Autotomy – just drop it and run

by Bill Bateman & Trish Fleming.  An organism only has to fail once in escaping a predator for its evolutionary fitness to be reduced to zero. Selection to avoid ending up as a meal is, therefore, intense. More intense than selection on avoiding missing a meal such that in the evolutionary arms race, prey tends to be ahead of predators – the ‘life vs. dinner’ differential.

An extreme way of avoiding being dinner is to sacrifice part of your body to escape predation: many taxa will voluntarily drop an appendage when caught or threatened by a predator, a process called ‘autotomy’ (self-cutting), often along a breakage plane to aid rapid shedding of the leg, tail, antenna etc.

One of our main study species – Lygodactylus capensis – the southern African dwarf gecko
One of our main study species – Lygodactylus capensis – the southern African dwarf gecko

Autotomy has fascinated us, since although the benefit (survival) seems huge, there are also costs. Losing a leg can make you slower; losing a tail can rob you of fat stores, or alter your locomotion.

We have now produced two major reviews (on invertebrates and lizards) and several empirical papers on how limb autotomy affects several species of invertebrate and how tail autotomy affects multiple species of lizard. A cricket that loses a leg may end up running slower and expending more energy in running, but its life is not shortened, nor do males become less competitive over females. Females, who mount the males to mate, often find it difficult to stay aboard and have to remount several times, but this is a price that is cheap in comparison to death. Equally, geckos that lose their tail run slower, and find it more difficult to climb vertical surfaces without their tail prop to help them, but again, this is better than being dead.

Most people are unaware that choosing to lose a limb or a tail really is a choice: crickets that have lost a leg take longer to sacrifice a subsequent one; female crickets that have mated will more rapidly autotomise a forelimb that bears their ear than will a virgin female that still needs to track down a singing male. Tame lizards can be handled without the risk of them autotomising their tail, and larger lizards that can fight with teeth and claws are less likely to rapidly shed a tail than are smaller defenceless ones.

This last observation was the main subject of a recent paper (‘Telling tails’) reviewing the link between intrinsic factors (body size and form) with the incidence of lost/regenerating tails and relative tail length. This review compared data for lizard species from across the globe. For 350 lizard species, we found that more delicate species have relatively longer tails. Longer-tailed lizards also tend to be climbing, active foraging, or diurnal species.

A long-tailed pygopodid – Pletholax gracilis
A long-tailed pygopodid – Pletholax gracilis

The species with the highest incidence of tail loss records were nocturnal lizards, larger skinks, climbing geckos and actively-foraging iguanids. Because caudal autotomy is an anti-predator defense, we assume that the incidence of lost or regenerating tails in natural populations reflects the different predation pressures that populations are subject to. This study clearly indicates that we need to also consider these intrinsic factors.

We are not done with autotomy: we still want to explore the multifarious costs and benefits that come with this extreme anti-predator escape response.

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