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Beaked whales

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Beaked whales


Temporal range: Miocene–Recent
Ziphius cavirostris
Genera
  • see text

Beaked whales are the members of the family Ziphiidae which consists of 21 species. These toothed whales are notable for their elongated beaks. Beaked whales are some of the world’s most extreme divers. They can dive for long periods—20 to 30 minutes is common, and 85 minute dives have been recorded—and to great depths: 1,899 meters (6,230 ft) and possibly more.[1]

Beaked whales are one of the least known groups of mammals because of their deep-sea habitat, mysterious habits, and apparent low abundance. [2] Only three to four of the 21 species are reasonably well-known. Baird’s and Cuvier's beaked whales were subject to commercial exploitation, off the coast of Japan, while the Northern bottlenose whale was extensively hunted in the northern part of the North Atlantic late in the 19th and early in the 20th centuries.

Physical characteristics

Beaked whales are moderate in size, ranging from 4 to 13 metres (13 to 43 ft) and weighing from 1 to 15 tonnes (0.98 to 14.76 long tons; 1.1 to 16.5 short tons). Their key distinguishing feature is the presence of a 'beak', somewhat similar to many dolphins. Other distinctive features include a pair of converging grooves under the throat, and the absence of a notch in the tail fluke. Although Shepherd's beaked whale is an exception, most species have only one or two pairs of teeth, and even these do not erupt in females. Beaked whale species are often sexually dimorphic — one or the other sex is significantly larger. The adult males often possess a dramatically bulging forehead.[3] However, aside from dentition and size, there are very few morphological differences between male and female beaked whales.[4]

Individual species are very difficult to identify in the wild, since body form varies little from one species to another. The observer must rely on often subtle differences in size, color, forehead shape, and beak length.

The blubber of these whales are almost entirely wax ester, a unique characteristic of this family.[5]

Teeth

Beaked whales are unique among toothed whales in that most species only have one pair of teeth. The teeth are tusk-like but are only visible in males, which are presumed to use these teeth in combat for female reproductive rights.[6] In females, the teeth do not develop and remain hidden in the gum tissues.

In December 2008, researchers from the Marine Mammal Institute at Oregon State University completed a DNA tree of all 21 known species of beaked whales. Among the results of this study was the conclusion that the male's teeth are actually a secondary sexual characteristic, similar to the antlers of male deer. Each species' teeth have a characteristically unique shape. Females are presumed to select mates based on the shape of the teeth, because the different species are otherwise quite similar in appearance. Females may also choose mates based on the size or shape of teeth or the scars they bear.[6]

These teeth also play an important role in competing to control a harem.

Taxonomy

Beaked whales comprise at least 21 species of small whale in the family Ziphiidae, which is one of the least-known families of large mammals: several species have been described only in the last two decades. Six genera have been identified.

The beaked whales are the second-largest family of Cetaceans (after the dolphins) They were one of the first groups to diverge from the ancestral lineage. The earliest known beaked whale fossils date to the Miocene, about 20 million years ago.


Evolutionary history

As many as eight genera antedate humans [7] [2] Some included ancestors of giant beaked whales (Berardius), such as Microberardius and Cuvier's beaked whale (Ziphius) had many relatives, such as Caviziphius, Archaeoziphius, and Izikoziphius. They were probably preyed upon by predatory whales and sharks, including Carcharocles megalodon.

Recently, a large fossil ziphiid sample was discovered off the South African coast, confirming the extant ziphiid diversity might just be a remnant of a higher past diversity. After studying numerous fossil skulls off the shore of South Africa, researchers discovered the absence of functional maxillary teeth in all South African fossil ziphiids, which is evidence that suction feeding had already developed in several beaked whale lineages during the Miocene. Researchers also found fossil ziphiids with robust skulls, signaling that tusks were used for male-male interactions (speculated with extant beaked whales).[2]

Ecology

Diving

Beaked whales are deep divers with extreme dive profiles.[8] They regularly dive deeper than 500 m to echolocate for food, and these deep dives are often followed by multiple shallower dives of less than 500 m.[9] Based on currently available data, beaked whales are thought to spend much of their lives below water.[10]

Deep-diving mammals face a number of challenges related to extended breath holding and hydrostatic pressure. Cetaceans and pinnipeds that prolong apnea must optimize the size and use of their oxygen stores, and they must deal with the accumulation of lactic acid due to anaerobic metabolism.[8] Beaked whales have several anatomical adaptations to deep diving: large spleens, livers, and body shape. Most cetaceans have small spleens. However, beaked whales have much larger spleens than delphinids, and may have larger livers as well. These anatomical traits, which are important for filtering blood, could be adaptations to deep diving. Another notable anatomical adaptation among beaked whales is a slight depression in the body wall that allows them to hold their pectoral flippers tightly against their bodies for increased streamlining.[4]

The challenges of deep diving are also overcome by the unique diving physiology of beaked whales. Oxygen storage during dives is mostly achieved by blood hemoglobin and muscle myoglobin.[9] While the whale is diving, its heart rate slows and blood flow changes. This physiological dive response ensures oxygen-sensitive tissues maintain a supply of oxygen, while those tissues tolerant to hypoxia receive less blood flow. Additionally, lung collapse obviates the exchange of lung gas with blood, likely minimizing the uptake of nitrogen by tissues.[4]

Feeding

The throats of all beaked whales have a bilaterally paired set of grooves that are associated with their unique feeding mechanism, suction feeding.[4] Instead of capturing prey with their teeth, beaked whales suck it into their oral cavity. Suction is aided by the throat grooves, which stretch and expand to accommodate food. Their tongue can move very freely. By suddenly retracting the tongue and distending the gular (throat) floor, pressure immediately drops within the mouth sucking the prey in with the water.

Dietary information is available from stomach contents analyses of stranded beaked whales and from whaling operations. Their preferred diet is primarily deep-water squid,[6] but also benthic and benthopelagic fish and some crustaceans, mostly taken near the sea floor.[10] In a recent study, gouge marks in the seafloor were interpreted to be a result of feeding activities by beaked whales.[11]

To understand the hunting and foraging behavior of beaked whales, researchers used sound and orientation recording devices (DTAGs) on two species: Cuvier’s beaked whale (Ziphius cavirostris) and Blainville’s beaked whale (Mesoplodon densirostris). These whales hunt by echolocation in deep water (where the majority of their prey is located) between approximately 200 and 1885 m and usually catch about 30 prey per dive. Cuvier’s beaked whale must forage on average at 1070 m for 58 minutes and Blainville’s beaked whales typically forage at 835 m deep for an average of 47 minutes.[8]

Range and habitat

The family Ziphiidae is one of the most widespread families of cetaceans, ranging from the ice edges at both the north and south poles, to the equator in all the oceans.[12] Specific ranges vary greatly by species, though beaked whales typically inhabit offshore waters that are at least 300 meters deep.

Beaked whales are known to congregate in deep waters off the edge of continental shelves, and bottom features, such as seamounts, canyons, escarpments, and oceanic islands, including the Azores and the Canary Islands.[10]

Life history

Very little is known about the life histories of beaked whales. The oldest one ever recorded was 84 years for a male Baird’s beaked whale, and the oldest recorded female Baird’s beaked whale is 54 years. For all other beaked whale species studied, the highest recorded age is between 27 and 39 years. Sexual maturity is reached between seven and 15 years of age in Baird’s beaked whales and northern bottlenose whales. Gestation varies greatly between species, lasting 17 months for Baird’s beaked whales and 12 months for the northern bottlenose whale.[13] There are currently no data available on their reproductive rates.

It is difficult to determine group size of beaked whales, due to their inconspicuous surfacing behavior. Groups of beaked whales, defined as all individuals found in the same location at the same time, have been reported as ranging from one to 100 individuals. Nevertheless, some populations’ group size has been estimated from repeated observations. For example, northern and southern bottlenose whales (H. ampullatus and H. planifrons), Cuvier’s beaked whales and Blainville’s beaked whales (Mesoplodon densirostris) have a reported maximum group size of 20 individuals, with the average ranging from 2.5 to 3.5 individuals. Berardius species and Longman’s beaked whales (Indopacetus pacificus) are found in larger groups of up to 100 individuals.[10]

Not much information is available about group composition of beaked whales. Only three species have been studied in any detail: northern bottlenose whales, Blainville’s beaked whales, and Baird’s beaked whales. Female northern bottlenose whales appear to form a loose network of social partners with no obvious long-term associations. In contrast to females, some male northern bottlenose whales have been repeatedly recorded together over several years, and possibly form long-term associations. Studies of Blainville’s beaked whales have revealed groups usually consist of a number of females, calves and/or juvenile animals. These whales are assumed to live in "harem-like" groups, where several females and young are accompanied by a single male.[6] Baird’s beaked whales are known to occur in multiple male groups, and in large groups consisting of adult animals of both sexes.

Conservation

For many years, most beaked whale species were insulated from anthropogenic impacts because of their remote habitat. However, there are now several issues of concern:

  • Studies of stranded beaked whales show rising levels of toxic chemicals in their blubber.[14]
  • As a top predator, beaked whales, like raptors, are particularly vulnerable to build-up of biocontaminants. They frequently ingest plastic bags (which do not break down and can be lethal).[10]
  • They more frequently become trapped in trawl nets, due to the expansion of deepwater fisheries.[15]
  • Decompression sickness

A major conservation concern for beaked whales (family Ziphiidae) is they appear to be vulnerable to modern sonar operations, which arises from recent strandings that temporally and physically coincide with naval sonar exercises.[16] Postmortem examinations of the stranded whales in concurrence with naval exercises have reported the presence of hemorrhaging near the ears or gas and fat emboli, which could have a deleterious impact on beaked whales that is analogous to decompression sickness in humans.[9] Gas and fat emboli have been shown cause nervous and cardiovascular system dysfunction, respiratory distress, pain, and disorientation in both humans and animals.[16] In the inner ear, gas embolism can cause hemorrhages, leading to disorientation or vestibular dysfunction.

Breath-holding divers, like beaked whales, can develop decompression-related problems (the “bends”) when they return to the surface after deep dives.[8] This is a possible hypothesis for the mass strandings of pelagic beaked whales associated with sonar-related activities. To illustrate, a diving beaked whale may be surfacing from a deep dive and must pass vertically through varying received sound levels. Since the whale has limited remaining oxygen supplies at the end of a long dive, it probably has limited abilities to display any normal sound avoidance behavior. Instead, the whale must continue to swim towards the surface to replenish its oxygen stores.[10] Avoiding sonar inevitably requires a change in behavior or surfacing pattern. Therefore, sonar in close proximity to groups of beaked whales has the potential to cause hemorrhaging or to disorient the animal, eventually leading to a stranding.

Current research reveals two species of beaked whales are most affected by sonar: Cuvier’s beaked whales (Z. cavirostris) and Blainville’s (M. densirostris) beaked whales. These animals have been reported as stranding in correlation with military exercises in Greece, the Bahamas, Madeira, and the Canary Islands.[17] The livers of these animals had the most damage.[18]

Though some evidence indicates sonar-related activities can actually lead to a form of decompression sickness in beaked whales, the topic is still up for debate. A significant limiting factor in determining the likelihood of bubble formation in whale tissues and the risk of decompression sickness is the lack of information on their normal diving and surfacing patterns.[8] More research is necessary to determine the extent of whale tissue damage caused by sonar exercises.

Four species are classified by the IUCN as "lower risk, conservation dependent": Arnoux's and Baird's beaked whales, and the northern and southern bottlenose whales. The status of the remaining species is unknown, preventing classification.[19]

Film

Tracking their silent voices (original title: Auf der leisen Spur der Schnabelwale) is a documentary about an Atlantic scientific expedition. A team of marine biologists tried to find beaked whales and to analyze their behavior using acoustic methods only.[20]

References

External links

  • The Beaked Whale Resource
  • Beaked whales - into the abyss - BBC News article
  • Diving with Beaked Whales - A Whale Trackers documentary video.
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