Baleen whales and Toothed whales, explained

What is a baleen whale and what is a toothed whale? What do they have in common? What are the differences between them?

The group of cetaceans is composed of whales, dolphins and porpoises. Dolphins and porpoises have teeth, but whales can have teeth or baleen plates depending on the species. Sometimes this creates confusion among people. In this post, we will try to explain the differences between baleen and toothed whales, what do they have in common, their origin, their phylogeny and their evolution. 

Phylogeny of Cetaceans 

Cetaceans, from the Ancient Greek huge fish (ketos), are the widest living group of aquatic mammals, they inhabit every ocean basin and some river systems. Their morphology and size are diverse, ranging in adult length from the Vaquita (Phocoena sinus) which is 1.45m to the Blue whale (Balaenoptera musculus) which grows up to 33m. 

They belong to the order of Cetartiodactyla, an order that gathers the old orders of cetaceans and artiodactyls. The ancestor of all cetaceans was a land artiodactyl named Pakicetus (figure 1, B), this hypothesis is well-supported by molecular and morphological analysis. Artiodactyls are those ungulate (even-toed hoofed mammals) who hold their weight on 2 fingers, the 3rd and 4th of each paw, but taxonomically it is an obsolete group. There are 4 suborders in the order of Cetartiodactyla: Tyolpoda (camels), Suina (boars), Whippomorpha (cetaceans and hippos) and Ruminantia (giraffes and deer). Thus, nowadays the closest land animal to cetaceans are hippos as shown in figure 2. 

 

Figure 1. A gallery showing reconstructions of Eocene/Oligocene archeoceti. A) Indohyus (Raoellidae, Raoellid), B) Pakicetus (Pakicetidae); C) Ambulocetus (Ambulocetidae); D) Remingtonocetus (Remingtonocetidae); E) Georgiacetus (Protocetidae); F) Dorudon (Basilosauridae); G) Janjucetus (Janjucetidae, Mysticeti) and H) Aetiocetus (Aetiocetidae, Mysticeti). (Gatesy 2013, Artwork is by Carl Buell) 

 

Figure 2. Phylogenetic position of Cetacea relative to other artiodactyls 
(Gatesy 2012, artwork by Carl Buell)

Focusing on the suborder of cetaceans, there are two groups: Archeoceti or primitive cetaceans, which are extinct and Neoceti or modern cetaceans, which has almost 90 species (87-89). Neoceti has 2 lineages: Odontoceti or toothed cetaceans which groups 9 families and Mysticeti or baleen whales which groups 4 families as shown in figure 2. Odontoceti and Mysticeti diverged 34 million years ago, by the end of Eocene as shown in figure 3. Conservative features reveal distant common ancestry between Odontoceti and Mysticeti. However, many relationships within Cetacea remain less certain: Mysticeti represents the most resolved and well supported region of the taxonomy tree, but in the case of Odontoceti the data show conflicting signals and are unclear on occasion. 

Cetacean diversity increased in the Miocene, probably thanks to decreasing paleo-temperatures, plankton diversity and the opening of the Southern Ocean that changed the circulation. Despite the radiation of the species, they all have common adaptations for aquatic life such as streamlined bodies, elongate tail stocks, horizontal propulsive tail, pectoral fins used as a rudder in locomotion with a rigid elbow, modified ear structure (also outer ears are absent) and eyes physiology to be able to sense directionally in and above the water, skin lacks sweat and sebaceous glands and their bodies are nearly hairless. Their body sizes are determined by rates of heat loss in water, the surface of the flukes and the feeding apparatus. However, Mysticeti and Odontoceti do have differences, in the following sections we will focus on each lineage.

Figure 3. Time-calibrated phylogenetic tree of cetacea (Mcgowen 2009)

Odontoceti

The 9 families that belong to Odontoceti lineage are: Physeteridae (Sperm whales, Physeter mocrocephalus), Kogiidae (Draw and Pygmi Sperm whales, Kogia sima and K. breviceps), Platanistidae, Pontoporiidae and Iniidae (river dolphins), Monodontidae (Narwals and Belugas, Monodon Monoceros and Delphinapterus leucas), Phocoenidae (Porpoises), Ziphiidae (Beaked whales) and Delphinidae or oceanic dolphins, which is a taxonomic hodgepodge for all those small to medium-sized Odontoceti that don’t fit in the other families. In there we find Dolphins and the “blackfishes” like Orcas (Orcinus orca), False Killer whales (Pseudorca crassidens), Pilot whales (Globicephala sp.), Melon-headed whales (Peponocephala electra) and others.

General characteristics 

Typically, Odontocetes are small or medium-sized cetaceans (except for the Sperm whale which can reach 18m), have sexual dimorphism and males are larger than females in almost all the species. They have large relative brain size and use tools. They have complex social behaviors and self-recognition. They lost the olfactory sense but they utilize the echolocation system to detect prey, navigate and socialize. They are also characterized by a single blowhole, an asymmetrical skull, a complex system of nasal sacs and a fatty organ in the forehead known as melon that play an important role in echolocation. In odontocetes we find the deep divers, like the Cuvier’s beaked whale (Ziphius cavirostris) that can dive to 3.000m deep or the Sperm whale that can dive deeper than 2000m and has the largest brain of any animal alive on Earth. 

Feeding 

The most obvious distinguishable feature is the presence of teeth throughout life. Odontoceti comes after the words tooth (odontos) and marine monster (ketus) from Ancient Greek. Their homodont dentition allows the animals to feed off fish, mollusks and crustaceans, which are preyed individually. Unlike other mammals, odontocetes do not replace their teeth. Some families (ex. Physeteridae) show a convergent tooth loss as a result of a reduced role for teeth in processing food: toothed whales swallow the preys entirely.

Echolocation

Echolocation is another characteristic feature of Odontocetes, it is a unique sensory innovation from all living and fossil odontocetes, it allows the animals to navigate, find food, avoid predators and communicate using specialized sounds that they produce (high-frequency clicks and whistles). They receive the sounds of these vocalizations as an echo and process them thanks to specific structures in their heads. 

Social behavior and ecology

Odontocetes typically live in stable social units (mother-calf pairs, juvenile subgroups or male alliances) that form wider associations that can go up to hundreds or even thousands individuals in some species. Calf protection and food finding are important functions of society. Some species live in matrilineal long-term units (a group that is lead by the older female) that can create long-term associations with other units.

Some species make seasonal movements in which males sometimes migrate separately from females and calves, which suggests that cultural retention of knowledge of movement patterns is important. Researchers talk about clans and evidence of culture. Thus, live-capture removals and intentional shooting or by-catch by fishermen have a disproportionally large effect on the population’s ability to adapt and flourish due to the cultural capital retained specially by older individuals. Females often have a role other than reproduction and we can find reproductive senescence or menopause in several species. In some others, such as the pilot whales, the strong social bonds cause an entire pod to be driven ashore. 

The complex and strong social structure of odontocetes is shown physiologically in the Limbic System, a complex set of brain structures highly involved in emotional processing, memory formation and learning skills. 

Mysticeti

The 4 families that belong to Mysticeti lineage are: Balaenidae (Right and Bowhead whales, Eubalaena sp. and Balaena mysticetus), Neobalaenidae (Pygmy right whale, Caperea marginata), Eschrichtiidae (Gray whale, Eschrichtius robustus), Balaenopteridae or rorquals, these are baleen whales with a dorsal fin and the only ones with throat grooves like blue, fin, humpback whales (Balaenoptera musculus, B. physalus, Megaptera novaeangliae) and others. 

General characteristics

Mysticetes are medium-sized or big cetaceans and females grow slightly bigger than males.  They have loose social bonds, adults wander the oceans alone or in small and temporary groups. They retained olfaction. They are also characterized by two blowholes or nasal openings. Baleen whales dive shallower than the toothed whales, diving less than 400 m deep. 

Feeding

They are characterized by an elaborate filter-feeding apparatus that features baleen, a keratinous sieve that continuously grows from their palate and is used to filter big amounts of food from the water at once. Mysticeti apparently comes after a translation error from the Ancient Greek of Aristotle’s “Historia Animalium” but it is related to the fact that the baleen plates look like a moustache. Modern mysticetes possess rudimentary teeth during fetal development that are reabsorbed before birth.  The biggest one is the Blue whale, it can engulf and filter a volume of prey and water that would fill a medium-sized swimming pool, it is described as the world’s largest biomechanical event. 

Critical modifications occurred in order to adapt their feeding apparatus. This modifications are related to how whales take water and filter preys. There are 3 filter-feeding modes as shown in figure 4: engulfment feeding off small schooling fish and crustaceans (Balaenopteridae or rorquals), benthic suction  off invertebrates from the bottom of the coastline (Gray whales), and skimming for feeding off aggregations of small and relatively immobile zooplankton (right and pygmy right whales). The rostrum of skimmers is characteristically narrow and arched, the rostrum of benthic suction feeders is moderately arched and the one from engulfment feeders is flat.

Figure 4. phylogeny of baleen whales and filter-feeding modes represented: a) engulfment, b) benthic suction, 
c) Skimming (Gatesy 2012, artwork by Carl buell)

Social behavior and ecology

Mysticetes have a more complex neocortex than odontocetes, but have a smaller Limbic system. This suggests that baleen whales have social complexity but little emotional involvement. Baleen whales use low-frequency sounds for long-range communication. Typically, they are not known to build bonds between individuals and the strongest association is between the mother and the calf for the first year after birth, while the mother is nursing the calf and teaching it how to survive. During this period of time, females appear to be highly protective with the offspring. While mating or preying, some species are known to build short-term associations between individuals and they can migrate in pairs or small groups specially if they are immature. Some species have been seen having empathetic behaviors such as protecting other species of mammals from predators attacks. Baleen an toothed whales are known to interact between them and also with other marine mammals such as sea-lions. 

Despite Odontocetes do migrate long distances, Mysticetes perform the longest migrations of this group of mammals. They tend to migrate between their feeding and breeding grounds. The longest migration belongs to the Eastern North Pacific Gray whale population, which can migrate up to 20.000 km roundtrip between their feeding grounds (between Alaska and Russia) and their breeding grounds (Mexican Pacific coast).

Author: Maria Serra

References

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