Hemipenes, amazing copulatory organs of snakes

hemipenes rat snakeThe snake’s hemipenes
The hemipenes, the copulatory organs of male snakes, are wonderful creations. Snakes (and lizards) have two of them, both hidden in the base of the tail. When the males are sexually aroused, these structures are ‘everted’ and will pop out of the vent (cloaca). During copulation, only one of them is inserted into the female cloaca.
The surface of the hemipenes is often adorned with sharp spines and projections, arranged in rosettes. It is believed that their function is to prevent the hemipenis from slipping from the partner’s cloaca. However, in snakes with more or less smooth hemipenes, such as the Siamese Spitting Cobra, intermission is not impaired. Yet, there is a close relationship between the form of the hemipenes and the female cloaca, as has been adeptly summarized in Chris Mattison’s beautiful Encyclopedia of Snakes.
‘These [spines and projections] are thought to help the male snake to locate the cloacal opening of the female and to fix the hemipenis in place once copulation has begun. The hemipenes of some species are branched and in these species the female’s cloaca is also branched. Similarly, if the hemipenes are covered with spines, the female’s cloaca has a thicker wall than in those species in which spines are reduced or absent. Dissection of females has shown that in every case the internal shape of the cloaca corresponds closely to the shape of the hemipenes of males of the same species. This system therefore forms an isolating mechanism, preventing unrelated species from mating and the shape and structure of the hemipenes are of considerable interest to taxonomists as they form part of a ‘lock-and-key’ mechanism.’(1)

hemipenes rat snakes and bronzebackThe hemipenes have no enclosed sperm duct but instead an external groove, the sulcus spermaticus. Sperm is produced in the male’s testes, which are located in the body cavity, a little behind the stomach and liver and close to the small intestines. From the testes the sperm moves through the sperm duct and is released in the male’s cloaca, where during copulation the sulcus spermaticus of the everted hemipenis forms a channel that leads the sperm into the female’s cloaca. Females of a number of snake species are able to store sperm for a considerable length of time. In species that produce more than a single clutch during a breeding season, sperm storage may be used in fertilizing the second clutch.

In the late 19th century, American herpetologist Edward Drinker Cope called attention to the importance of the hemipenes in taxonomy. (2) The form and ornamentation of snake hemipenes may help clarifying the phylogenetic relationships between various groups. In this regard, some taxonomists even consider hemipenal structure as one of the major morphological characters.
Smith (1943) emphasized the importance of hemipenal morphology in the systematics of South-East Asian snakes. (3) Hundreds of snake studies have been devoted to the structure of snake hemipenes, partly summarized in Myers and Cadle, 2003 (4).
In general the tails of male snakes are relatively longer than the tails of females of the same species, as hemipenes and retractor muscles are accommodated within the tail. At the base, near the vent, the tails of males tend to be more robust. In many species the tail length/total length is a rather reliable indicator for the sex. For example, in northern Thailand’s spotted slug snakes (Pareas margaritophorus and P. macularius) the ratio usually ranges 13-17 per cent in females and 17-22 per cent in males. Such a difference, however, is less noticeable in arboreal snakes.

hemipenes pit-viper and cobraIt has become a standard procedure to measure tail length and total length in fresh (road-killed) specimens, as well as gently squeezing the base of the tail and massaging it towards the vent. In males, the latter will usually result to everting one or both hemipenes, that will pop out of the vent. A more refined method is to inject water or formalin with a hypodermic syringe into the base of the tail.
The presence of hemipenes, of course, is proof that the specimen is male. However, the absence of hemipenes during these procedures is no proof of the female gender. In damaged or dry specimens the hemipenes will not pop out. Juveniles usually have little developed hemipenes. On the other hand, the caudal scent gland papillae may be large in females and appear similar to hemipenes. These papillae, however, are always smaller; they may have a red tip but no visible blood vessels. Occasionally eggs squeezed out of the snake’s body are evidence that the individual is female.

As can be seen in the pictures, there is a truly amazing variety of forms and ornamentations of the hemipenes. The hemipenes of a number of species are forked. In the spotted slug snake Pareas macularius they are forked for about half of their length, but in the picture above (number 5) the forked parts still stick together. In the related species Pareas margaritophorus the appearance and forking of the hemipenes are rather similar.
Picture 7 below shows the completely everted hemipenis of Oligodon fasciolatus, a common kukri snake in northern Thailand. The forking is very evident.

hemipenes kukri snake and blackheadIn species within the same genus the hemipenes may show considerable variability. On the other hand, the hemipenes of truely related species are often quite similar. There is no relationship between the size and appearance of the hemipenes and the disposition or danger of the snake. For instance, the males of the gentle, slender Common Blackhead (Sibynophis collaris) possess rather fearsome hemipenes with a number of large, awe inspiring, hooked spines (picture 8). On the other hand, the much feared and deadly venomous Siamese Spitting Cobra (Naja siamensis) has small and completely smooth tubular hemipenes (picture 6).

©SJON HAUSER: text and pictures

Notes:
(1) Chris Mattison, 1998. The Encyclopedia of Snakes. Blanford, London.
(2) Edward Drinker Cope, The classification of the Ophidea. Trans. Amer. Phil. Soc., n.s. 18 (part 2, article 3), 1895: 186-219.
(3) Malcolm A. Smith, 1943. The fauna of British India, Ceylon and Burma, including the whole of the Indo-Chinese subregion. Reptilia and Amphibia. Vol. III, Serpentes. Taylor & Francis, London.
(4) Charles W. Myers and John E. Cadle, 2003. On the snake Hemipenis, with Notes on Psomophis and Techniques of Eversion: A Response to Dowling. Herpetological Review 34 (4): 295-302.