IS CAUDAL LURING A HAZARD FOR YOUNG DEATH ADDERS?

Raymond Hoser
PO Box 599
Doncaster, Victoria, 3108, Australia
Phone: +61 3 9812 3322, Fax: +61 3 9812 3355, Mobile: +61 3 0412-777211, E-mail: adder@smuggled.com

Originally published in Monitor – Journal of The Victorian Herpetological Society 16(1)(June 2007):21-22.

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.

All 15 recognised species have a relatively stout body, ambush predation and use of a caudal lure (Hoser 1995).

In Death Adders this 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).

Observations of captive Death Adders over a 30 year period to end 2006 has indicated that small and neonate Death Adders (Genus Acanthophis) do not use their tail as a caudal lure to anywhere near the same extent as large, heavier adults.

While the bulk of my observations have been primarily with the species A. antarcticus, I have observed a similar trend in individuals of all described species from Australia (as per Hoser 2002) as well as in some from outside Australia.

Included here are long-term captives and snakes raised from birth to adult.

By way of example, none of three newborn female death adders (A. antarcticus) born on 4 February 2002 were seen using their tails as lures until August 2002.  Even then the observation in one the snakes was unusual in that luring was not repeated in that snake commonly and it was some months before others also regularly used their tails as caudal lures.

By early 2003 when the three snakes were nearly 50 cm in length, caudal luring became fairly routine and became even more pronounced as the snakes approached 70 cm in mid 2003.  (The snakes were growing at about 5 cm a month).

As of July 2003, two neonate Death Adders (A. antarcticus) and four neonate Top-end Death Adders (A. cummingi) (two died in June from a virus since identified via electron microscopy as a reovirus), all born in February 2003 had not yet been seen using their tail as a lure.

Worrell (1972) and others have noted young or neonate Death Adders using their tail as a lure when hungry.

Notwithstanding these printed claims, my own observations indicate that while neonate Death Adders will sometimes twitch and wriggle their tail as a lure to attract food when hungry, this is relatively uncommon and remains so for some time after birth.

It's also been observed by myself that while Death Adders of all ages will sit in a characteristic position of tail near head, this situation is far more commonly adhered to in adults than young specimens.

An adult not seen in this position can as a trend be deduced not to be hungry.  By contrast, neonates not in this position may be.

Notwithstanding this, caudal luring becomes common as Death Adders approach maturity (at around 60 cm total length).

In the captive situation, Death Adders soon learn that the keeper is the source of their food and will increase caudal luring in response to the keeper looking in the cage, moving their hand over the cage, moving the cage itself or similar.

In fact adults will frantically twitch and wriggle their tails in anticipation of food, while this is relatively rare in young snakes.

The question is; why the difference between the ages?

Some other facts of relevance follow:

It is also noted that in the wild and sometimes in captivity, Death Adders have an ontogenetic shift in dietary preferences.

Many young Death Adders have a strong preference for lizards, shifting towards a preference for rodents and birds as they age.  This is masked somewhat in captivity as most snakes eat what the keeper feeds them (even if this is by force or assist feeding).

However the trend towards warm-blooded food in wild specimens is evident from my own inspections of road-killed Death Adders in various parts of Australia.

It is also a reflection of food availability in terms of the size of the Death Adder itself.

Another notable aspect of Death Adder behavior gained from wild specimens is seen in the activity patterns.

Adult females are sedentary and rarely move. Adult males are likewise, but are more commonly seen active at night.

The presumed difference is because the males are looking for mates.

In terms of Sydney Death Adders, this has been confirmed by myself in the 1970's and 1980's on the basis of the following:

·         Wild male Death Adders are active and move about in tandem with their counterparts in captivity.  The movement is often in response to weather conditions and time of year, noting that temperature cycles in captivity ran more-or-less in tandem with those in the wild.

·         These wild snakes when brought into captivity have no interest in food, but will immediately mount and mate with any available females.  Hence feeding being ruled out as a reason for the movement.

·         The same mating activity is seen in captive adult males that become unusually mobile.

·         In other words, and with rare exceptions, the only activity commonly seen in adult Death Adders in the wild stems from mating activity and not active foraging for food.

Contrary to this, young immature Death Adders are commonly seen active at night.  While the sex ratio of adults found crossing roads at night is heavily weighted in favor of males, further corroborating the assertions of fact above, the sex ratio of immature snakes is 50:50.

As mating cannot be a consideration for these snakes the only remaining consideration can be that movement is due to a search for food.

This hypothesis, in part goes against what's known about Death Adders being ambush predators.

We also know that thermoregulation is not at issue as the snakes are active at night, meaning that the wide-ranging activity observed is not immediately a result of a need to find warmer or cooler sites and that the general environmental temperature at ground level tends to be fairly even.

Could the increased foraging by young Death Adders indicate that these snakes are not always ambush predators? Does it indicate that they may also actively stalk and capture their prey?

My opinion is yes. 

This is also reflected in younger captive Death Adders which will when hungry be seen actively moving about their cages.

Hungry adults by contrast rarely do.

All the above indicates that the lack of observation of caudal luring in smaller Death Adders is not something anomalous in terms of my observations or the captive situation, but rather an actual appraisal of the reality in both captivity and the wild.

Assuming this to be so, it becomes clear that there must have been some selective pressure forcing Death Adders to tend to curtail caudal luring when young and increase it as they gain size and weight.

The question then becomes what is this pressure?

My guess is that a small neonate snake using it's lure will run the risk of being pounced upon by a predator either too large for it to eat or otherwise a danger to the snake.

By way of example a 100 gram bird pouncing on a young Death Adder's tail, (noting the snake may weigh less than 10 grams for some months after birth), may well grab the tail (and snake) and cause fatal injury to it.  A small snake biting the head of the incoming bird will not be able to stop the juggernaught of a 10 times larger bird heading straight for the tail.

While the snake may bite the bird, perhaps ultimately causing it's death, this would probably not stop the snake receiving one or more potentially fatal bites itself.

By contrast a 100 gram bird tackling the tail of a Death Adder itself weighing two or three times this is likely to find itself easily grounded by a sharp bite to the head.

In terms of even larger birds or mammals, these are not usually insectivorous and hence a caudal lure on a Death Adder of any size is unlikely to attract their attention in terms of potential food.

Alternatively, larger Death Adders may also be able to better fend off unsuitable animals attracted by their caudal luring.

In other words the only conclusion that can be drawn is that caudal luring may be a hazard for small Death Adders.  There appears to be no other basis for explaining why younger specimens don't do it as often as larger ones.

REFERENCES

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.

Worrell, E. R. 1972. Dangerous snakes of Australia and New Guinea. Angus and Robertson, Sydney, Australia. 65 pp.