Seals are marine mammals that belong to the scientific order Pinnipedia. This order contains three families: Phocidae (true seals), Otariidae (fur seals and sea lions), and Odobenidae (walruses). Seals live mostly in cold and temperate waters and spend a significant portion of their lives in the water rather than on land. Their bodies are adapted for an aquatic lifestyle with streamlined torpedo-shaped torsos, short fur, and limbs modified into flippers. This brings up the question – do seals have tails or feet? The answer is that seals have flippers rather than feet, and some species have short tails while others do not.
Do seals have tails?
When it comes to tails, there is variation among seal species. True seals (family Phocidae) do not have external tails. Their tailbones are present internally but are not visible. In contrast, most fur seals and sea lions (family Otariidae) do have small flaps of skin at the ends of their bodies that contain a few vertebrae. These are visible externally and could be called tails, although they are very short and stubby. Sea lions have the most prominent tails that can measure up to 10 inches long in males. Fur seals have extremely short tails only about an inch long. Meanwhile, walruses (family Odobenidae) lack a tail externally or internally.
So in summary:
- True seals – No external tail
- Fur seals & sea lions – Short stubby external tail present
- Walruses – No tail
The function of the tail in fur seals and sea lions is not entirely clear. It may improve swimming performance and maneuverability in the water. The tails contain cartilage rather than vertebrae and do not show much movement. Overall, the tail is not a significant feature in any seal species compared to aquatic animals that use their tails for propulsion like otters and dolphins. The tail’s small size indicates it does not play an essential role.
Do seals have feet?
Instead of feet, all species of seals have flippers to propel themselves in the water. Their limbs are oriented horizontally rather than vertically underneath their bodies like feet. However, the skeletal structure of seal flippers still contains components found in land mammals with feet. The flippers consist of shoulder/upper arm bones, elbows, forearms, wrists, and phalanges akin to fingers. This five-digit arrangement is a hallmark of mammals descended from terrestrial quadrupeds.
There are key differences that make seal flippers distinct from land mammal feet:
- Flippers are stiff with little flexibility or muscle control
- Phalanges are embedded in dense tissue with no separate digits
- Outer edge of flipper has smooth skin rather than distinct toes
- Hindflippers are larger and more muscular than foreflippers
- A major part of propulsion comes from entire body undulation
When seals are on land, their flippers are oriented horizontally pointing forwards. This makes it impossible to use the limbs for walking. Instead, seals belly-crawl by wriggling their bodies aided by their hindflippers. Their locomotion is clumsy and slow on land compared to agile swimming enabled by their streamlined flippers.
Differences between seal species
While all seals share the general traits of lacking external hind limbs and having modified forelimbs as flippers, there are some subtle differences in flipper anatomy across species.
True seals tend to have shorter foreflippers relative to body size compared to fur seals and sea lions. Their hindflippers are proportionally larger than their foreflippers. True seals frequently orient their hindflippers underneath them when resting. In contrast, fur seals and sea lions have longer foreflippers and smaller hindflippers. Their foreflippers serve as the main source of propulsion. True seals are generally slower and less agile swimmers compared to fur seals and sea lions.
| Seal Group | Foreflipper Size | Hindflipper Size |
|---|---|---|
| True seals | Smaller | Larger |
| Fur seals | Larger | Smaller |
| Sea lions | Larger | Smaller |
Looking more closely at the skeleton, true seals have smaller hand-like bones in their foreflippers compared to fur seals and sea lions. Their phalanges bones are also thicker. True seals tend to have 1-2 more vertebrae in their hindflippers than foreflippers. These modifications enhance true seal’s rearward thrust. Fur seals and sea lions have more elongated foreflipper bones with longer phalanges. Their flippers serve more like oars for paddling through the water.
On land movement
When seals come onto land, their lack of hind legs and modified forelimbs makes terrestrial locomotion difficult. Different types of seals have varied strategies for hauling themselves around. True seals rely more on their hindflippers for generating forward propulsion on land. They thrust their hindflippers backward and upward in a crawling motion. Their foreflippers are held close to the body to reduce drag. Fur seals and sea lions depend more on their larger foreflippers to pull themselves along on land. The foreflippers can also act as stabilizers by digging into the ground. Their smaller hindflippers trail behind and provide less propulsion.
Sea lions are able to raise their trunks higher vertically off the ground compared to other seals. This makes their movement somewhat more effective on land. Sea lions can rotate their hindflippers forward under the pelvis which allows them to assume a posture resembling a quadruped. This enables them to walk on all fours which is unique among seals. However, sea lions still lack true feet and their “walking” is awkward and slow.
Swimming propulsion
A seal’s lack of hind legs and possession of flippers makes them very different from terrestrial mammals in terms of swimming. For efficient aquatic movement, seals rely on three major mechanisms:
- Foreflipper paddling
- Hindflipper thrusting
- Whole body undulation
The foreflippers generate most of the forward propulsion. They are powered by strong chest and shoulder muscles and act as oars to pull the seal through the water. The hindflippers provide rearward thrust and steering control. Their motion is like a sculling paddle. Perhaps most importantly, seals swim with lateral undulation of their entire flexible trunk and tail section. This serpentine body waving accounts for a major part of the propulsive force.
Different seal species use these techniques in varying proportions based on their flipper sizes and skeletal adaptations as described previously. True seals do more sculling with their large hindflippers while fur seals and sea lions dominate with foreflipper paddling. But all species rely significantly on sinuous whole-body motion as well. This makes seals swimming locomotion fundamentally different from other marine mammals like whales, manatees, and otters that mostly use tails or hindlimbs for propulsion. The lack of a prominent vertical tail fin underscores how seal bodies are specialized for lateral undulation.
Diving adaptations
Since seals need to hunt underwater, their physiology has adaptations to enable prolonged diving. Oxygen stores in their blood and muscles allow seals to hold their breath for a long time when submerged. Depending on the species, seals can dive for anywhere from 5 minutes to over an hour. Their flexible ribcages allow the lungs to fully collapse under pressure at depth without injury. Seals also have physiological adaptations to conserve oxygen usage when diving. Their heart rate slows down, blood flow is restricted from the skin and extremities, and muscles can tolerate increased lactic acid levels.
A seal’s streamlined body shape, lack of external appendages, and modified flippers help reduce drag while diving. The smooth flat profile of their flippers aligned with their bodies decreases turbulence. Compared to terrestrial mammals with legs and feet, seals evolve fewer energy costs and can dive deeper and longer. Their musculoskeletal system is designed for efficient vertical stroking motions for fast descents and ascents. Seals’ adaptations for an aquatic life free their movement from the constraints faced by land animals.
Comparison to other aquatic mammals
How do a seal’s flippers and lack of a tail compare to adaptations seen in other marine mammals? Here is a brief overview:
Whales & dolphins – Have a horizontal tail fluke used for propulsion rather than vertical flippers. The fluke moves up and down rather than side-to-side. Their tails enable high speeds. Lack hindlimbs.
Manatees & dugongs – Possess large, elongated foreflippers for paddling. Also have small vestigial hindlimbs. Use whole body undulation minimally. Slow swimmers.
Otters – Have webbed feet and lengthy tails that are used cooperatively for propulsion. Forepaws are often used for steering. Limbs are oriented vertically.
Polar bears – Limbs are structured like a terrestrial mammal for walking. But have partially webbed feet for swimming propulsion rather than modified flippers.
Sea turtles – Have forelimbs modified into large flippers that are the main source of propulsion. Lack distinct hindlimbs or a tail. Are not mammals.
Seals possess a unique body plan optimized for their marine environment that combines limbless hindquarters, modified forelimbs specialized as flippers, and a flexible spine for undulation. This distinguishes them from both terrestrial and fully aquatic mammals. The seal’s body form represents an evolutionary compromise that facilitates movement both in the water and to some extent on land.
Conclusion
In summary, seals do not possess either tails or feet like terrestrial mammals. All species lack external hindlimbs and tails are either non-existent or very small. Instead, a seal’s body has adapted for an aquatic lifestyle with streamlined torpedo-shaped trunks and limbs modified into horizontal flippers. The foreflippers contain remnants of mammal hand bones but are encased in stiff tissue to create effective paddles. While locomotion on land is clumsy, seals are graceful in the water. Their specialized bodies allow different types of efficient underwater propulsion through use of flippers and undulation. Seals’ adaptations emphasize how mammal body forms can be shaped by environmental pressures. By becoming expertly suited for the marine environment, seals have departed from standard mammalian anatomy – trading tails and feet for flippers and flexibility.
