HEAD RETRACTION AS A DEFENCE AND OTHER HABITS OF DEATH ADDERS, GENUS ACANTHOPHIS (SERPENTES:ELAPIDAE)
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Park Orchards, Victoria, 3114, Australia.
Originally published in December 2003 in The Herptile:Journal of the International Herpetological Society 28(4):137-140.
Death Adders (Genus Acanthophis) are unique among the family Elapidae in that their appearance and biological habits have evolved in a manner convergent with viperid snakes.
Included is the stout body, ambush predation as a feeding strategy and use of a caudal lure as part of their ambush predation (Hoser 1995).
In Death Adders the lure takes the form of a modified scale at the end of the tail terminating in a spine. The latter part of the tail is often of a different colour to the rest of the snake, including being sometimes white, cream, or black (see Hoser 1989 for photographs of examples).
The use of a caudal lure in Death Adders is well-known and has been repeatedly documented in the literature (including for example Carpenter, Murphy and Carpenter, 1978, Chiszar, Boyer, Lee, Murphy, and Radcliffe, 1990).
In essence the luring involves the wriggling and other sometimes slow and deliberate movement of the tail, sometimes in a subtle manner, but when the snake is particularly aroused this may become a frenzied wriggle and usually remains so for as long as the snake is aroused or aware of a food item in the vicinity. In the captive situation this is often when the keeper makes an appearance near the snake's cage, the snake obviously realizing the connection between keeper and food.
Such observations are typical in captive adults, including my own spanning a 30 year period.
In captivity and in the wild Death Adders like to hide their body under cover from where they will rest and feed.
Typically the resting position is with the snake in loose horse-shoe shape or variant of it, the end result being the placement of the head immediately adjacent to the tail, with the tail placed in front of the head, from where a strike can be made.
As a potential food item seeks the movement of the tail the snake will strike at the prey and grab it. That's the theory anyway and is probably what occurs in the wild.
Notwithstanding this, in captivity, domestic rodents or fish are unlikely to be attracted to a Death Adder's lure, even if wriggling, especially if the rodent or fish is already dead!
The speed of the strike is usually so fast as to prevent the incoming animal from actually getting a grip on the wriggling tail.
In captivity Death Adders feed by day. Based on the food items taken both in captivity and in the wild, in particular small lizards it is evident that Death Adders also feed during the day in the wild as these prey species are only diurnally active and are not otherwise 'stalked' by the snakes.
Nocturnal movement by Death Adders is observed by herpetologists, and in stark contrast to the lack of diurnal movement activity by these snakes.
While the literature indicates that these snakes are nocturnal, this may not in fact be so. Instead the situation may well be that these snakes are both nocturnal and diurnal and that the major factor for the snakes is the type of activity done by day or night.
Based on my own records nocturnal movement over distances, including roads, usually appears to relate to two main activities:
The thick-set relatively clumsy build of Death Adders would make them particularly vulnerable to predation and hence the propensity of these snakes to move about under cover of darkness.
In captivity, the resting and feeding behavior of Death Adders can be readily observed.
Notable is that while the preferred resting position is with the body completely concealed, the head may or may not be concealed. For the head to be exposed is particularly common in hungry specimens resting in leaf-litter cover (as is sometimes paralleled in wild specimens seen).
The reason is presumed to be so that the snake can get an uninterrupted strike at it's prey.
In the period 2002-2003, I held a number of Death Adders of various species as described by Hoser (2002), including A. antarcticus, A. cummingi, A. hawkei and A. woolfi in 30 cm long plastic containers known as "Click Clacks" (manufactured in New Zealand).
The snakes ranged from adult to juvenile.
The substrate of the cages was hardened clay (as in a rock-type surface), overlain with some loose gum leaves. Each cage had a heat gradient, via underneath Thermofilm heat mat and also had a water container.
The "Click Clack" containers themselves are coloured (opaque) save for the lids which are clear (see through).
It is only through these lids that either myself or the snakes can see one another.
When hungry the snakes would rest with their heads exposed and in the typical "adder position" with tail more or less in front of the head.
Usually all but the head and tail would be concealed under the leaves, but often the head was not, or it was only partly concealed.
However the following was noted.
If I were to pass my hand over the containers and within less than 15 cm of the snake's heads, all the Death Adders would withdraw their heads backwards (tightening any S-shape) and thereby putting it completely under the leaves.
In doing so the snake would rarely if ever move the leaves and no other part of the snake would move.
It was clear that this head-retraction was a defensive move to enable the snake to be completely concealed from a potential enemy.
The snake would not however puff itself up or flatten out in a defensive posture. In other words the snake would be doing it's best to remain completely invisible.
In other words, this head-retraction defence shouldn't be confused with the defensive strike posture, which occurs when a snake is agitated and pulls it's head back into a tight S-shape from which it can launch into a defensive strike.
By contrast when food was waved in front of a hungry Death Adder, the snake would flick it's tongue with increased frequency and move it's head and neck forward towards the food item. This would often lead to both the neck and head moving out from under cover.
Another fact previously unreported in relation to Death Adders is the excellent ability of a hungry snake to estimate it's strike range.
If for example a dead mouse is dropped in front of a hungry Death Adder's head it will only strike at it if it can actually bite it. Otherwise it won't bother attempting to, even if the rodent is dropped a few cm out of range.
However if the rodent is withdrawn and dropped within range the snake will bite with lightning speed.
Within this ambit is the ability of a Death Adder to bite an item travelling in the air and before it actually hits the ground.
In terms of the ability of a Death Adder to be able to accurately estimate it's strike range, I videotaped a feeding 62 cm Death Adder (A. antarcticus) in June 2003. A half-grown rat dropped just out of strike range was not struck at. The snake did however continue to wriggle it's tail. The snake did not move it's body.
However, as the rodent was withdrawn by use of tongs, the snake moved forward, but still didn't attempt to bite.
The tail twitching by this stage was frenzied. It was all wriggling on none of the more subtle, side movement of the tail as is sometimes seen.
When the rodent was dropped a bit closer to the snake's head and just within strike range, it took the item with the speed of strike Death Adders are well-known for.
If the rodent is left out of range, most hungry Death Adders will eventually crawl up to it and bite it, but it's not uncommon for this to take up to 30 minutes after it's been placed just a few cm out of range.
Hence the two newly reported facts in terms of Death Adder biology are as follows:
In terms of wild Death Adders, the following observations are important.
The fact that Death Adders do as a matter of course quietly withdraw their head under cover, thereby concealing themselves completely, goes a long way to explaining why these snakes are nearly impossible to find in the wild during the day.
Put simply they aren't seen.
Many people, myself included, have spent many hours unsuccessfully searching for these snakes during the day in the wild state.
The fact that stationary Death Adders won't attempt to strike unless they know they are going to bite confirms how well they are adapted to an ambush predation lifestyle and how important it is for them to get their food item on the first bite as if they fail, then there is little time it will stay around long enough for the snake to get a second chance.
Carpenter, C. C., Murphy, J. B. and Carpenter, G. C. 1978. Caudal luring in the Death Adder (Acanthophis antarcticus (Reptilia, serpentes, elapidae)). Journal of Herpetology 12:574-577.
Chiszar, D, Boyer, D., Lee, R., Murphy, J. B. and Radcliffe, C. W. 1990. Caudal luring in the southern death adder, Acanthophis antarcticus. Journal of Herpetology 24(3):253-260.
Hoser, R. T. 1989. Australian Reptiles and Frogs. Pierson and Co., Sydney, NSW, Australia:238 pp.
Hoser, R. T. 1995. Australia’s Death Adders, Genus Acanthophis. The Reptilian 3(4):7-21 and cover, 3(5):27-34.
Hoser, R. T. 2002. Death Adders (Genus Acanthophis): An updated overview, including descriptions of 3 new island species and 2 new Australian subspecies. Crocodilian - Journal of the Victorian Association of Amateur Herpetologists 4(1):5-11,16-22,24-30, front and back covers.
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