With their rotund bodies and stout legs, hippos can appear at first glance to resemble giant hairless pigs. This seeming similarity leads many to wonder – are hippos and pigs actually related?
If you’re short on time, here’s a quick answer to your question: while hippos and pigs share some superficial physical traits, they are not closely related. Hippos are large semi-aquatic mammals native to sub-Saharan Africa. They are most closely related to cetaceans like whales and dolphins.
In contrast, pigs are omnivorous farm animals that belong to the pig family Suidae. The two species belong to completely different branches of the mammalian family tree.
In this in-depth article, we’ll take a close look at the evolutionary history and biological classifications of hippos and pigs to understand their relationship in detail. We’ll compare their anatomy, behavior, genetics, and fossil record to get to the bottom of this question once and for all.
Taxonomy and Evolutionary Origins
Hippopotamus Classification
The hippopotamus belongs to the family Hippopotamidae, which includes two extant species: the common hippopotamus (Hippopotamus amphibius) and the pygmy hippopotamus (Choeropsis liberiensis). Hippos are part of the order Artiodactyla, making them close relatives of pigs, sheep, deer, antelopes and giraffes.
Their closest living relative is the pygmy hippopotamus, while their closest extinct relatives include species like Hexaprotodon that were very common in Europe and Asia during the Miocene.
Common hippos likely split from pygmy hippos around 8 million years ago during the late Miocene epoch when climate change caused forests to expand, isolating pygmy hippos in West Africa while the common ancestor of common hippos spread across sub-Saharan Africa.
Pig Classification
Pigs belong to the Suidae family, which contains pig-like species that originated around 40 million years ago during the late Eocene in what is now South and Southeast Asia. Today, there are 17 extant species divided into between 5-7 genera depending on the taxonomic classification used.
This includes feral and domestic pigs (Sus scrofa) like those found on farms.
Pigs are part of the Cetartiodactyla order along with hippos, whales and dolphins. However, they are placed in the Suina suborder rather than Tylopoda like hippos. This reflects the evolutionary split between pigs and hippos over 55 million years ago despite some similar ecological niches and behaviors.
When Did Hippos and Pigs Diverge?
Researchers have traced back the evolutionary split between Suidae (pigs) and Hippopotamidae (hippos) to around 55 million years ago during the late Paleocene or early Eocene based on genetic evidence and the fossil record.
Back then the common ancestors of both groups were small, deer-like forest dwellers. Their lineages then diverged and adapted to new environments over tens of millions of years.
While hippos later adapted to life in rivers and lakes across Africa around 35 million years ago, early pig-like creatures spread across Afro-Eurasia and diversified. This long evolutionary separation is why hippos and pigs are classified differently despite some behavioral and biomechanical similarities.
Anatomy and Morphology
Skeletal Structure
The skeletal structures of hippos and pigs share some similarities but also have distinct differences. Both animals have a tetrapod skeletal structure with front and hind limbs. However, the hippo skeleton is much more dense and robust to support their enormous body mass, which can reach over 3,000 lbs.
Hippos have shorter, thicker limb bones compared to pigs. Their skulls also differ, with hippos having elongated snouts and jaws compared to the more tapered, pointed snouts of pigs.
Teeth and Jaws
Hippos and pigs have adapted their teeth and jaws to suit their distinct diets. Hippos are herbivores that graze on grass and aquatic plants. They have large, wide molars to grind up tough plant matter. Their jaws also hinge open up to 150 degrees.
In contrast, pigs are omnivores with a mix of incisors and molars suited for an omnivorous diet. Their jaws have a more restricted range of motion compared to hippos.
Skin and Body Covering
Both species have thick skin and limited hair covering their bodies. A hippo’s skin can be up to 2 inches thick, helping protect them from cuts and abrasions. This thickness also helps regulate body temperature and buoyancy in the water. Pigs have bristly hairs covering most of their body.
However, pigs do not rely on hair for insulation like some mammals. Instead, they mostly rely on subcutaneous fat to help conserve heat. Both animals also possess sebaceous glands that secrete an oily substance providing waterproofing and sun protection.
Habitat and Behavior
Diet and Feeding
When it comes to their dining habits, hippos and pigs have some similarities but also key differences. Both species are omnivorous and spend much of their time grazing and foraging for food. However, the hippo is much more aquatic, spending up to 16 hours a day in rivers and lakes and feeding mostly on grass.
According to a National Geographic report, hippos can consume over 80 lbs (35 kg) of grass each night! In contrast, pigs tend to feed more on roots, fruits, seeds, fungi, and even small animals when available.
An interesting feeding adaptation in hippos is their wide mouth and specialized lips that help them efficiently clip grass while mostly staying submerged. Pigs have a superior sense of smell to help sniff out food sources.
So while the two species have varied diets, they employ special techniques to obtain plenty of nutrients.
Social Structure
When it comes to group living, hippos tend to be much more social and gather in larger pods called bloats. These can consist of up to 100 hippos! The groups are matriarchal, led by the dominant female. Pigs form smaller sounders of around 20 individuals with variability in leadership between males and females.
One advantage to the hippo’s larger groups could be safety in numbers against predators in their river habitats. 😱
The young of both species also show similarities and differences. Baby hippos will nurse for 8 months and can suckle underwater, a feat no other mammal can match! Piglets are also ready to rummage around for food quite early.
So family ties and nurturing of the young seem important to both species’ social structures.
Mating and Reproduction
When it comes to making little hippos and pigs, there are some amusing parallels. Male hippos battle fiercely for territory and access to females when it’s time to mate. Dominant 800-pound males can hold prime river stretch territory where females will come to give birth.
Think of these spots as ancient hippo singles bars! 😆 Male pigs also fiercely compete for sows in heat and the largest boars typically dominate.
In terms of reproduction, the hippo again shows its affinity for the water. Females give birth underwater to a 25-60 pound calf that needs to paddle to the surface to take its first breath! Piglets also emerge ready to move around just hours after birth.
So both species invest heavily in mating battles and births produce fast-developing young.
Genetics and the Fossil Record
Genetic Analysis
Recent genetic analysis has shed light on the evolutionary relationship between hippos and pigs. By sequencing and comparing the genomes of modern hippopotamuses and pigs, scientists have found evidence that the two species share a common ancestor from around 55 million years ago (Hernandez Fernandez and Vrba, 2005).
This ancestor was likely a small, pig-like omnivore that foraged in forests. Over time, different populations of this ancestral species adapted to different ecological niches and evolved into the distinct species we know today.
One key genetic difference between hippos and pigs is in a gene called SOX9, which controls development of the skeleton. Mutations in this gene led to major changes in the skulls and teeth of ancient hippos compared to pigs (Nikaido et al., 1999).
Hippos also lack a gene called TLR5 that is present in pigs and allows them to recognize certain bacterial proteins. This may be related to the different diets adopted by each lineage (Zhang et al., 2016).
While hippos and pigs look quite different today, their genomes reveal they are in fact evolutionary cousins. Genetic analysis allows us to peer into the distant past and uncover the hidden biological ties between distinct species over millions of years.
The high degree of molecular similarity between hippos and pigs provides firm genetic evidence that they shared a common ancestor not too far back in evolutionary time.
Ancient Ancestors
The fossil record also preserves intriguing evidence of the evolutionary relationship between hippos and pigs. While the earliest definitive hippo fossils date to around 16 million years ago, molecular clock estimates suggest the hippo lineage diverged from other artiodactyls around 55 million years ago (Boisserie et al., 2005).
The identity of the specific ancestral species remains mysterious, but several small, four-toed, omnivorous mammals from this period could be candidates.
One of the most intriguing is Anthracotherium, an early pig-like ungulate that inhabited Eurasia and Africa during the Eocene, between 56 and 33.9 million years ago. It had an elongated snout and small semiaquatic body adapted to living in swamps and riparian forests (Pickford, 2006).
Genetic studies suggest this animal was close to the common ancestor of hippos, pigs, and ruminants (Hassanin et al., 2012). Its mix of pig-like and hippo-like traits makes it a fascinating potential transitional fossil between these two lineages.
Other fossils provide snapshots of the evolutionary transition from dog-like omnivores to semi-aquatic grazers. Kolpochoerus was an early pig that lived 23 to 5.3 million years ago in Africa and the Middle East.
It had an elongated skull and small hippo-like teeth for grazing river plants (Haile-Selassie and Simpson, 2013). These and other fossils help reveal the step-wise progression of adaptations that eventually gave rise to the mighty hippopotamus.
The fossil record fills in gaps in our understanding of hippo evolution and substantiates their close genetic relationship with pigs. Together, fossils and DNA tell the story of how a small omnivorous mammal adapted to a semi-aquatic lifestyle and evolved into one of the most unique large mammals of today.
The hippo’s evolutionary journey closely parallels many other stories of adaptation and divergence revealed through paleontology and genetics.
Conclusion
While hippos and pigs may look somewhat alike at first glance, a deeper investigation reveals they are only distant relatives. Hippos are closely related to cetaceans, while pigs belong to a separate branch of artiodactyls.
Major differences in their anatomy, genetics, behavior, and evolutionary origins confirm they diverged tens of millions of years ago.
So the next time you see a portly hippo lounging in the water, remember – despite appearances, it’s not just a giant bald pig! With their unique semi-aquatic lifestyle and evolutionary history, hippos are a fascinating species in their own right.
