestrial mammals.

Williams is first author of a paper on the new findings published December 8 in Science. Narwhals, known as "unicorns of the sea" for the large tusks on the males, live year-round in Arctic waters. They have been relatively isolated from human disturbances until recently, when declines in Arctic sea ice have made the region more accessible to shipping, oil exploration, and other human activities.

Narwhals monitored after release gradually returned to more typical behavior and normal heart rates. But Williams said she worries that the stress from human disturbances could cause behavioral responses in narwhals that are inconsistent with their physiological capacities. Their natural escape response to avoid killer whales and other threats typically involves moving slowly either to great depths or into shallow coastal areas beneath ice cover where killer whales can't follow. "This is not a speedy animal," she explained.

A decreased heart rate (called bradycardia) is a normal part of the mammalian dive response, along with other physiological changes to conserve oxygen. In narwhals, the researchers measured resting heart rates at the surface of about 60 beats per minute. During normal dives (after the escape period), their heart rates dropped to between 10 and 20 beats per minute, depending on exercise level. Heart rate normally rises with increased exercise, even during a dive.

"That's what is so paradoxical about this escape response -- it seems to cancel out the exercise response and maintains extreme bradycardia even when the whales are exercising hard," Williams said.

The extremely low heart rates that Williams observed in fleeing narwhals are similar to those seen in animals with a "freeze reaction," one of two mutually exclusive responses animals can have to perceived threats, the other being a "fight or flight" response that revs up heart rate and metabolism. The narwhals, in their response to a stressful situation, seem to combine elements of a physiological freeze reaction with a behavioral flight reaction, with potentially harmful consequences.

"For terrestrial mammals, these opposing signals to the heart can be problematic," Williams said. "Escaping marine mammals are trying to integrate a dive response on top of an exercise response on top of a fear response. This is a lot of physiological balancing, and I wonder if deep-diving marine mammals are designed to deal with three different signals coming to the heart at the same time."

The same phenomenon may occur in other deep-diving whales when they are disturbed by human-generated noise in the oceans, which has been associated with strandings of deep-diving cetaceans such as beaked whales, she said.

"The disorientation often reported during strandings of deep-diving whales makes me think something has gone wrong with their cognitive centers," Williams said. "Could this result from a failure to maintain normal oxygenation of the brain?"

She calculated that the escape dives her team monitored in narwhals required 97 percent of the animal's oxygen supply and often exceeded its aerobic dive limit (meaning depletion of oxygen stores in the muscles, lungs, and blood, followed by anaerobic metabolism). Normal dives of similar duration and depth used only about 52 percent of a narwhal's oxygen store, the study found.

The study was conducted in Scoresby Sound on the east coast of Greenland, where coauthor Mads Peter Heide-Jørgensen, a research professor at the Greenland Institute of Natural Resources, has been studying narwhals since 2012. Native hunters in the area set out nets to catch fish, seals, and other animals, including narwhals. Heide-Jørgensen developed a collaboration with the hunters to allow scientists to tag and release narwhals caught in the nets. He has been using satellite tags to study the movements of the East Greenland narwhal population.

Williams's group at UC Santa Cruz developed unique tagging technology for marine mammals that enables researchers to monitor exercise physiology during dives by recording electrocardiograms, swimming movements (stroke rates), and other data. The tags function much like the Fitbits people use to monitor their daily activities. For this study, resting heart rate was measured in nine narwhals, and five were monitored during dives after release. The instruments were attached to the narwhals with suction cups and fell off after one to three days, floating to the surface where they could be recovered by the scientists.

In previous studies, Williams has used the instruments to study exercise physiology and dive responses in bottlenose dolphins, Weddell seals, and other species. "This was our first opportunity to put the tags on a deep-diving whale to monitor its physiological and behavioral responses," Williams said. "It all began with the work on dolphins in our facilities at Long Marine Laboratory."

Among the findings of her earlier studies was a surprising frequency of heart arrhythmias in dolphins and seals during intense exercise at depth. The new findings add to her concerns about the effects of disturbances that cause an escape response in deep-diving marine mammals.

"Unlike threats from predators like killer whales, noise from sonar or a seismic explosion is difficult to escape. Problems can start if the whales try to outrun it," Williams said. "The implications of this study are cautionary, showing that the biology of these animals makes them especially vulnerable to disturbance. This technology has given us a window into the narwhal's world, and what we see is alarming. The question is, what are we as humans going to do about it?"

In addition to Williams and Heide-Jørgensen, the coauthors of the paper include Susanna Blackwell of Greeneridge Sciences in Santa Barbara, California; Beau Richter at UC Santa Cruz; and Mikkel-Holger Sinding at the Natural History Museum of Denmark, University of Copenhagen. This work was funded by the U.S. Office of Naval Research and National Science Foundation and the Greenland Institute of Natural Resources.

Story Source:

Journal Reference:
Terrie M. Williams, Susanna B. Blackwell, Beau Richter, Mikkel-Holger S. Sinding, Mads Peter Heide-Jørgensen. Paradoxical escape responses by narwhals (Monodon monoceros). Science, 2017; 358 (6368): 1328 DOI: 10.1126/science.aao2740

Until recent declines in Arctic sea ice levels, narwhals (Monodon monoceros) have lived in relative isolation from human perturbation and sustained predation pressures. The resulting naïvety has made this cryptic, deep-diving cetacean highly susceptible to disturbance, although quantifiable effects have been lacking. We deployed a submersible, animal-borne electrocardiograph-accelerometer-depth recorder to monitor physiological and behavioral responses of East Greenland narwhals after release from net entanglement and stranding. Escaping narwhals displayed a paradoxical cardiovascular down-regulation (extreme bradycardia with heart rate ≤4 beats per minute) superimposed on exercise up-regulation (stroke frequency >25 strokes per minute and energetic costs three to six times the resting rate of energy expenditure) that rapidly depleted onboard oxygen stores. We attribute this unusual reaction to opposing cardiovascular signals—from diving, exercise, and neurocognitive fear responses—that challenge physiological homeostasis. 
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  • Claudiu Constantin Nicolaescu
Narwhals' acoustic behavior described using audio tagging

June 13, 2018, Public Library of Science

[Image: narwhalsacou.jpg]

The clicking, buzzing and calling behavioral patterns of elusive East Greenland narwhals have been described thanks to in-depth recordings, in a study published June 13, 2018 in the open-access journal PLOS ONE by Susanna Blackwell from Greeneridge Sciences, Incorporated, United States of America, and colleagues.

Climate change is predicted to increase human activity in the Arctic, including remote areas of Greenland where narwhals live. However, little is known about the whales' acoustic behavior or their reactions to anthropogenic sounds. Previous studies have mostly relied upon underwater microphones, which are limited in their ability to record spatial and temporal variations.

The author of the present study captured six narwhals in East Greenland and tagged them with acoustic and satellite instruments. The researchers were able to record 533 hours of audio and analyzed their recordings to describe how the whales' acoustic behavior varied by location and time.

The researchers found that the narwhals produced three types of sounds: clicks, buzzes and calls. Clicks and buzzes were produced during echolocation for feeding, while the authors presume that calls served communication purposes. Calls were typically produced at depths of less than 100 meters, with over half being produced less than 7m from the surface. However, buzzes were produced at much greater depths of between 350 and 650 meters. The authors even used their recordings to identify a likely preferred feeding area: a particular fjord which had especially high buzzing rates. They also noted a possible stress response to capture and tagging: the narwhals were silent afterwards for around a day, reinforcing the need to record over larger timespans.

While much remains unknown about narwhal acoustics, this work provides new insights into where and when these elusive whales produce sound and could establish a baseline to help assess future impacts of climate and anthropogenic changes on narwhals.

Susanna Blackwell says: "Wide-scale changes are taking place in the Arctic, with warmer temperatures leading to shrinking summer ice coverage. More ice-free water means easier access for vessels and industrial operations, such as exploration for oil and gas. The inhospitable pack-ice environment that is narwhals' home for much of the year has for millennia kept them in relative isolation—even from biologists. Now new amazing tools allow us to take a multi-day, virtual ride on the back of a narwhal!"

Journal Reference:
Blackwell SB, Tervo OM, Conrad AS, Sinding MHS, Hansen RG, Ditlevsen S, et al. (2018) Spatial and temporal patterns of sound production in East Greenland narwhals. PLoS ONE 13(6): e0198295.

Changes in climate are rapidly modifying the Arctic environment. As a result, human activities—and the sounds they produce—are predicted to increase in remote areas of Greenland, such as those inhabited by the narwhals (Monodon monoceros) of East Greenland. Meanwhile, nothing is known about these whales’ acoustic behavior or their reactions to anthropogenic sounds. This lack of knowledge was addressed by instrumenting six narwhals in Scoresby Sound (Aug 2013–2016) with Acousonde™ acoustic tags and satellite tags. Continuous recordings over up to seven days were used to describe the acoustic behavior of the whales, in particular their use of three types of sounds serving two different purposes: echolocation clicks and buzzes, which serve feeding, and calls, presumably used for social communication. Logistic regression models were used to assess the effects of location in time and space on buzzing and calling rates. Buzzes were mostly produced at depths of 350–650 m and buzzing rates were higher in one particular fjord, likely a preferred feeding area. Calls generally occurred at shallower depths (<100 m), with more than half of these calls occurring near the surface (<7 m), where the whales also spent more than half of their time. A period of silence following release, present in all subjects, was attributed to the capture and tagging operations, emphasizing the importance of longer (multi-day) records. This study provides basic life-history information on a poorly known species—and therefore control data in ongoing or future sound-effect studies. 
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  • Claudiu Constantin Nicolaescu
Beluga whales and narwhals go through menopause

Date:  August 27, 2018
Source:  University of Exeter

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Female narwhal swimming at the surface, Baffin Island, Canada.
Credit: © wildestanimal / Fotolia

Scientists have discovered that beluga whales and narwhals go through the menopause -- taking the total number of species known to experience this to five.
Aside from humans, the species now known to experience menopause are all toothed whales -- belugas, narwhals, killer whales and short-finned pilot whales.
Almost all animals continue reproducing throughout their lives, and scientists have long been puzzled about why some have evolved to stop.
The new study, by the universities of Exeter and York and the Center for Whale Research, suggests menopause has evolved independently in three whale species (it may have evolved in a common ancestor of belugas and narwhals).
"For menopause to make sense in evolutionary terms, a species needs both a reason to stop reproducing and a reason to live on afterwards," said first author Dr Sam Ellis, of the University of Exeter.
"In killer whales, the reason to stop comes because both male and female offspring stay with their mothers for life -- so as a female ages, her group contains more and more of her children and grandchildren.
"This increasing relatedness means that, if she keeps having young, they compete with her own direct descendants for resources such as food.
"The reason to continue living is that older females are of great benefit to their offspring and grand-offspring. For example, their knowledge of where to find food helps groups survive."
The existence of menopause in killer whales is well documented due to more than four decades of detailed study.
Such information on the lives of belugas and narwhals is not available, but the study used data on dead whales from 16 species and found dormant ovaries in older beluga and narwhal females.
Based on the findings, the researchers predict that these species have social structures which -- as with killer whales -- mean females find themselves living among more and more close relatives as they age.
Research on ancestral humans suggests this was also the case for our ancestors. This, combined with the benefits of "late-life helping" -- where older females benefit the social group but do not reproduce -- may explain why menopause has evolved.
Senior author Professor Darren Croft said: "It's hard to study human behaviour in the modern world because it's so far removed from the conditions our ancestors lived in.
"Looking at other species like these toothed whales can help us establish how this unusual reproductive strategy has evolved."
Although individuals of many species may fail to reproduce late in life, the researchers looked for evidence of an "evolved strategy" where females had a significant post-reproductive lifespan.
The study was funded by the Natural Environment Research Council, and the team included researchers from the University of York and the Center for Whale Research.

Story Source: University of Exeter. "Beluga whales and narwhals go through menopause." ScienceDaily. (accessed August 27, 2018).

Journal Reference:
  1. Samuel Ellis, Daniel W. Franks, Stuart Nattrass, Thomas E. Currie, Michael A. Cant, Deborah Giles, Kenneth C. Balcomb, Darren P. Croft. Analyses of ovarian activity reveal repeated evolution of post-reproductive lifespans in toothed whales. Scientific Reports, 2018; 8 (1) DOI: 10.1038/s41598-018-31047-8
In most species the reproductive system ages at the same rate as somatic tissue and individuals continue reproducing until death. However, females of three species – humans, killer whales and short-finned pilot whales – have been shown to display a markedly increased rate of reproductive senescence relative to somatic ageing. In these species, a significant proportion of females live beyond their reproductive lifespan: they have a post-reproductive lifespan. Research into this puzzling life-history strategy is hindered by the difficulties of quantifying the rate of reproductive senescence in wild populations. Here we present a method for measuring the relative rate of reproductive senescence in toothed whales using published physiological data. Of the sixteen species for which data are available (which does not include killer whales), we find that three have a significant post-reproductive lifespan: short-finned pilot whales, beluga whales and narwhals. Phylogenetic reconstruction suggests that female post-reproductive lifespans have evolved several times independently in toothed whales. Our study is the first evidence of a significant post-reproductive lifespan in beluga whales and narwhals which, when taken together with the evidence for post-reproductive lifespan in killer whales, doubles the number of non-human mammals known to exhibit post-reproductive lifespans in the wild.
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  • Claudiu Constantin Nicolaescu
Narwhals spend at least half time diving for food, can fast for several days after meal

March 14, 2019, Public Library of Science

[Image: narwhalsspen.jpg]
A group of narwhals in Scoresby Sund, East Greenland. Credit: Carsten Egevang.

Narwhals—enigmatic arctic whales known for their sword-like tusk—spend over half their time diving to find food but are also able to last up to three days without a meal, according to a study by Manh Cuong Ngô and colleagues from the University of Copenhagen in Denmark, published in PLOS Computational Biology.

Narwhals are deep-diving whales that feed on halibut, cod and squid. The researchers analyzed 83 days of continuous tagging data from a single male Narwhal (Monodon monoceros) in the arctic waters east of Greenland. They also recorded sudden drops in the whale's stomach temperature, which indicate that the whale has fed on cold arctic prey, for the first 7 days. They found that swimming could be divided into three types—near surface swimming down to 50m, shallow foraging dives to between 50 m and 350m, and deeper dives for prey at depths between 350m and 900m. Although the authors used data from a single individual, the dataset—which includes over 8500 dives—is the longest and most detailed record of whale diving activity studied to date. Such long-term records are hard to obtain because tracking devices often detach after just a few days or weeks.

Narwhals are thought to be impacted by reductions in sea-ice caused by warming waters, competition with fisheries, and disturbance by shipping vessels. This study is the first step towards understanding the diving behavior of these elusive creatures, which could ultimately help design conservation strategies to help protect them as the climate changes. Future studies should compare this baseline data with records from narwhals exposed to high levels of human activity, the authors suggest.

"Advanced statistical approaches provide a unique insight into a cryptic deep diving whale in pristine parts of the Arctic," says study senior author Susanne Ditlevsen. "We found complex relationships between characteristics of diving patterns during different behavioral states, which has not been discovered before in the research of cetaceans."

Journal Reference:
Manh Cuong Ngô et al, Understanding narwhal diving behaviour using Hidden Markov Models with dependent state distributions and long range dependence, PLOS Computational Biology (2019).DOI: 10.1371/journal.pcbi.1006425

Diving behaviour of narwhals is still largely unknown. We use Hidden Markov models (HMMs) to describe the diving behaviour of a narwhal and fit the models to a three-dimensional response vector of maximum dive depth, duration of dives and post-dive surface time of 8,609 dives measured in East Greenland over 83 days, an extraordinarily long and rich data set. Narwhal diving patterns have not been analysed like this before, but in studies of other whale species, response variables have been assumed independent. We extend the existing models to allow for dependence between state distributions, and show that the dependence has an impact on the conclusions drawn about the diving behaviour. We try several HMMs with 2, 3 or 4 states, and with independent and dependent log-normal and gamma distributions, respectively, and different covariates to characterize dive patterns. In particular, diurnal patterns in diving behaviour is inferred, by using periodic B-splines with boundary knots in 0 and 24 hours.
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