Beaked whale


Beaked whales are members of the cetacean family Ziphiidae, noted as being one of the least-known groups of mammals because of their deep-sea habitat, reclusive behavior and apparent low abundance. Only three or four of the 24 existing species are reasonably well-known. Baird's beaked whales 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 in the late 19th and early 20th centuries.
Reports emerged in late 2020 of the possible discovery of a new beaked whale species off the coast of Mexico, the taxonomy of which had not been determined.

Physical characteristics

Beaked whales are moderate in size, ranging from and weighing from. 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 large bulging forehead, some to an extreme feature. However, aside from dentition and size, very few morphological differences exist between male and female beaked whales.
Individual species may be very difficult to identify in the wild, since many species appear similar. The observer must rely on size, shape, and placement of teeth and often subtle differences in size, color, forehead shape, and beak length. In collected specimens, the expansion of the premaxillary process in the skull can be a key feature to identification.
The blubber of these whales is almost entirely composed of wax ester instead of the more usual triacylglycerols, a unique characteristic of this family.

Dentition

Beaked whales are unique among toothed whales in that most species have only one pair of teeth. The teeth are tusk-like, but are visible only in males, which are presumed to use these teeth in combat for females for reproductive rights. 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 13 of 15 known species of Mesoplodon 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. In some cases, these teeth even hinder feeding; in the strap-toothed whale, for example, the teeth curve over the upper jaw, effectively limiting the gape to a few centimeters. Females are presumed to select mates based on the shape of the teeth, because the different species are otherwise quite similar in appearance.

Taxonomy

As of 2024, the Society for Marine Mammalogy Committee on Taxonomy recognizes 24 extant species of beaked whales in six genera. Several species have only been formally described in the last two decades, most recently in 2021.
The beaked whales are the second-largest family of cetaceans after the oceanic dolphins. Beaked whales were one of the first extant clades to diverge from the ancestral lineage. The earliest known beaked whale fossils date to the Miocene, about 15 million years ago.
A 2016 study split the beaked whales into the basal extinct Messapicetus clade and the crown Ziphiidae which include all of the living members of the family as well as other extinct forms. Both clades share some key characteristics of the family including thick skull bones and the trend toward loss of teeth. In 2020, a molecular study further resolved the relationships among the crown Ziphiidae and placed Shepherd's beaked whale, the only living species with a full set of erupted teeth, between Berardiinae, whose extant forms have four erupted teeth, and Ziphiinae, whose extant form has two erupted teeth.
The name Ziphiidae was coined from the genus Ziphius by J. E. Gray in 1865 to move the beaked whales from the family Delphinidae into a new family. Gray noted in 1866, however, that Hyperoodontidae should have priority for the new beaked whale family name owing to earlier usage, but Gray preferred Ziphiidae due to an apparent confusion between the upper and lower jaw in the naming of Hyperoodon.
Hyperoodontidae was preferred in a 1968 phylogeny, which stated that Gray's objection did not qualify as an exception under the International Code of Zoological Nomenclature. Hyperoodontidae is indeed currently marked as the valid name by the Integrated Taxonomic Information System which states no successful petition for Ziphiidae had been made to the ICZN as of 2023. In contrast, Smithsonian researchers J.G. Mead and Robert Brownell Jr. argued in 1993 that due to being the "name of choice for over 100 years", Ziphiidae should be given exception under the ICZN Article 23.12. In addition, several authorities, including the Society for Marine Mammalogy Committee on Taxonomy and IUCN Red List of Threatened Species among others continue to use Ziphiidae.
A further, unrelated confusion has arisen, as noted on the ITIS, due to the propagation of an incorrect citation of "Gray, 1850" for Ziphiidae.

Evolutionary history

As many as 26 genera antedate humans. These include ancestors of giant beaked whales, such as Microberardius, and ancestors of Cuvier's beaked whale ; they had many relatives, such as Caviziphius, Archaeoziphius, and Izikoziphius. They were probably preyed upon by predatory whales and sharks, including Otodus 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 Iberia and 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.

Ecology

Diving

Beaked whales are deep divers with extreme dive profiles. They regularly dive deeper than to echolocate for food, and these deep dives are often followed by multiple shallower dives less than 500 m. This pattern is not always followed, however. Animals have been observed spending more than an hour at or near the surface breathing. Beaked whales are often seen surfacing synchronously, but asynchronous surfacing has also been observed. In March 2014, a study by Cascadia Research revealed that Cuvier's beaked whales were recorded to dive at least 2992 m in depth, a mammalian record. Another study, published in 2020, reported a Cuvier's beaked whale making a dive that lasted 222 minutes, another mammalian record.
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. 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. However, they are not invulnerable to the effects of diving so deep and so often. Cascadia Research shows that the deeper the whales dive, the less often they dive per day, cutting their efforts by at least 40%.
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. 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.