The weight of life: Cattle 40% of all mammals by mass, wild animals 6%

By, New Delhi
Mar 02, 2023 04:59 AM IST

Almost 40% of mammals by mass in the world are cattle, and with India accounting for around 30% of the world’s cattle according to the US Department of Agriculture, Indian cattle should account for around 12% of the mammalian biomass in the world

Almost 40% of mammals by mass in the world are cattle, and with India accounting for around 30% of the world’s cattle according to the US Department of Agriculture, Indian cattle should account for around 12% of the mammalian biomass in the world.

For Representation
For Representation

That may come as a surprise, but a new census shows a vanishing natural world in which it is humans who are throwing their weight around, literally. In addition to the cattle, humans themselves account for 36% of the world’s total mammalian biomass, and other domesticated animals for 18%. Only the remaining 6% is wild mammals.

To be sure, mammalian biomass is just a fraction of total animal biomass (6.5%), which itself is a fraction (0.47%) of the biomass on Earth; plants account for the most (over 80%). And within animals, insects account for the most (45%). There may be a scientific basis to the giant bugs that dominate B-movies after all.

But neither insects, nor Indian cows is the subject of the global census of wild mammal biomass, described by the Weizmann Institute of Science in Israel as the first of its kind, which was published on Monday in PNAS. The metric it uses is wet biomass, as opposed to dry biomass, which would have measured the biomass after removal of moisture content.

Wet biomass was calculated from population counts for some species and modelled for others, using an artificial intelligence-based tool.

“We were able to attain global population reports accounting for about half of the total biomass of wild land mammals. To infer the total mass for the remaining species, we used the available global population report to train a machine learning model based on species-specific properties, such as ranges, body size and diet,” professor Ron Milo, who heads a lab in Weizmann’s Plant and Environmental Sciences Department, said in an email response.

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The human hand

In 2020, a team from Milo’s lab calculated the mass of all human-made objects, ranging from skyscrapers to newspapers, and found that the total surpassed the total biomass on the planet, plants and animals combined.

The new study has now found that the biomass of domesticated animals has reached 630 million tonnes – 30 times the weight of all wild terrestrial mammals (approximately 20 million tonnes) and 15 times that of wild marine mammals (40 million tonnes, mostly whales of various species). The domination of domesticated animals, combined with the total human biomass of 390 million tonnes, now offers yet another perspective of humanity’s impact on the planet.

“The more we’re exposed to nature’s full splendour, be it through films, museums or nature tourism, the more we might be tempted to imagine that nature is an endless and inexhaustible resource. In reality, the weight of all remaining wild land mammals is less than 10% of humanity’s combined weight, which amounts to only about 6 pounds (about 2.7kg) of wild land mammal per person,” Milo said.

“In other words,” he added, “our research shows, in quantifiable terms, the magnitude of our influence and how our decisions and choices in the coming years will determine what’s left of nature for future generations.”

Cattle dominate

Of the 630 million tonnes domesticated mammal biomass, cattle alone contribute 416 million tonnes. Other mammals reared for meat and milk, such as buffaloes (68 million tonnes), sheep (39 mn tonnes), pigs (38 mn tonnes) and goats (32 mn tonnes) also figure high on the list, followed by pack animals such as horses, camels, and donkeys.

The total of 630 mn tonnes does not include pet dogs and cats, Milo said. Dogs have a total mass above 20 mn tonnes, similar to the combined biomass of all wild terrestrial mammals. Domestic cats come lower at about 2 mn tonnes, but even that is much higher than the combined biomass of all African savanna elephants (1.3 mn tonnes).

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Bambi leads in the wild

The reason cats outweigh the world’s largest land mammal is, to state the obvious, their higher numbers. African elephants, in fact, are only third among wild mammals in the biomass list.

The white-tailed deer, the species featured in the Walt Disney classic Bambi (1942), heads the list at 2.7 million tonnes for 45 million individuals. It is followed by the wild boar (1.9 million tonnes, 30 million individuals). The 1.3 million tonnes for African elephants comes from half a million individuals.

Indian elephants are not among the top 10 wild terrestrial mammals, whose biomass accounts for 40 % of the 20 million tonnes. “The total biomass of Asian elephants is about one tenth that of African elephants,” Milo said.

The biomass of some other large Indian mammals, too, is low. “There are only a few thousand Royal Bengal tiger individuals, as well as a few thousand Indian rhinoceroses. Their contribution to the total biomass of wild land mammals is therefore smaller than one might expect given their world renown,” Milo said.

In sheer numbers, bats (1,200 species) account for a fifth of all land mammal species, and two-thirds of all wild land mammal individuals, but make up only % of the biomass of this section.

Rodents, which account for a higher number of species but fewer individuals than bats, combine for 16% of the total wild mammal biomass. Even-hoofed mammals, which include the Bambi species, account for nearly half the 20 mn tonnes.

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    Puzzles Editor Kabir Firaque is the author of the weekly column Problematics. A journalist for three decades, he also writes about science and mathematics.

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