Original Description: A. Gunther in 'Annals and Magazine of Natural History' 1895; Vol. 15. Series 6. "Notice of Reptiles and Batrachians collected in the Eastern Half of Tropical Africa" (523-529).
GenBank: Thrasops jacksonii
Identification: A big black tree snake with a large dark eye and short head (Gunther,1895). Thrasops jacksonii is rear fanged and mildly venomous. It is a large, slim, black snake which is arboreal and looks like a black Boomslang (Dispholidus typus). Restricted to the extreme northwest of Zambia. (Broadley, 2003)
Distribution: Tanzania, Kenya, Uganda, Rwanda, Burundi, E Democratic Republic of the Congo (Zaire), Congo (Brazzaville), Central African Republic, NW Zambia, Cameroon
Type locality: Kavirondo, Kenya.
Identification: It smells strongly of liquorice, especially if freshly sloughed. The pupil is round. The iris is black, so the whole eye appears black. The body is laterally compressed; the tail is long, a third of the total length. (Spawls, 2002)
Adults are uniform glossy black above and below; the throat may be grey or white. (2) Uniform black with a velvety appearance, they may have a few grey markings on the head shields and labials, with the chin and throat yellowish. (Broadley, 2003)
Juveniles are checked black and orange or yellow, above and below; the tail is spotted yellow; the head and neck are olive-green or brown. The juvenile colour changes when the snake is between 40 and 60 cm long, and some young adults may retain traces of the juvenile pattern. (Spawls, 2002)
- Juvenile measureing 445 mm, from Chambura (Bunyaruguru), Western Ankole; head above, olive, below pale greenish and greenish-yellow; first ten scale-rows behind head, olive; above generally, alternately mottled black and light brown or fawn, three to four scales width; below, black marked alternately deep yellow or creamy; tail above, black dotted dull yellowish. (Pitman, 1974)
- Another juvenile, from Kilembe (Eastern Ruqenzori), top head olive, neck yellow with black-tipped scales, sides orange; body above, black predominates with grops of yellowish or orange-yellow scales arranged alternately, more or less vertically barred appearance; throat bright yellow extending to about twelfth ventral; thence ventrals marked boldly black and yellow, alternately disposed each successive ventral, producing chequered effect; each subcaudal round yellow spot alternately inner and outer part scale. (Pitman, 1974)
- Kampala juvenile irregular yellow mottling on dorsum. Some quite large half-grown examples still retain juvenile variegated coloration and pattern. (Pitman, 1974)
Entire length 65 inches, of which the tail takes 20. (Gunther, 1895)
Maximum size: about 2.3 m, average 1.4 to 1.8 m (Spawls, 2002); may reach a maximum length of about 230 cm. Adults regularly reach 180 cm. (Broadley, 2003)
- "Entebbe and Mabira Forest specimens each measured more than 1829 mm.
- Female from Lake Nabugabo 1803 (1263 + 540) mm.
- Unsexed from Mt. Elgon, with tail tip missing 1950 + (1390 + 560 + ) mm.
- Ionides (1963) records neighboring Western Kenya (Kakamega) female 1851 (1304 + 547) mm.
- Ashe (Nairobi) largest measured 1854 mm.
- Largest handled, not precisely measured c. 2100 mm.
Schmidt (1923) largest (Belgian Congo) female 2160 (1550 + 610) mm and male (Belgian Congo) 1900 (1320+ 580) mm." (Pitman, 1974)
Hatchlings 32 to 35 cm. (2)
Sexual dimorphism: No significant difference between male (191-205) and female 184-209 ventral counts; significance in marked disparity between males 134-149 and females 125-142 subcaudal counts. No apparent difference in tail ratio between sexes. (Pitman, 1974)
Key to Central and West African species of the genus Thrasops:
- More than 188 ventrals –> 2
Fewer than 188 ventrals –> 3
2. 13 or 15 dorsal scale rows –>T. flavigularis
17 to 21 dorsal scale rows –>T. jacksonii
3. Dorsal scales keeled –>T. occidentalis
Dorsal scales smooth –> 4
4. 13 dorsal scale rows; cloacal scale single –>T. batesii
15 to 19 dorsal scale rows; cloacal scale divided –>T. aethiopissa (Chippaux, 2012)
Thrasops jacksonii Species Characteristics:
Ventrals: 178 to 211 (187-214, more than 180 west of the Gregory rift valley). (Spawls, 2002)
Dorsal Scale Rows: 19 (rarely 17 or 21) rows at midbody (Spawls, 2002); dorsal scales strongly overlapping, with single apical pits. (Broadley, 2003)
Midbody scales: 17 to 19 rows at midbody (sometimes 21). (Spawls, 2002)
Cloacal scales divided; subcaudals 129-155 (Spawls, 2002)
Preocular 1; postoculars 3; temporal 1+1; upper labials 8 (rarely 9), the fourth and fifth entering eye; lower labials 9-12; the first 4-5 contacting anterior chin shields. (Broadley, 2003)
The body scales are usually keeled in East African specimens. (Spawls, 2002)
Similar species: The juveniles are unmistakable. The only other snake with which adults could be confused is the black colour form of the Boomslang Dispholidus typus, from which it may be distinguished by the absence of large fangs under the eye (the Boomslang has such fangs). Although black Boomslangs are rare in East Africa, they occur in the same areas as Jackson’s Tree Snakes; Boomslangs are highly dangerous so suspected Jackon’s Tree Snakes should be treated with great caution until their identity is confirmed. (Spawls, 2002)
Forest, forest islands, woodland and reverine forest, from 600 to over 2400 m altitude. (Spawls, 2002)
Essentially arboreal, frequenting primary forest. (Pitman, 1974)
Diseases and Parasites
Parasites: Ticks Aponomma latum (Koch) and linguatulid Raillietiella boulengeri (Vaney and Sambon). (Pitman, 1974)
Life History and Behavior
Diurnal and arboreal, a superb climber, ascending high into trees, to 30 m or more. If approached or threatened in a tree and unable to slide away, it will launch itself off into space and its long light body enables it to move sideways while falling; when it lands it quickly slides away. Mara River specimens were seen to jump from trees into the water. If cornered, it will inflate its neck and anterior body like a Boomslang, move sideways and make huge, lunging strikes, looking very large and threatening. (Spawls, 2002)
Loveridge (1944) describes: ‘According to Christy this snake distends its neck like a cobra.’ Cansdale (Penguin 1955) ‘when annoyed or frightened it inflates the front half of its body to several times its size.’ Villiers (1963) also refers this behavior of West African Species Thrasops. Ashe (pers. Com.) noticed Nairobi Snake Park captive specimens are sharp-tempered, easily provoked, and the strike appears to aim at the person’s face rather than the hand about to grab it; confirms jacksonii blows out neck, possibly with less provocation than the boomslang. Inflated neck is considerably smaller than the boomslang, but impressive. (Pitman, 1974)
Diet: It is a generalist feeder; known prey items include arboreal lizards (especially chameleons), and mammals including bats. It raids nests for eggs, nestlings and adult birds, and it has been seen to drop out of a tree to catch a frog. (Spawls, 2002)
- In open-air Nairobi Snake Park, captive specimen fell deliberately 5 feet to ground to chase, seize and swallow frog (in 30 seconds). (Pitman, 1974)
Bird remains found in stomachs examined; also preys on arborieal rats and mice, chameleons and lizards. Evidently takes frogs if given the opportunity. At Busingiro, on the outskirts of the Budongo Forest, a large example was killed on a bungalow as it was hunting roosting bats (however, this may be a case of mistaken identity. Thasops jacksonii is not normally associated with dwellings and the large snake killed on the bungalow could have likely been Boiga blandingii which often enters houses andis a well-known bat hunter). Ionides (perrs. Com.) 1851 female (Kakamega) disgorged one fledged bird, one fledgling and 3 adult chameleons (chamaeleo b. bitaeniatus Fischer). (Pitman, 1974)
It lays between 7 and 12 eggs, roughly 1.5 x 3 cm. (Spawls, 2002)
Four gravid females taken Mt. Elgon between 14 December and 11 January contained respectively 7,8,10, and 12 eggs, varying in size 19 x 8 mm to 35 X 18 mm. (Pitman, 1974)
The eastern subspecies has a very restricted range, and much of the forest it inhabits is being rapidly felled, so it may need monitoring. (Spawls, 2002)
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