Reptiles
Spider-tailed Horned Viper
Pseudocerastes urarachnoides · also called spider-tailed horned viper, Persian spider viper, false-horned viper
The spider-tailed horned viper wears a spider on the end of its tail — a bulbous knob fringed with waving leg-like scales that is a good enough copy to fool a hunting bird into swooping down for a meal, and directly into the viper’s strike. It was so strange that for decades scientists assumed the “spider” was a deformity.
Overview
The spider-tailed viper is known only from the western Zagros Mountains near the Iran–Iraq border, on gypsum and limestone rock. It is 50–70 cm, sandy and lumpy-scaled to match the stone, with raised “horns” over the eyes. It hunts almost exclusively birds — especially small migratory songbirds passing through — by ambush.
The spider tail
The last few centimetres of the tail form a swollen oval bulb (a spider’s “abdomen”) flanked by 5–6 pairs of long forked scales that dangle and wave like legs. The buried snake holds its body dead still and crawls the “spider” across the rock; a bird stoops to grab it and is struck from centimetres away. Resident birds learn to avoid it, so the trick works best on naive migrants.
Fooled the experts
Described from a single 1968 specimen, the tail was long dismissed as a tumour or parasite. It took field observation and video in 2006–2015 to confirm it as a real, functional mimic — now considered one of the best cases of aggressive mimicry in any vertebrate.
Status
The spider-tailed viper is Data Deficient, with a tiny range, specialised habitat, and rising illegal collection for the exotic pet trade all cause for concern.
Spider-tailed Horned Viper: common questions
How does the spider-tailed viper's lure work?
The snake is an ambush predator that hunts birds, and its tail has evolved into a fishing lure shaped like a spider or a solifugid. It buries most of its body against a rock, matching the stone so well it is nearly invisible, and holds completely still except for the tail. The tail tip is a bulbous knob edged with long, dangling, leg-like scales, and the snake jerks and crawls it across the rock surface so that, to a bird, it looks exactly like a spider or small arthropod scuttling in the open. Insect-eating birds, particularly small migrants passing through the Zagros on migration, drop down to snatch the "spider" and are struck by the viper from just centimetres away, with almost no chance to react. Birds that live in the area year-round seem to learn to avoid it, so the lure works best on naive migrants.
Why did scientists not recognise this snake for so long?
The species was described in 1970 from a single specimen collected in 1968, and its bizarre tail was assumed to be an abnormality rather than a real feature. Various herpetologists over the years suggested the "spider" was a tumour, a birth defect, or the result of a tick or other parasite attached to the tail, and it was largely treated as a curiosity or a possible junior synonym of the closely related Persian horned viper. It was not until the 2000s that researchers found more animals in the field, confirmed that the tail structure is consistent, symmetrical, and species-wide, and then filmed the snake actually using the tail to lure and catch birds. Video published around 2015 removed any doubt that this is a genuine, functional caudal mimic, and it is now considered one of the most remarkable examples of aggressive mimicry in vertebrates.
Is the spider-tailed viper dangerous to humans?
It is a true viper with a venomous bite, and its venom is presumed to be similar in effect to that of other Pseudocerastes vipers, causing local pain, swelling, and tissue damage and some effects on blood clotting. There is very little clinical data because the snake is rare, lives in a remote and politically sensitive border region, and rarely encounters people. It is not aggressive and its whole strategy is to stay hidden and motionless. The realistic risk is to a herpetologist or collector handling one; ordinary travellers are very unlikely to meet the species. As with any viper bite, it would need proper medical treatment.
Is the spider-tailed viper endangered?
It is officially listed as Data Deficient, meaning there is not enough information to assign a proper threat category, but there is real concern for it. It has a very small known range, it depends on a specific rocky habitat and on migratory bird prey, and vipers in general are slow to mature and produce few young. Since it became famous, it has been targeted by the international exotic reptile trade, with animals smuggled out of Iran and offered for very high prices, and this collecting pressure on a small, localised population could be damaging. Iran protects the species and researchers have called for better survey work and enforcement.