Topics Covered in this Lecture:
Slideshow1: Whales 1
- Evolution of Whales
- Feeding Strategies of Whales
- Adaptations of Whales
- Reproduction of Whales
The whales are arguably the most majestic and awe-inspiring creatures ever to inhabit the earth. They have captured the interest and imagination of humans for millennia, yet they have not always been treated with respect. In this lecture, we will examine the evolutionary roots of these aquatic mammals, and look at the development of two profoundly different feeding styles. From these strategies arose much of the behavior of the whales. From their many adaptations to aquatic life to their intriguing style of romace, the whales always inspire a deep reverence for nature.
Evolution of Whales
The ancestry and evolution of whales is not well known. However, enough pieces have been cobbled together to give us a general idea of the early ancestors of whales. Remember, that whales are mammals, and from this most basic premise we can infer that whales began their existence on land. Thus, to find the ancestors of whales, we must look to early mammals with similar characteristics.
Analyses of the biochemical and genetic characteristics of whales suggests that they are close relative of the ungulates, hoofed mammals that populated North America more than 50 million years ago. Among these was a small dog-sized mammal known as Mesonyx. This animal, which looks a lot like a wolf, had five small hooves per limb, instead of claws. Its habit of feeding on fish in shallow waters, its apparent skill at swimming, and its long, whiplike tail set the stage for the purely waterborne animal that it was to become. A few million years later, it developed eyes and kidneys adapted to saltwater, it lost hair and developed an insulating layer of blubber, it evolved the ability to hear underwater, and it developed nasal plugs to close its nostrils while it was diving.
From these wolf-like beginnings, a prototype toothed whale arose. The oldest whale fossils, dating back 50 to 53 million years ago, were discovered in Pakistan and Egypt. Like modern whales, they were equipped with a slender upper jaw with a blowhole, simple teeth, widely separated eyes, and an elongated brain case. These whales also had developed an ability to hear.
Closer to home, 28 giant vertebra were discovered in Louisiana in 1832, and initially identified as remains of a giant reptile. Subsequent discoveries in England and elsewhere identified this animal as an ancient marine mammal. Interestingly, the teeth of these ancient mammals consisted of canines and incisors, adapted for grasping prey and moshing small bones. It is likely that these ancient whales foraged in shallow waters, eating fish and possibly, soft-bodied mollusks, like squid. Examinations of dozens of specimens since that time have enabled scientists to piece together a good picture of the early ancestral whale, which looks much like a dolphin or other toothed whales living today.
From these ancestral marine mammals arose the whales that we know today. Evolutionary pressures apparently related to food supply split the whales into two major groups: the toothed whales, including the killer whales, the sperm whale, and the dolphins; and the baleen whales, including the blue whale, the gray whale, and the humpback whale. The toothed whales came to be known as the Odontocetes, and the baleen whales came to be known as the Mysticetes. Both belong to the Order Cetacea, in the Phylum Chordata, Class Mammalia. Collectively, all whales, including dolphins and porpoises, are known as cetaceans.
While we're on this subject, you should know that dolphins are whales belonging to the family Delphinidae in the suborder Odontoceti. Members of this family, which have beaks and conical teeth, include the bottlenose dolphin (aka Flipper), the pacific white-sided dolphin, and the killer Whale. Porpoises, which lack a beak and have spade-shaped teeth, belong to the family Phocoenidae. They include the Dall's Porpoise, a beautiful little white-bellied porpoise that typically travels in schools, and the common or harbor porpoise.
Feeding Strategies of Whales
The separation of the cetaceans into two suborders, the Odontoceti, the toothed whales, and the Mysteceti, the baleen whales, represents an interesting evolutionary solution to the problem of getting food. Obviously, the toothed whales are predators, feeding on a variety of fishes, squid, sea lion pups, and even other whales. Baleen whales, on the other hand, strain their food from the water. They accomplish this by using baleen plates, which are hard, fibrous, hornlike filters that hang within the mouth of the whale. By taking great gulps of water and straining it through the plates, these whales consume many types of zooplankton and krill (shrimp-like organisms in great abundance in polar oceans). Gray whales, a species of baleen whales, even filter the mud at the bottom of the sea to obtain the large amounts of amphipods (like beach hoppers) that live there. But feeding differences aren't the only differences between the Odontoceti and the Mysteceti. Let's compare and contrast these two groups more closely.
As shown in the table below, the Mysteceti lack teeth, but have baleen instead. Their skulls are symmetrical with paired external nasal openings (or blowhole). Note also that Mysteceti lack a sternum, except on the first pair of ribs. In contrast, the Odontoceti, have teeth but no baleen. Their skulls are not symmetrical, and they only have a single external nasal opening. In addition, their rib cage is fused with a sternum on several ribs.
The Odontoceti and Mysteceti also differ in another important way: the Odontoceti use echolocation to find their prey. Echolocation is a kind of sonar for these whales. By emitting a series of clicks and listening for the return signal, these whales can locate prey and different objects. Recent work on the feeding behavior of these whales also indicates that they are able to stun their prey using a burst of sound, that temporarily paralyzes the victim. We will talk more about sound production in whales in our next lecture.
|Teeth lacking (except as embryonic vestiges)||Teeth present (although is some species they do not emerge through the gum)|
|Baleen plates present||No baleen plates|
|Skull symmetrical||Skull asymmetrical|
|External nasal openings paired||Single external nasal opening|
|Sternum composed of single bone, which articulates with the first pair of ribs only||Sternum composed of three or more bones, which articulate with three or more pairs of ribs|
|No echolocation but employs sophisticated forms of communication||Echolocation and sophisticated forms of communication|
|Up to 100 feet long||Up to 60 feet long|
|Social and/or solitary||Social and/or solitary|
While the Mysteceti appear to lack the ability to echolocate (there is some debate about this, however), they can employ an equally effective means for obtaining food. Humpback whales (and possibly other species of whales) can create curtains of bubbles that surround and concentrate zooplankton. This technique, called bubble netting, begins with the whale circling and rising while emitting air and creating bubbles. After a sufficient interval, the humpback rises vertically in the center of the "bubble net" and engulfs the "trapped" food.
Both groups also achieve great sizes: the blue whale, a Mysteceti, is the largest animal on earth achieving lengths of more than 100 feet and weights reaching 130 tons. The heart of a blue whale is the size of a small car, necessary to pump the more than 9 tons of blood in the circulatory system of this giant beast. Of the Odontoceti, the sperm whale is the largest, measuring close to 60 feet in length, and weighing from 32 to 45 tons. The sperm whale is renown for its ability to dive deep in the ocean, at times to depths exceeding a mile or more. At these depths, the sperm whale finds its favorite prey, the giant squid!
Both groups of whales also appear to migrate over great stretches of the ocean. The migrations on the gray whale from Alaska to the tip of Baja are well known to Californians. The humpback whale also migrates from Alaska, making its winter resting grounds in Hawaii. Other humpbacks spend the summer in Antarctica and winter at various sites in Australia and the South Pacific, along the southern coasts of Africa, and along both coasts of South America. Among the Odontoceti, the sperm whale spends winters in equatorial waters and summers in the rich and productive waters of polar regions.
Both groups of whales have members who are social or solitary. Gray whales typically travel in groups when young, but older males may travel alone. Killer whales are notorious for traveling in packs, and the communication that has developed among these packs can be quite complex and extraordinary. Both groups appear to employ remarkable degrees of communication and we will examine this behavior in greater detail in our next lecture.
Various other characteristics can be attributed to different species, regardless of the group to which they belong. Such behaviors include spyhopping, raising their head up into the air as if to look around; breaching, leaping out of the water and landing with a great splash; and tail-sailing, standing on their head with their tail out of the water as if to sail in the wind. Why whales perform these behaviors is anyone's guess, but my guess is that they do it because they can!
To summarize, the biggest differences between these two groups of whales appear to be related to feeding. Despite anatomical and a few behavioral differences needed for feeding, other characteristics of these whales appear quite similar. It is interesting to think about the entire evolutionary process of whale. The mere fact that a wolf-like mammal was "forced" into the sea should tell something about the degree of competition for food in these times. Whatever the reason, we can be very glad that these great and beautiful mammals exist on our planet today.
Adaptations of Whales
No study of whales would be complete without a brief survey of the incredible adaptations that whales employ to survive in the oceanic environment. As we discussed with intertidal and sandy beach organisms, physical factors play a large part in the life of marine organisms, and whales are no exception. Water resistance (which affects swimming), temperature (don't forget that they are mammals), pressure (during diving), oxygen supply (while underwater or diving), buoyancy regulation, sound transmission, and visibility are just a few of the factors that whales must contend with in their everyday lives. Let's take a brief look at how whales solve these problems.
To feed on fish, one must swim faster than the fish, and no animal has solved this problem as elegantly as the dolphin. Capable of speeds up to 30 knots or more, dolphins have evolved a swim stroke that allows it to swim much faster than should be possible based on body shape alone. Initial experiments of the hydrodynamics of dolphin shape (using models) indicated that dolphins would need muscles that generated ten times the power of other mammals to achieve the speeds at which they were observed to swim. However, subsequent observations of swimming behavior revealed that the tail stroke of a dolphin acts to create a laminar flow along the surface of the dolphin, reducing the drag on its body, and thereby enabling it to swim with a higher efficiency and faster speeds.
Like other mammals, whales are warm-blooded or endothermic; that is, they maintain a constant internal body temperature, somewhere around 37 degrees C. To accomplish this in the cold waters of the ocean, they maintain a thick layer of blubber (up to 50 centimeters) beneath their skin. This blubber acts as an insulator to prevent loss of heat. However, blubber alone is not totally effective against the cold. In a marvel of circulatory engineering, whales employ what is known as "countercurrent exchange." Arteries leaving the heart are placed within the inner part of the limbs and blood returning to the heart runs in veins on the outside of the body surface. In this way, warm arterial blood leaving the heart exchanges heat with cold venous blood returning to the heart and temperature is conserved. Many organisms, including aborigine humans, also employ this strategy for conserving heat.
Pressure becomes a problem for whales only when they dive to great depths, a phenomenon known among whalers as sounding. Because water pressure increases by one atmosphere for every 33 feet increase in depth, whales diving to thousands of feet must have some means for dealing with pressure. It appears that the vascular cavities and lungs of whales are compressible. Air may be forced out of the lungs but it manages to escape into passages leading to the blowhole. In this way, a supply of oxygen is always on hand. As for the rest of the body, the tremendous pressures appear to make no difference because a large proportion of a whale's body is composed of water.
Oxygen supply can also be a problem on long dives. Sperm whales and bottlenose whales have been clocked for as long as 90 minutes and 120 minutes, respectively, beneath the surface. How do whales hold their breath this long? The answer is not in their lungs. The lung capacity of most whales is relatively smaller than land mammals and the best diving whales have the smallest lungs. Whales do have a higher proportion of blood in their bodies, 10-15% by body weight compared to 7% in humans. They also have a greater concentration of red blood cells and they have myoglobin in their muscles. Both adaptations increase the ability of whales to carry oxygen but, as it turns out, even these mechanisms are insufficient to explain the breath-holding capabilities of whales.
The answer may lie in a phenomenon known as bradycardia. Bradycardia is a process by which an animal slows its heartbeat while underwater. By slowing its heartbeat, the supply of oxygen is conserved. Bradycardia was first observed in ducks, but it is also employed by marine iguanas. In fact, marine iguanas have been shown to be able to stop their heart beat for a few moments when underwater. While it is doubtful that whales actually stop their hearts, there is some evidence that they slow their hearts while underwater.
Buoyancy regulation is another problem faced by whales, especially deep divers. Whales are negatively buoyant and the deeper a whale dives, the more negatively buoyant it becomes. While the maintenance of buoyancy in whales is little understood, it has been suggested that the oil that fills the whale's tissues, particularly in the sperm whale, changes composition under pressure. As a result, positive buoyancy is established as the oil becomes more buoyancy under pressure. A 30 ton sperm whale may contain as much as 2.5 tons of oil, and this function of this oil may be somehow associated with deep diving. Nonetheless, any ideas concerning buoyancy control in these animals can only be considered speculative.
We will talk more about sound transmission and hearing in our next lecture, but suffice it to say that whales have adapted keen mechanisms to focus sound and developed sophisticated behaviors for transmitting sound. Perhaps the most interesting behavior is their use of the sofar channel, a homogeneous layer of water resting between 3000 and 4000 feet that acts to channel sound around the world oceans. Whales will swim in these channels and sing their songs repeatedly. You'll have to wait to learn why they do this.
Finally, whales have evolved a pair of eyes that suits them both in water and out. Having an elliptical shape, as compared to the spherical shape of human eyes, the muscles around this eye can bend it to suit vision either in air or water. In addition, because light intensity is generally weak in the oceans, whales have evolved very large pupils, which allow them to see better in the dim light of oceanic waters. It is also likely (at least I would hypothesize) that their eyes have evolved to see better in blue light. However, I should note that dolphins seem to prefer object that are red and yellow, which means at least that they can distinguish colors.
Examination of the many fascinating adaptations of whales only increases our appreciation for the level of sophistication of these animals. I will only leave our discussion here with one thought: the brain of a dolphin is somewhat larger than the human brain and has a convoluted cortex, suggestive of intelligence. Are dolphins smart? We'll talk more about this subject in our next lecture.
Reproduction of Whales
The reproductive prowess of whales is, as you might expect, equivalent to their size. No less advanced are their mating behaviors, which are quite liberal by human standards. We will review a few of the more salient features here.
The genitals of all whales are contained within the body. On both males and females, a genital slit is located near the anus. Other than two mammary slits next to their genitals, their is little else to distinguish the females from the males.
Like other mammals, male whales have one testis and a penis located within the body cavity. In mature males, the testis may be as long as a forearm and weight several pounds. The penis varies in size among species, the largest apparently going to the males humpback whales, who are reputed to have penises up to fourteen feet in length. Like other mammals, the penis is composed of spongy tissue that becomes erect when filled with blood, but the penis of whales also contains a large amount of tough, fibrous tissue, which gives it elasticity.
The reproductive structure of female whales is quite similar to other mammals. Two ovaries discharge eggs into their respective uterine tubes that feed into the main body of the uterus. One curiosity of whale ovaries is that they leave a record of ovulation. A "protruberance" called a corpus albicans, is left behind after each ovulation. Why this occurs is unknown.
The breeding habits of whales are not well known. However, recent studies of humpback whales appear to suggest that two males will take part in copulation with a female. The males "surround" the female, one male providing a stable platform while the other male copulates. There appears to be some evidence that many males will mate with a single female; competition among males has been reduced to a battle of the sperm, rather than the physical battles that many male mammals display before mating.
Gestation periods for most whales is around 11 months. One species, the long-finned pilot whale, has a gestation period of 16 months! The harbor porpoise, on the other hand, has a nine-month gestation period, just like humans.
The birth of an infant whale is generally accompanied by a group of females, who support the mother and provide protection for the baby. Whales are typically born tail-first and the fetus is expelled directly into the water. Newborn whales can be quite large; baby blue whales can weigh up to 150 tons! Once born, it will be helped to the surface to breathe and cared for with the utmost protection. Suckling of the young whale can last from 4-11 months with both mother and child swimming close to the surface so both can breathe easily. This motherly care is quite important to whales; given their long gestation periods, significant investments are made in the child long before it is born. These long gestation periods also contribute to slow population growth, a factor that lead to the near extinction of many species living today.
A Few Concluding Remarks
Our discussion of the whales is brief at best, but I hope that you have a better appreciation for the complex and intricate beauty of these magnificent creatures of the sea. Their adaptations to life at sea are remarkable, considering that they are mammals and probably better suited to land. Yet these mammals returned to the sea, something that many human has considered on occasion. Perhaps our wistful longings while standing by the sea are the same longings that whales felt some 50 million years ago. Perhaps someday humans too will return to the sea.
from The Remarkable Ocean World