What is the largest size an animal can be?
Mike Follows
Sutton Coldfield, West Midlands, UK
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It is usually claimed that the blue whale is the biggest animal that has ever lived on Earth. The largest recorded mass of a blue whale is around 190 tonnes, though a bigger individual has been observed that might have tipped the scales at 250 tonnes. However, this might yet be surpassed by a marine reptile that lived at the same time as the dinosaurs, called Ichthyotitan severnensis.
The so-called square-cube law, something first observed by Galileo Galilei in 1638, sets an upper limit to the size of land animals. If the size of an animal is scaled up, the cross-section of its bones increases by the square of the scaling factor and its volume (and hence its mass and weight) would be cubed, doubling the stress on its bones. This can be visualised with one die alongside eight dice arranged as a cube (two by two by two).
For example, doubling the height of an animal would mean the cross-section of its bones would increase by a factor of four, but its weight would increase by a factor of eight. Further increasing the size of the animal means that a threshold would be reached where the bones of an animal would be unable to support its body weight without breaking 鈥� or the animal would need to modify its behaviour, which is why elephants avoid jumping.
While coloniality does allow some organisms to reach huge sizes, the trade-off is that such life forms lack the means to move
In contrast, marine animals like the blue whale can take advantage of their buoyancy to scale up in size without significantly increasing the stress on their bones. Reptiles have slower metabolisms compared with mammals, so can divert a bigger proportion of their food intake to increase their body mass, which is why there were so many big dinosaurs.
In 1976, Edwin Salpeter and Carl Sagan co-wrote a speculating about the possibility of life in the atmosphere of Jupiter. The imagined ecosystem included 鈥渇loaters鈥�, kilometres in diameter, which would hitch a ride on the convection currents there.
While floaters are unlikely to be discovered on Jupiter, scientists in search of extraterrestrial life need to be open to the possibility of life forms that elude our imaginations.
Neale Monks
Berkhamsted, Hertfordshire, UK
Colonial organisms, formed from genetically identical but morphologically specialised units working together, can be very large. Arguably, some can be seen from space. For example, some corals can end up growing into massive reefs.
Likewise, social insects achieve similar end results by working cooperatively. In the case of these animals, such as bees and termites, the colony or nest is arguably the organism. If you learned the requirements of life at school, you may recall it is the colony, not the individual insect, that satisfies all seven criteria of life (movement, respiration, sensitivity, growth, reproduction, excretion and nutrition). The same holds true for all colonial organisms, including coral, sponge, bryozoan and siphonophore species.
The biggest corals are massive, with at least one exceptionally large Porites coral reaching 22 metres high and 8 m wide. Although corals do have a simple nervous system, it isn鈥檛 as fast or well coordinated as that in many other animals, so communication between polyps is relatively small and limited to simple reflexes. As they don鈥檛 move about and tend to rely on filter feeding or even symbiotic algae for nutrition, this probably isn鈥檛 a major issue for them. But it is probably fair to say that while coloniality does allow some organisms to reach remarkably large sizes, the trade-off is that such life forms lack the means to move around.
Social insects can form colonies containing mind-blowing numbers of individuals, stretching across dozens or hundreds of miles. But coordination between the individual insects, let alone separate colonies, is relatively slow. An ant that has discovered food has to walk back home and tell its sisters, then those ants will head back out following some sort of chemical trail.
While certainly clever, this isn鈥檛 anything like as fast as the hand-eye coordination we take for granted when we see something edible and lift it up to our mouth.
So, is there an uppermost limit on animal size? Probably, and one thing to consider is surely the rate at which information can be spread throughout the organism if it is going to be quick enough to elicit a useful and timely response.
Herman D鈥橦ondt
Sydney, Australia
If we move outside the animal kingdom, much larger organisms can be found.
Sequoias can be . An aspen tree in Utah called Pando cloned itself to form a 430,000-square-metre forest. A Posidonia australis 鈥渟eaweed鈥� in Australia鈥檚 Shark Bay cloned itself into a 180-square-kilometre patch of seagrass.
If life exists on another planet, it will be affected by that world鈥檚 gravity, with lower levels allowing animals to grow larger. And it is possible that evolution on other planets would come up with different body systems, with sizes that are anyone鈥檚 guess.
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