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Why do almost all vertebrates have tails, even when they don鈥檛 seem useful, but apes and frogs don鈥檛?

Stephen Johnson
Eugene, Oregon, US

All non-aquatic vertebrates descend from a common ancestor, a tetrapod of approximately 360 million years ago 鈥� which possessed a tail. The existence of the tail was primarily for balance, particularly when using only the rear legs, such as when feeding in trees. However, the tail evolved to have many uses, for example allowing primates extreme agility in the arboreal portions of forests.

It isn鈥檛 true that apes and frogs don鈥檛 have tails. Frogs have five different metamorphic life stages and possess a tail when they are tadpoles, tadpoles with legs and froglets. In fact, frogs couldn鈥檛 survive without their tails (and gills) before they move to their final stage on land.

All primates have tails, including humans. Human tails aren鈥檛 external, but consist of three to five fused vertebrae known as the coccyx, or tailbone.

Some might think the coccyx is vestigial and serves no purpose, but, in fact, it has several important uses. Many muscles attach to the coccyx, and it provides support for important structures, such as the anus and the vagina. It also provides support of the rest of the body when sitting.

We may not be able to wag our tails when happy, but we are very unhappy if anything happens to our tailbone.

Nicole Keshav
Cambridge, UK

Apes and frogs have vestigial tails in the embryonic stages. The tails don鈥檛 develop because they aren鈥檛 needed by the adults.

Will this change in the distant future? It is certainly possible!

Garry Trethewey
Arkaroola, South Australia

There are several approaches to answering this question.

Let鈥檚 look at 鈥渆ven when they don鈥檛 seem useful鈥�. Evolutionary theory describes a great game of trade-offs. Overall, any feature that persists must have more benefits than costs.

As an example, a bearded dragon incurs the cost of growing and maintaining an extra 10 per cent of its body (due to its tail) and the cost of having to run further into a crack in a rock to be safe from capture.

It appears to me that being tailless would be an advantage to a bearded dragon. But that only indicates my lack of knowledge.

Let鈥檚 look at why apes and frogs don鈥檛 have tails. They actually do possess much reduced tails. Humans develop a tail until about eight weeks of gestation, after which it stops developing and remains as a coccyx 鈥� our tailbone. This is a group of three to five vertebrae fused into one to five bones. This lack of definition suggests very minimal function.

Back to evolutionary theory. At some time, and in some environmental circumstances, apes and frogs with smaller tails did better than those with bigger tails.

Once the machinery for developing a functional tail had disappeared, it is unlikely that it would re-evolve from scratch.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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