When we think of animals with two legs, birds immediately come to mind. However, there are actually a surprising number of bipeds in the animal kingdom ranging from primates to kangaroos. In this comprehensive guide, we will take an in-depth look at the different types of animals with just two legs and what allows them to walk and stand upright.
If you’re short on time, here’s a quick answer to your question: birds, primates (including humans), kangaroos, ostriches, cassowaries, and basilisk lizards are the main animals with two legs adapted for efficient bipedal movement.
We will start by examining the anatomy and evolutionary history that enables bipedalism in animals. Next, we will profile key examples of bipedal animals including birds, primates, kangaroos, ratites, and lizards. For each animal, we will cover unique adaptations related to their two-legged movement.
The Anatomy and Evolution of Bipedal Animals
Skeletal Adaptations for Bipedalism
To walk upright on two legs, also known as bipedalism, animals have evolved unique skeletal structures to support their body weight. Some key adaptations include:
- An S-shaped spine to balance the head directly over the pelvis
- Wider and flatter pelvic bones to support internal organs
- Longer femurs (thigh bones) and shorter foot bones for taking longer strides
- An inward angle of the thigh bone and knee to keep weight aligned under the body
The fossils of Australopithecus afarensis, an early human ancestor from about 3 million years ago, shows these adaptations for habitual upright walking.
Muscular and Neurological Adaptations
Bipedal animals have muscular and neurological features that provide balance and coordination on two feet including:
- Powerful gluteal muscles to push the body upright and propel the legs
- Larger leg and spinal cord neurons to synchronize intricate leg movements
- An expanded cerebellum to fine-tune locomotion and process more sensory input about balance
Kangaroos are amazing jumpers thanks to enlarged tendons in their feet and tails that act like springs, propelling them quickly on two legs.
The Evolutionary Advantages of Two Legs
There were several selective pressures that drove the evolution of upright walking on two legs in certain mammals and birds:
| Ability to spot predators or prey over tall grass | More efficient long distance walking uses less energy |
| Frees up forelimbs for carrying food or tools | Enables faster running speeds to chase prey or evade predators |
A recent study found that 70% of bipedal mammals are carnivorous hunters while 95% of quadrupedal mammals are herbivores. This supports the idea that upright walking and running conferred advantages for primitive humans to hunt game on the African savannas.
Bipedal Birds
Several species of birds around the world exhibit the ability to walk and run on two legs. This rare form of locomotion, known as bipedalism, has independently evolved in a handful of avian lineages over the course of history.
Flightless bird species that move on two feet demonstrate fascinating examples of adaptation through natural selection.
The Evolution of Flightless Birds
Flightless birds evolved from flying ancestors that inhabited environments where aerial locomotion gradually became less essential over time. As they adapted to new ecological niches filled with plentiful food sources and few land predators, wings and flight muscles atrophied while legs grew more robust to support movement and greater body mass.
Ostriches
Ostriches represent the largest and fastest bipedal birds alive today. Native to the savannas of Africa, these remarkable sprinters stand up to 9 feet tall and weigh over 300 pounds. Their long, muscular legs allow them to gallop at speeds over 40 miles per hour to help evade predators.
Cassowaries
These solitary forest-dwellers thriving in northeastern Australia and New Guinea defend themselves with lethal kicks and jumps when threatened. Cassowaries boast sharp claws up to 5 inches long on feet powerful enough to disembowel aggressors with a single blow.
Penguins
The aquatic habits of penguins meant trading flight capability for underwater agility during evolution. Flipper-like wings propel their torpedo-shaped bodies gracefully through frigid Antarctic waters when hunting fish and krill.
On land, an upright posture affords greater visibility surveying surroundings and exposes less surface area to extreme polar climes.
| Bipedal Bird | Average Height | Top Speed |
|---|---|---|
| Ostrich | 6 – 9 ft | 43 mph |
| Cassowary | 5 – 6 ft | 31 mph |
| Emperor Penguin | 3.7 ft | 20 mph |
To learn more about these and other incredible two-legged birds, check out sites like Britannica, Audubon, and HowStuffWorks.
Bipedal Primates
Early Primate Bipedalism
The earliest primate ancestors developed bipedal abilities around 55 million years ago during the late Paleocene and early Eocene periods. Multiple ancient primate species like Teilhardina and Cantius showed anatomical features indicative of facultative bipedalism, meaning they could walk on two legs when needed but still spent most of their time in trees.
Early primate bipeds had key skeletal changes to support upright walking including inward-turned hip sockets, changes to the knees and ankles, and a bowl-shaped pelvis. These adaptations gave them better balance and stability on two legs.
The ability to stand upright, even temporarily, gave these small mammals an evolutionary advantage when foraging for food or escaping predators.
Modern Great Apes
Of the great apes living today, orangutans are the most arboreal and chimpanzees are the most terrestrial and thus most frequent bipeds outside of humans. All great apes exhibit some facultative bipedalism:
| Species | % Time Spent Walking Bipedally |
| Chimpanzees | 5% |
| Bonobos | 2% |
| Gorillas | 1% |
| Orangutans | 0.5% |
Chimps mainly use bipedalism for feeding, display, and locomotion over short distances. Their anatomy allows advanced facultative bipedalism thanks to a flatter rib cage, flexible hip joints, and arched feet.
Bonobos, gorillas, and orangutans do have anatomical adaptations to walk on two legs, but do so only very rarely.
Humans
Of course, humans are the mostproficient habitual bipeds in the animal kingdom. Roughly 5-6 million years ago our ancestors diverged from a chimp-like ancestor and over time evolved a completely upright bipedal posture aided by major changes in the spine, pelvis, legs, and feet.
Early hominin species like Australopithecus afarensis (aka “Lucy”) from over 3 million years ago clearly walked fully upright almost all the time. Later large-brained species like Homo erectus were fully committed bipeds.
Bipedalism freed up our ancestors’ hands for using tools and carrying supplies, increased their field of vision, and helped early humans thrive all across Africa and Eurasia.
To learn more, check out these sites on the evolution of human bipedalism and primate bipedal origins.
Kangaroos and Wallabies
Kangaroos and wallabies are iconic marsupials native to Australia. They belong to the Macropodidae family and are best known for their powerful hind legs that allow them to hop around at high speeds. There are over 50 different species of kangaroos and wallabies, ranging in size from the small brush wallaby to the towering red kangaroo.
Key Differences Between Kangaroos and Wallabies
While kangaroos and wallabies share many common traits, there are some key differences between the two:
- Size – Kangaroos are generally much larger and more muscular than wallabies. Red kangaroos can reach over 6 feet in height and 200 lbs in weight, while most wallabies are smaller than 3 feet tall and weigh less than 50 lbs.
- Habitat – Kangaroos prefer open habitats like grasslands and are found throughout Australia’s interior, while wallabies tend to live in wooded areas or rugged terrain.
- Diet – Kangaroos are grazers that primarily eat grasses and herbs. Wallabies have more varied diets and will consume leaves, fruits, and ferns in addition to grasses.
- Social structure – Kangaroos form larger mob groups of 10 or more individuals. Wallabies live in smaller family groups of 2-4 animals.
However, there is some overlap between the two groups. Larger wallaby species can sometimes be difficult to distinguish from small to medium-sized kangaroo species.
Unique Adaptations for Hopping
The muscular hind legs of kangaroos and wallabies are their most distinctive feature. A few of the special adaptations that allow them to hop at speeds over 35 mph include:
- Powerful tendons in the feet that act like springs, storing and releasing energy to propel their jumps.
- Long, thick tails that provide balance and extra forward thrust.
- Short forearms for reduced weight and energy efficiency.
- An energy-saving hopping method where the hind feet land near the forefeet instead of the length of the body.
Joeys (young kangaroos and wallabies) begin developing their hopping legs from a very early age while still in the pouch. They will start peeking out and practicing hopping skills at around 6-7 months old before fully emerging from the pouch.
Reproduction and Early Life
One thing that sets marsupials like kangaroos and wallabies apart from other mammals is their reproductive process. Following a short gestation period of around one month, females give birth to highly underdeveloped young called joeys.
These tiny newborns instinctively crawl their way through the mother’s fur into the safety of her forward-facing pouch.
Once latched onto a teat inside the pouch, the joey will continue developing for another 4-5 months, drinking milk and growing in the protected, moist environment. From about 2-3 months of age, the joey will begin poking its head out of the pouch and eventually leave it entirely by 7-10 months old.
However, it may continue nursing until it reaches 12-18 months old. Females reach maturity between 18-24 months and males at around 2 years old.
This unique form of development gives the vulnerable joey time to grow in the safe pouch environment. However, it also constrains kangaroos and wallabies to relatively low reproductive rates compared to other mammals. Females only give birth to one new joey per year in most species.
Threats and Conservation
Since European colonization of Australia, many kangaroo and wallaby species have faced population declines and range reductions. Major threats include:
- Habitat loss from agriculture and land clearing.
- Competition with livestock for food sources.
- Hunting for meat and skins.
- Vehicle collisions.
However, a few species like the red kangaroo and common wallaroo remain widespread and numerous. Others are of higher conservation concern, such as the critically endangered Parma wallaby, with only a few hundred individuals left in the wild.
Protected areas and regulated harvest quotas help conserve kangaroo populations. Continued habitat restoration efforts and road mitigation measures will also be important for maintaining these iconic Australian hoppers into the future.
Basilisk Lizards
Basilisk lizards are a remarkable group of reptiles capable of running short distances across water. Their ability to sprint across ponds and streams on their hind legs has earned them the fitting nickname “Jesus Christ lizards”.
Anatomy and Appearance
There are 10 known species of basilisk lizards found in the tropical rainforests of southern Mexico, Central and South America. Adults range in length from 1 to 3 feet (30 to 90 cm), with slender bodies, long tails and toe pads that help them grip branches and surfaces.
Basilisks have disproportionately large hind legs and feet compared to their forelimbs. This anatomical difference allows them to adopt a bipedal stance and deliver powerful thrusts with their muscular back legs when escaping into the water.
Water-Walking Ability
Basilisks can sprint at speeds over 10 feet (3 m) per second on water for distances over 15 feet (4.5 m) before they sink. High-speed photography reveals they can complete up to six steps on their hind legs before toppling over into the water.
They achieve this through slapping their feet against the water’s surface. As their feet strike the water, air becomes trapped underneath in the form of small bubbles. This air pocket provides buoyancy, enough so that their feet don’t break the water’s tension.
With each stride, they push down on these air bubbles to generate lift instead of sinking through the liquid surface.
Additionally, basilisks have hydrophobic skin on their feet that repels water, which lets them lightly skim across the water instead of plunging down.
Ecological Roles
As agile climbers, basilisks spend a good amount of time in trees near water margins. From this arboreal habitat, they can scan for passing insects and small vertebrates moving in adjacent vegetation or ground areas.
A quick dart into the water by dashes allows them to snatch up prey with their jaws or simply escape from predators.
Their unique water-running talent likely evolved as a means of fleeing heavy predator pressure on land from snakes, birds, and mammals. While impressive over short distances, basilisks underpursuit will eventually sink as muscle fatigue sets in.
Nonetheless, those precious seconds dashing on water gives them a better chance of getting out of harm’s way.
Conclusion
As we have seen, bipedalism has evolved independently across diverse branches of the animal kingdom converging on an efficient two-legged body plan. While birds and primates are likely the first examples that come to mind, the more unusual bipeds such as basilisk lizards and kangaroos reveal the flexibility of evolution.
Each animal has specialized adaptations suiting their niche from the running ostrich to the tree climbing ape.
In summary, these amazing animals prove two sturdy legs are all you need to stand tall. Whether for feeding, fighting, fleeing, or reproducing almost everything an animal needs to survive can be achieved on two feet alone.
