Male hippopotamuses are an uncommon dimension. Amongst mammals, males are often a lot bigger than females, however in hippos the sexes have surprisingly related sized our bodies. All besides the jaws and tusks, that are a lot larger in males.
Our latest analysis, utilizing information uncovered from 60 years in the past on almost 3,000 hippos, means that their semi-aquatic behaviour could play a key function in minimising the scale variations between the sexes.
Comparable huge mammals like elephants, lions and gorillas all show huge variations in dimension between the sexes. The record-holding male southern elephant seal can weigh as much as a staggering 3700kg, six instances bigger than the feminine (round 600kg).
Not like these species, male hippos contest territory and struggle over females within the water. (Even elephant seals fiercely defend their harems on land.) Water reduces the benefit of huge physique dimension in a territorial contest. And visually sizing up your rival is difficult when they’re nearly totally submerged. So male hippos generally sign dominance by yawning and displaying the gape of their jaw and the scale of their tusks, which look like extra necessary than physique dimension.
Hippos are one of many largest land mammals however devour comparatively small quantities of meals because of a gradual digestion price, with analysis suggesting that they might have already got reached the utmost physique dimension that’s bodily doable with their physiology. In addition they generally reside in massive teams, and meals sources could be scarce. Evolution might due to this fact favour females which are bigger bodied to allow them to compete for meals and water.
The hippo is among the most well-known massive mammal species, and could be recognized by youngsters at an early age. However hippos are difficult animals to check. They spend most of their time in water solely rising at night time to feed. Particular person identification is tough, and they’re notoriously aggressive. In addition they have a fearsome repute throughout the African continent.
As a part of our analysis, we had been lucky to accumulate a uncommon dataset on hippo physique dimension, following an electronic mail from David Walton, a scientist on the British Antarctic Survey. His pal, the famend zoologist and Cambridge College professor Richard Legal guidelines, died in 2014.
Legal guidelines left a powerful scientific legacy from 60 years of analysis starting from Antarctica to the east African savannahs. His workplace was stacked excessive with notebooks and maps, which had been vulnerable to being thrown out if we didn’t need them.
We drove to Cambridge and got here residence with piles of notebooks containing uncommon and detailed measurements of two,994 hippos collected from 1961 to 1966 in Uganda, all neatly recorded utilizing imperial models. Legal guidelines had beforehand used the information to ascertain the age of every animal utilizing their tooth dimension and put on.
We used the information to precisely discover the expansion and physique sizes of the sexes for the primary time. Two grasp’s college students at Bangor College labored to digitise and convert it to metric models and start the evaluation.
The final time Earth was this sizzling hippos lived in Britain (that is 130,000 years in the past)
On condition that male hippos compete fiercely for entry to females, we totally anticipated that grownup males can be considerably bigger in physique dimension and have larger tusks. It got here as a shock to find that males solely weighed on common 5% greater than females. Examine this with the African elephant, the place mature grownup males typically weigh twice that of a completely grown feminine.
Intriguingly whereas physique sizes had been very related, the information confirmed male hippos had a lot bigger jaws and tusks than the females. These can be utilized to lethal impact throughout fights over territory. The tusks of an grownup male hippo had been nearly double the scale of these of an grownup feminine (typically exceeding 2kg).
Male mammals are thought to have developed a lot bigger physique and weapon (tusks, horns, antlers) dimension than that of females as a result of totally different mating methods of the sexes. Females make investments an enormous period of time and vitality on being pregnant, lactation and weaning, whereas male enter often ends with profitable copulation.
In species the place a single male can mate with a number of females, essentially the most dominant particular person will possible father essentially the most offspring. Male elephant seals generally have harems numbering 40-50 females. As such, massive physique dimension is a wonderful solution to get an edge over your rivals.
Nonetheless, on account of discovering this 60-year-old dataset, we now know much more about why the enigmatic hippo differs on this respect and this fascinating side of evolutionary biology. Our findings have revealed that intense male competitors doesn’t all the time lead to bigger physique dimension. In reality, the variations between men and women are additionally very a lot managed by their atmosphere, weight loss plan, and physiology.