MadSci Network: General Biology

Re: How can ants carry so much weight in proportion to their size?

Date: Thu May 20 16:33:35 1999
Posted By: Rob Campbell, PhD Candidate, Biological Oceanography, University of British Columbia
Area of science: General Biology
ID: 926309712.Gb

Hi Josh-

That's a great question! If you look around on the internet, you'll find numerous references to ants being able to "lift up to 50 times their own weight", but nobody ever explains how, or where that number of "50 times" comes from. I haven't been able to find out where the number came from, but I can tell you why ants can lift so much.

The reason that ants can lift so much is because of scaling: ant muscles are no stronger than human muscles on a pull-for-pull basis, but the small size of ants gives them an advantage on how much muscle force they can produce.

A muscle is basically a bundle of fibres which can contract and create a pulling force- the amount of force produced by a muscle is proportional to the cross sectional area of that bundle of fibres. Consider a bundle of bungee cords- one bungee cord has a small cross sectional area, and doesn't exert much force (you wouldn't jump off a bridge with just one bungee cord to stop you!); a bundle of bungee cords, such as is used for bungee jumping, has a much greater cross sectional area, and exerts much more force (enough force to stop a bungee jumper from hitting the ground!). In human terms, you can think of a human bicep- the bigger the bicep, the larger the cross-sectional area, and the more force (or strength) that can be applied by that bicep. A fellow by the name of Wigglesworth, in 1972, looked at the strength of insect and vertebrate muscles, in terms of force per square centimetre, and found that they both exerted similar forces, so it's not that ant muscles are somehow stronger.

The reason that ants can lift so much is because body size (in terms of volume, which is closely related to mass) increases as a cube of length- while the cross sectional area of muscles increases as the square of length. So, as the size of an organism increases, its mass increases at a much greater rate than the cross-sectional area of its muscles, so those muscles have proportionately more mass to lift.

So, the reason ants can lift so much is because their small size means they don't have a large body mass that they must carry around- they have proportionately more muscle (in terms of that cross-sectional area) that they can use to lift heavy things. Conversely, humans are proportionately more massive, and have less muscle that can be applied to lifting heavy things. The diagram below will explain this:

You can see that as size increases from big to small, the difference between mass and the amount of force that can be produced by the muscles gets greater and greater. In effect then, we humans have all this extra mass we must lug around, and we have proportionately less muscle to apply force on it, which keeps us from being able to lift as much of our body weight as ants can.

Hope that helps!

Rob Campbell, MAD Scientist

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