G l a z e r | 0

|

SCN3615 Hippopotamus behavior

An analysis of the behavior of the Hippopotamus while taking a closer look at the underlying theme of aggression in all aspects of hippo life.

December 5, 2012

G l a z e r | 1

Index

Introduction2

Literature Review

Background2

Habitat & Ecology3

Movement & Feeding 4

Communication 4

Sexual Behavior, Reproduction & Parental Care5

Social, Territorial & Anti-Predator Behavior6

Aggression 7

Discussion 8

Conclusion 11

G l a z e r | 2

Introduction

The hippopotamus amphibious, also known as just the hippo, is an

African mammal living only on the continent of Africa with many unique

behaviors. Hippopotamus derives from the Greek word, “river horse” which

can be attributed to the large mass of their bodies similar to a horse and

also their days spent in the water (National Geographic Online). The hippo

is the third largest animal, behind elephants and white rhinos (African

Wildlife). I chose to take a deep dive in to the behavior of this animal after a

friend of mine shared a story about how she was on safari in South Africa

when a herd of hippos stampeded their vehicle. This was a surprise to me,

as I originally thought hippos were calm creatures. After my research and

hearing this personal account, I have come to find they are if not actually

the most aggressive, among the most aggressive species in the animal

kingdom.. This analysis will cover a general background of the origin

species, physical characteristics, habitat & ecology, movement, feeding,

communication, reproduction, sexual behavior, social behavior, territorial

behavior, aggression & anti-predator behavior. While all facets of the

hippopotamus lifestyle will be investigated, particular attention will be

focused on their aggression which is an underlying theme through many

behaviors.

Literature Review

G l a z e r | 3

The hippopotamus amphibious, a part of the hippopotamide family, has

evolved from the still existing choeropsis liberiensis, otherwise known as

the pigmy hippo (Estes, 1999). The pigmy hippo is a forest dwelling

creature that only resides on land; it has been found that pigmy hippos have

common ancestors with pigs (Estes, 1991). Over time, the pigmy hippo

developed new attributes, such as complete aquatic abilities. This is a

perfect example of evolution as animals will adapt to their environment in

order to survive. Being able to function in the water has come to be very

important to the hippo for examples such as feeding, mating, etc. While

looking at both the pigmy hippo and the (water) hippo, they do appear

physically similar in some ways. The hippo is characterized by their barrel-

shaped, massive, brown or grayish purple smooth and naked skin, with

short legs (Estes 1999). The head’s features include an enormous muzzle,

eyes, nostrils and small ears at the top of the head. In addition, hippos have

short, muscular paddle shaped tails that play a large role in aggression that

will be further explained later. Hippos are so large that males can be as

heavy as 7,040 lbs and 66” tall. Typically, males weigh between 3,529-7,040

pounds while females range between 1,440 and 5,157 pounds (Estes 1999).

Habitat & Ecology

In the wild, the hippo is only found in very specific regions on the

continent of Africa, from the South of the Sahara from Guinea to the Ivory

Coast (Estes, 1999). This is also an example of evolution because at an

G l a z e r | 4

earlier time, hippos were more dispersed on multiple continents (Estes,

1999). Within these specific regions, hippos will find habitats where water

is plentiful and grazing can be accommodated. This usually means edges of

swamps or rivers where the water is deep enough to cover a good portion of

their bodies when submerged (Estes, 1999). Hippo days are spent in the

water for a very specific purpose; to avoid dehydration. There is a myth that

hippos “sweat blood” which derived from the red liquid that secretes itself

from glands on the body (Out of Africa). Though hippos release this fluid,

they do not have sweat glands to monitor their body temperature, thus

overheating very easily and needing the water’s benefits. Another reason

that living part time in the water is so crucial is because it is much easier

for a mother to nurse and protect their child. For this reason they seek out

bodies of water with gently sloping, firm bottoms.

Movement & Feeding

Hippos are in the rhythm of feeding on land at night, then returning to

the water to digest and rest during the daytime (Estes 1991). Though the

hippo spends the entirety of their day in the water, the species is still

classified as a land animal. On land, the mammal tends to travel in herds

from 10-15 at a time. Don’t let the short limbs and large body of this animal

fool you, research has shown that hippos have been tracked moving as fast

as 30 mph, which is faster than an Olympic sprinter (National Geographic).

On average, hippos will gallop up to 18 mph when threatened and half the

speed on a more normal basis. At night when feeding, the average hippo

G l a z e r | 5

will travel 6-10 miles to maximize grazing (National Geographic). With the

exception of dependent calves on their mothers, feeding is an independent

practice for the hippo. They are primarily herbivores and will consume

about 88 pounds of grass every evening with the muscular lips, around 50

cm wide (Estes, 1991). In water, the hippo will also gallop, yet mostly

stationary among groups for the majority of the day. Adult hippos can stay

underwater for up to 5 minutes if necessary (Estes, 1999). The animal has

also adapted to rise in her sleep involuntarily when needed (Estes, 1999).

Communication

In line with aggressive nature of the hippopotamus, it would make

sense if they administered loud noises, generally associated with anger in

society. In fact, the hippo has been recorded evoking sounds as loud as 113

or more decibels which are comparable to a jet engine or standing at the

stage of a loud rock concert (San Diego Zoo). Research shows that hippos

do the lion’s share of their communication in water as opposed to on land.

Specifically these sounds are referred to as grunts, chuffs and honks (Out of

Africa). Another technique that the hippo is known to do is to have their

jaw submerged in water while calling out a sound that has the ability to

travel through both water and to land (Out of Africa). While a deeper

significance to the purpose of these sounds is still unknown, researchers

speculate they are used to warn off predators like most other animals do.

Sexual Behavior, Reproduction & Parental Care

G l a z e r | 6

The next portion of the essay will touch on sexual behavior, which

leads to reproduction and ultimately the need for parental care. The process

begins when a bull, male hippo, will wander through a herd of females in

search of a mate. In order to show his interest, he will sniff their backside

and be sure to move carefully and show submission if the herd appears

disturbed (Estes, 1991). In some cases, female hippos will yawn to threaten

the males (see Aggression). Courtship is not apparent from the male as he

will immediately bring his new mate to the water where the entirety of

sexual behavior is executed. From there, the hippos will first clash jaws

before the bull forces the female face down underwater and mounts his

partner; this aggression is one of many examples of how hippos establish

dominance. The male will become quite aggressive and snap if the female

resists from being underwater, thus forcing her back down.

Female hippos do not conceive until 9 years of age. Unlike many other

animals whose breeding period aligns with the abundance of resources,

hippos have a relatively stable environment. For this reason, seasons do not

dictate their mating schedule, though there is a natural tendency for dry

season to be the primary period of gestation and thus rainy season with the

most births (Estes 1991). Once fertile, females may calve at 2 years

intervals with 8 month gestation periods (Estes 1991). One way that females

can show they are in heat is from a ritual called urine testing. (Estes 1991).

Females can show their reproductive status by urinating on command for

bulls, and the males are trained to identify the urine accordingly.

G l a z e r | 7

Once the hippo endures the 8 month gestation period, they prepare to

give birth to one baby hippo (Estes 1991). The female will isolate herself

during the birth period either on land or in shallow water. Within 10-14

days, the mother and newborn will rejoin their herd to proceed with normal

behavior (Estes 1991). The mother is the sole participant in parental care,

and has a very strong affinity with the calf. Their relationship is externally

visible through licks, nuzzles and other nurturing behavior.

Social, Territorial & Anti-Predator Behavior

The social and territorial behavior will be explored next. Much of this

subject is lacking information that is still continuing to be researched.

Hippos are solitary at night when foraging, though very social during the

day when in water. There is very close contact among pods of hippos, often

resting on one another they are so close. During dry season, there is heavier

crowding where packs increase drastically in size.

Male hippos are very territorial over their claim to land and water.

Mature bulls will control large section of river and lakes with exclusive

mating rights where no other males may copulate, for extended periods of

time: 8 years for lakes and 4 years for rivers (Estes 1999). Bulls will allow

other bachelors to enter as long as they pose no threat and agree to no

sexual activity in the alpha male’s domain. One example of territorial

behavior that the hippo will exhibit is called dung showering (Estes, 1991).

This aggressive behavior consists of the hippo extracting their feces then

G l a z e r | 8

proceeding to shower them over territory with help of the muscular tale for

power. This tells others to stray from this region unless they are ready for a

fight. Conversely, cow (female) herds have been observed and have an

entirely different dynamic that has yet to be explained.

Cow herds may not be as easily explained as bull herds, but one thing

is for certain in terms of female behavior; a mother will protect her young.

Females with offspring will fight to the death to keep predators, (primarily

lions, crocodiles & hyenas), sometimes even male hippos, away from their

calves (Estes 1999). The jaws of an angry hippo are not something a

predator would want to come in contact with; they are actually strong

enough to snap a crocodile in two with one chomp if in the right position to

do so. For this reason, predators will attack from the back and stray from

approaching anywhere near the jaw. Predators can only do so much from

the rear, so these fights can often lead to a prolonged standstill while they

compete for the upper hand. Hippos may not be as agile on land, thus

intentionally will attempt to redirect the fight to water to gain advantage.

Aggression

Arguably the most aggressive mammal of the animal kingdom, the

hippopotamus has many ways of demonstrating aggression both among

their kind and also to ward off predators. Oddly enough, hippos are

primarily herbivores and do not actually use their jaws to hunt, they are

solely for threat and dominance. Already discussed have been examples of

G l a z e r | 9

aggression in communication with roaring and grunting, dominant sexual

behavior as well as dung showering for territorial advertisement. Further

demonstrations of aggression are exhibited through rearing, lunging, water

scooping/head shaking, infanticide, yawning and tusk clashing (Estes 1991).

Tusk clashing is the act when two hippos will open their jaws for jaw to jaw

contact, and compete for who has their larger and stronger gape (Estes

1999). Yawning is another sign of aggression which is when a hippo will tilt

their head back and open their jaws as wide as possible exposing the

maximum span, implying their strength and intending to pose a threat;

females will often do this when defending their offspring (Estes 1999).With

canines up to 20 inches, hippos have been held responsible for more human

deaths than any other large animal because they are threatened so easily

and respond with such aggression to small disturbances (Estes 1999).

Discussion

After extensive research on the different behavior of the

hippopotamus, including communication, feeding, sexual behavior,

reproduction, parental care, social/territorial/anti-predator behavior and

aggression, it is clear that aggression is apparent in many contexts of the

day to day life for the hippo. The personal account I have of the previously

mentioned safari story is completely supported by all of the sources I have

used. I do not believe that any source could make an argument for why

hippos could be seen as tranquil creatures, with perhaps the exception of

the nurturing actions of the mother-child relationship. Whether or not

G l a z e r | 10

aggression is warranted will be uncovered in this discussion. Similar to

many societies, it appears that males tend to exhibit these dominant

characteristics more through territorial advertisement, though females are

not far behind when it comes to defending the young. In reading many

articles, I found that majority of the public is unaware to the ferocious

nature of these animals. ABC Nightline’s article touched on this briefly

saying not to mistake them for “coach potatoes, docile, or harmless” despite

their “bathtub” like structure (Harris, 2009). Reflecting on all of the

research I have gathered, I cannot fully understand why an herbivore would

be so hostile with no reason. It makes me wonder, why are they threatened

so easily? Why do they exhibit these threat displays even among their kind?

Hippo on hippo kills not common that I question why they feel the need to

have these dominant displays. The fact that females will show threat

displays to males when approaching their young was interesting to me as

this doesn’t seem cohesive with the rest of the social structure. Though

some research does show that hippos kill within their own, the incidents are

usually justified by disease or overpopulation as opposed to violence with no

justifiable reason attached.

On the same note relating to the concept of feeding, when the most

prominent feature of the hippo is their jaws which could kill with such ease,

why and how do they remain herbivores? I understand that some research

has shown that a hippo stomach is not necessarily designed for meat

consumption, but I argue that if that was the only source of food available,

G l a z e r | 11

wouldn’t their stomachs adapt to being able to ingest this type of food over

time? From all the reading I have done on feeding, it seems as though

hippos are in no way considering the optimality theory, where animals will

eat what provides the greatest fitness benefits with the least amount of

effort. Every night, hippos spend hours and walk miles in order to consume

88+ pounds of grass which is extremely time consuming and not remotely

efficient. This is so time consuming that they need to spend the whole day

relatively stationary and digesting to recuperate from the previous night’s

forage. Considering the jaw meets all necessary hunting qualifications and

they are already experienced in aggression, I found it odd that they do not

seek game that would provide more fitness benefits; even smaller animals

such as fish. I ponder how they have evolved to stray from what seems to be

the obvious choice of choosing meat over grass.

Another topic the literature review skimmed the surface of was the

evolution of the aquatic abilities of the common hippo. From the older

published research, circa 1990s, the only evolution of species mentioned

was the pig and pigmy hippo. Newer studies show that in fact the hippo may

actually be in closest relation to the whale (Feldhake 2005). In terms of the

methodology that was used to arrive at this conclusion, the author does a

good job explaining the different stages of evolution used to arrive here.

For me, it still seems like a stretch to suggest they could have evolved from

the same animal, when this said common ancestor died out during the ice

age and there is no complete information (UC Berkley, 2005). At first glance

G l a z e r | 12

I would say that the whale and hippo are an example of divergent evolution,

as they seem to become more and more dissimilar. At closer analysis,

because pieces are missing from the evolutional lineage it is hard to make

that conclusion, especially because hippos and whales do still have some

similar characteristics.

It is clear through my readings that the social structure and hierarchy

of cow herds are still very much left up for discussion. Much of the

observations that are being done are within captivity, which presents the

challenge of whether or not observing non-wild hippos would provide

different results. Why are they so hard to observe in the wild? I find it

particularly interesting that there is so much information on the males and

their territory but none on the females. I understand they have very

different roles in the hierarchy, but what are the cows doing that is so

different that no conclusions can be made? Most of the research I have

found simply states female social structure is hard to understand, but I did

not find many follow up studies on anyone trying to delve deeper or going

forward. For one, I think an obvious behavior that female herds execute is

the dilution effect. The way that they congregate in the water pods during

the day to protect the young shows that they are trying to lessen their

chances of being picked by any predators, in this case most susceptible to a

crocodile.

Conclusion

G l a z e r | 13

Hippopotamuses are clearly complex creatures that have very specific

behaviors that cannot be seen in any other species. This can be attributed to

their unique physical characteristics that allow them to do so (ie. tail /dung

showering or jaw/tusk clashing). Overall, within every category of behavior,

aggression of some kind makes an appearance. Though many would not

expect it, the seemingly adorable creature is very easily disturbed and will

react accordingly. With more research on the concentration of female social

systems, maybe we will gain more insight as to why there is a distant

relationship between males and females and also more about how females

operate in general.

G l a z e r | 14

References

Blowers, TE., J. Waterman, C. Kuhar and T. Bettinger, 2010. Social Behaviors within a group of captive female Hippopotamus amphibious. Volume 28 Issue 2 pp 287-294, Journal of Ethology < http://link.springer.com/article/10.1007%2Fs10164-009-0184-6?LI=true>

Estes, R.D. 1991. Chapter 13 Hippopotamuses. Pages 222-226 in The Behavior Guide to African Mammals: including hoofed mammals, carnivores, primates. University of California Press, Berkeley and Los Angeles, California and University of California Press, Ltd., Oxford England.

Estes, R.D. 1999. Chapter 17 Hippotamus. Pages 185-189 in The Safari Companion: A Guide to Watching African Mammals. Chelsea Green Publishing Company, White River Junction, Vermont.

Feldhake, Glenn, 2005. Hippos: Natural History & Conservation. Voyageur Press, Inc.

Harris, Dan. Up Close and Personal with the Hippos of Uganda. Aug 21, 2009. ABC Nightline http://abcnews.go.com/Nightline/story?id=8376416#.UL7o-uQ1mSo

Hippo. Out of Africa <http://www.outtoafrica.nl/animals/enghippo.html>

Hippopotamus. National Geographic Online <http://animals.nationalgeographic.com/animals/mammals/hippopotamus/>

Hippopotamus. African Wildlife Foundation <http://www.awf.org/content/wildlife/detail/hippopotamus>

How Do Hippos Communicate with Each Other and What Type of Sounds Do They Make? Big Site of Amazing Facts. <http://www.bigsiteofamazingfacts.com/how-do-hippos-communicate-with-each-other-and-what-type-of-sounds-do-they-make>

Mammals: Hippopotamus San Diego Zoo<http://www.sandiegozoo.org/animalbytes/t-hippopotamus.html>

UC Berkeley, 2005. Scientists find missing link between the whale and its closest relative, the hippo. Phys Org. <http://phys.org/news28 06.html>