We used 60-year-old notebooks to find out why hippo males have bigger tusks than females

The male hippopotamus is an unusual size. In mammals, it is usually males larger than in females, but in hippos the sexes have surprisingly similar sized bodies. All except the jaws and stitches, which are larger in males.

Our recent research, using data discovered from 60 years ago on nearly 3,000 hippos, suggests that their semi-aquatic behavior may play a major role in minimizing size differences between the genders.

Similar large mammals such as elephants, lions and gorillas all show large variations in size between the sexes. The record-holding male southern elephant seal can weigh up to a staggering 3700kg, six times larger than the female (around 600kg).

A male southern elephant seal in Whakatane, New Zealand.
User: Air55

Unlike these species, male hippos compete for territory and fight females in the water. (Even elephant seals fiercely defend their harems on the ground.) Water reduces the advantage of large body size in a territorial contest. And visually the size of your rival is challenging when they are almost submerged. So male hippos usually indicate dominance by yawning and showing the gap of their jaw and the size of their tusks, which appear to be more important than body size.

Hippos are one of the largest mammals on earth but eat relatively small amounts of food due to a slow rate of digestion, with research suggesting they already have reaches maximum body size that is possible in their physiology. They also usually live in large groups, and food resources can be scarce. Evolution may therefore favor women with larger bodies to compete for food and water.

The top of the hipp’s head came out of the water with a savannah background
Hippo is almost completely submerged under water, Amboseli National Park, Kenya.
Graeme Shannon

The hippo is one of the most popular large mammal species, and can be recognized by children at an early age. But hippos are challenging animals to study. They spend most of their time in the water that only rises at night to feed. Individual identification is difficult, and they are known to be aggressive. They also have a scary reputation throughout Continent of Africa.

Rival of the male hippo in the territory.

As part of our research, we were fortunate to obtain a rare dataset on the body size of the hippo, following an email from David Walton, a scientist at the British Antarctic Survey. His friend, renowned zoologist and Cambridge University professor Richard Laws, died in 2014.

The laws have left a fascinating scientific legacy from 60 years of research from Antarctica to the eastern savannahs of Africa. His office is highly stacked with notebooks and maps, which are in danger of being thrown away if we don’t like it.

Old notebooks containing measurements
The notebooks contain details of body size measurements for nearly 3000 hippos collected between 1961-1966 by Richard Laws and his research team.
Graeme Shannon

We drove to Cambridge and came home with piles of notebooks containing rare and detailed measurements of 2,994 hippos collected from 1961 to 1966 in Uganda, all neatly recorded using imperial units. Laws previously used the data to establish the age of each animal used the size and wear of their teeth.

We used the data to accurately detect the growth and body size of the sexes for the first time. Two master’s students at Bangor University worked to idolize and convert it into units of measurement and start analysis.

Read more:
The last time the Earth was these hot hippos lived in Britain (that was 130,000 years ago)

Since hippo males are fiercely competing for access with females, we highly expect adult males to be larger in body size and have a larger tusk. It was a surprise to discover that men weighed only on average 5% more than women. Compare this to the African elephant, where adult males tend to weigh twice as much as a fully adult female.

Intriguing while the body sizes were similar, the data showed that male hippos had larger jaws and tusks than females. It can be used to deadly effect during a territorial dispute. The tusks of an adult male hippo are almost twice the size of those of an adult female (often exceeding 2kg).

Male mammals are thought to develop larger body and weapon (tusks, horns, horns) size than females due to different approaches to gender integration. Women invest a large amount of time and energy in pregnancy, lactation and prevention, while male input usually ends in successful copying.

In species where a single male can marry multiple females, the most dominant individual is likely to father the most offspring. Male elephant seals are common the harems numbered 40-50 women. As such, large body size is a great way to gain an edge over your rival.

However, as a result of finding this 60-year-old dataset, we already know a lot about why the enigmatic hippo is different in this respect and the fascinating aspect of evolutionary biology. Our findings revealed that intense male competition does not always result in larger body size. In fact, the differences between men and women are also controlled by their environment, diet, and physiology.

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