Little rattlesnake, Sistrurus miliarius barbouri
|Garman , 1883|
The dwarf rattlesnakes ( Sistrurus ) are a genus within the pit vipers ( Crotalinae) and thus also within the vipers (Viperidae). Depending on the source, there are two or three species that are only found in North America and have a body length of around 50 centimeters. With the rattlesnakes ( Crotalus ) they share the characteristic feature of the tail rattle , a structure at the end of the tail consisting of horn rings, with which rattling noises can be produced as warning sounds. The main distinguishing feature of the rattlesnakes is the scaling of the head, which in the dwarf rattlesnake consists of large shields and in the rattlesnake from many individual scales.
The types of the dwarf rattlesnakes differ in their physique only insignificantly from the rattlesnakes, but remain significantly smaller than most members of this genus. Miniature rattlesnakes normally reach a body length of around 50 centimeters, the Massassauga ( S. catenatus ) can reach a maximum length of one meter, the other two species can reach a maximum length of around 75 centimeters. The males are usually longer than the females, while the latter are often more massive and therefore heavier.
Strongly keeled back and flank scales surround the middle of the body. The basic color of all dwarf rattlesnakes is adapted to the habitat and mostly gray to brown, sometimes bluish or reddish. They often have dark, rectangular spots on their backs that interrupt a reddish-brown to orange-colored dorsal ligament in the common dwarf rattlesnake ( S. miliaris ). Other spots can appear in the neck area or on the head. As with most snakes, the ventral side consists of a series of non-keeled abdominal scales (ventralia) and is usually light in one color. There is no sexual dimorphism besides size, but the northern populations of the Massassauga have a high proportion of melanistic animals.
The rather flat head stands out clearly from the slender neck. It is mostly triangular and has its widest point behind the eyes, but not as pronounced in this form as in rattlesnakes. The very large poison glands are located at this widest point . The snout is usually more or less rounded. In contrast to the rattlesnakes, the scaling of the skull consists of nine large head shields. With rattlesnakes, the upper head is covered with small scales with the exception of the supraocularia (over-eye shields ) and only in the area of the front snout do they have other shields such as the unpaired rostral directly above the mouth opening and the two nasals (scales around the nostril opening) that cover the nostrils . The scales and shields of the dwarf rattlesnakes are largely uniform without any horn-like outgrowths. According to the color of the body, the head is usually uniformly dark, although only in the case of the dwarf rattlesnake a clearly separated temple band extends over the eyes to the corner of the mouth.
The tail is very short compared to other snakes. Since dwarf rattlesnakes are ground-dwelling, they don't need a long tail to be used when climbing. Also, a short tail can be made to vibrate more easily to use the tail rattle. The tail usually becomes lighter towards the tip, there is no darkening in front of it. The tail end is formed by the rattle, which is significantly smaller than that of the rattlesnake and only produces a relatively low sound, similar to the hum of a large insect. These are the former scales on the tip of the tail, which are the only ones that are not shed during the moult. Accordingly, the rattle is lengthened with each moult . In the wild, the end links of the rattle occasionally break off, so that the number only corresponds to the previous number of molts in young snakes.
The eyes are specially adapted for night vision, so the vertical pupils are narrowed in the form of slits during the day. The iris mostly corresponds in its color to the color of the head or the color stripe that passes over the eye, and is accordingly dark brown to black in the dwarf rattlesnake and the Massassauga and light in the Mexican dwarf rattlesnake ( S. ravus ). Jacobson's organ corresponds to this other scale creepers . It lies in the upper jaw and analyzes the molecules that are brought to it by the two tips of the tongue. Nerve fibers that are connected to the brain via the olfactory nerve end in the sense organ .
A special feature of the pit vipers and thus also the dwarf rattlesnakes is the pit organ located on both sides between the nostrils and the eyes , with the help of which heat radiation is perceived. With the help of the pit organs, dwarf rattlesnakes and rattlesnakes can detect temperature differences of 0.2 to 0.4 ° C and in this way recognize the mostly warm-blooded prey animals very well. Even lizards can be identified because they are usually a bit warmer than its surroundings.
The venomous apparatus of the dwarf rattlesnake consists of the long fangs and the venomous glands , which are located in the head behind the eyes , as with the rattlesnakes . In all vipers, the poison fangs sit at the front end of the greatly shortened upper jawbone and, when at rest, are folded back into the mouth, they are unfolded when the mouth is opened. They lie in a fleshy sheath that retracts when unfolded and reveals the teeth. The teeth contain a venom canal that leads to the venom gland, with an outlet near the tip of the tooth (tubular tooth, solenoglyphic tooth). The poison glands are surrounded by muscles that push the poison out of the glands when bitten. Dry bites, i.e. bites that do not release poison, are rather rare in rattlesnakes.
distribution and habitat
The distribution area of the Massassauga stretches diagonally through the USA and extends in the south to beyond the eastern area of the Mexican border and in the north, in the area of the Great Lakes , also to southern Ontario , Canada . To the southeast is the range of the dwarf rattlesnake, which extends to the southern and eastern coasts of the USA. The Mexican dwarf rattlesnake, on the other hand, is only found in an isolated area in the highlands of southern Mexico.
All three species are ground dwellers, they are only rarely found in trees or bushes or in water, although the Massassauga in particular can swim well. The eastern subspecies of the Massassauga ( S. catenatus catenatus ) lives mainly in the east of its range in forest and swamp areas, the other two subspecies as well as the other species of the dwarf rattlesnake prefer drier areas and can be found in steppe to desert areas. The dwarf rattlesnake occurs in parts of its range in wetlands, but usually prefers dry microhabitats . The Mexican dwarf rattlesnake is a distinct highland species.
Way of life
As with all cold-blooded vertebrates, the activity of dwarf rattlesnakes is very dependent on temperature and corresponds to that of rattlesnakes. The times of activity change accordingly, especially in areas with distinct seasons. In its northern distribution area, the Massassauga is particularly active at night and at dusk during the warmest season. In autumn and spring this activity shifts into the early morning hours or even into the daytime hours when solar radiation is needed for warming. During the winter, however, they hibernate and retreat to a suitable hiding place.
In spring, an increased overall activity can be determined in all species, since this is the time of mating time and the males look for potential sexual partners.
In contrast to the mostly larger rattlesnakes, dwarf rattlesnakes feed mainly on lizards and amphibians, but only very rarely on nesting birds or small mammals.
The dwarf rattlesnakes hunt their prey like the rattlesnakes as ambulance hunters . They wait in suitable places until a prey animal of the right size comes by. The prey is perceived and localized by the sensory organs of the head, whereby the recognition of cold-blooded animals is much more difficult than that of mammals, which are very warm compared to the environment. When attacking, the snake pushes its front body forward and opens its mouth, whereby the poison fangs are unfolded and then struck into the prey. The prey is usually held afterwards, as lizards leave no heat trail and therefore cannot be tracked.
Rattlesnakes move around like other snakes, especially by snaking, with parts of their bodies pushing each other off the unevenness of the ground, or by crawling on the ventral scales, always pushing the front body forward and then pulling the back body .
Reproduction and development
All dwarf rattlesnakes are viviparous ( ovoviviparous ). The main differences between the species are the size of the litter and the mating and birth times. The mating season falls in spring. The young snakes are born in summer, a second generation can follow in spring after wintering.
Both males and females mate with as many partners as possible during the mating season. As with rattlesnakes, ritualized competition can occur between males to mate individual females. You can find the females via a scent trail of pheromones that they follow. In mating fights, the competing males wrap their front bodies and try to push the opponent to the ground. As with other snakes, mating occurs when the male inserts his hemipenis into the female's cloaca and releases his sperm.
The ovulation occurs only after mating . In the case of species that mate in summer or autumn, there can be a relatively long time between the two events during which the sperm are stored in a special chamber in the female genital tract.
After ovulation, the eggs are fertilized and the young snakes begin to develop. The pregnant females spend significantly more time sunbathing and thus warming up their bodies, and in some species they gather in particularly suitable places. At birth, the young snakes are only enclosed in a thin egg shell, from which they break out after a few minutes and move away from the place of birth. Brood care is unknown in dwarf rattlesnakes.
As with all other snakes, it also comes with the sistrurus to regular molts to enable growth. The first molt occurs at the age of a few days, after which the young snakes molt several times a year. After reaching adulthood, the number of moults decreases to an average of two to three per year, with the first moult usually taking place in spring after hibernation . In contrast to all other snakes, the scales at the tip of the tail of rattlesnakes and dwarf rattlesnakes are not shed, and these form the tail rattle that grows longer with each molt. In the phase before molting, the horny layer of the tail scale first thickened, and the new scale formed underneath. The older one gets caught in the new scale and therefore cannot be thrown off.
Predators, threatening and defensive behavior
Like rattlesnakes, dwarf rattlesnakes have a number of enemies that they kill and eat despite their highly potent venom, despite their effective defense capabilities. These include carnivorous mammals such as foxes , coyotes and also domestic dogs and cats , various birds such as falcons and the cuckoo ( Geococcyx californianus ) and various types of snakes. The latter include above all the non-poisonous king snake ( Lampropeltis getula ), which is immune to the poison of the dwarf rattlesnakes and kills them by entangling them, as well as larger rattlesnake species.
The main defense strategy of the rattlesnakes is their camouflage , which they get through their coloring as well as their behavior. In addition, they often hide under stones or in bushes. When this passive defense does not work, an active and aggressive defense occurs, which is mainly used against large mammals. They then curl up on the ground and use their tail rattle to produce a clear warning sound, while fixing the potential opponent and, in extreme cases, biting. The warning given by the very loud rattle is particularly effective for ungulates, which tend to come across and trample dwarf rattlesnakes.
The closest relatives of the dwarf rattlesnakes are probably the rattlesnakes , the only other snake genus to have developed a tail rattle. They share other characteristics, including, for example, a way of life that is very well adapted to dry and warm habitats. As a main distinguishing feature they have a different signage on the head, which in the dwarf rattlesnakes consists of several large shields and in the rattlesnakes is broken up into numerous individual scales.
The triangular-headed adder ( Agkistrodon ) and the American lance-snake ( Bothrops ) are also closely related to the rattlesnakes and dwarf rattlesnakes . A possible cladogram of the close relatives of the rattlesnakes is accordingly:
|American pit vipers *||
*: Crotalinae; only specified genera
In addition to these studies, there are also some working groups that question the monophyly of the rattlesnakes and consider the dwarf rattlesnakes to be part of the group. This is justified by the fact that the only essential difference lies in the scaling of the head, which was already developed in the form of large scales in the ancestors of both groups and must therefore be considered plesiomorphic in the dwarf rattlesnakes . This assumption is partly confirmed by molecular genetic studies, but partly also refuted.
Within the dwarf rattlesnakes, only three species with a few subspecies are known:
- Little Rattlesnake ( Sistrurus miliarius ( Linnaeus , 1766 ))
- Massassauga ( Sistrurus catenatus ( Rafinesque , 1818 ))
- Mexican pygmy rattlesnake ( Sistrurus ravus ( Cope , 1865 ))
According to more recent studies, however, the Mexican dwarf rattlesnake is regarded as a sister group of all rattlesnakes and classified accordingly as Crotalus ravus . According to these studies , the genus Sistrurus only has two species.
The poison of the dwarf rattlesnake corresponds in its basic composition to that of the rattlesnake and like most viper poisons it is hemotoxic , i.e. it destroys blood cells and the walls of the blood vessels. It differs from the paralyzing neurotoxins that are found mainly in poisonous snakes . Hemotoxins lead mainly to tissue destruction, internal bleeding and swelling and are very painful, but compared to most neurotoxins they kill less quickly. Unlike the rattlesnakes, there are no species among the dwarf rattlesnakes that also produce neurotoxic components. The exact composition of the poison is still unknown and varies between the species. In contrast to the poisons of the rattlesnakes, those of the dwarf rattlesnakes are far less well researched.
Humans and rattlesnakes
Toxic effect on humans
The venom of dwarf rattlesnakes is similar to that of most rattlesnakes. However, due to the rather small amounts of poison injected by the animals, it is comparatively harmless. In addition, there are the short poison teeth that do not allow deep penetration into the tissue. Due to the often rapid availability of medical help and various antidotes , there is usually only a very painful swelling of the bite site with local blood cells and tissue destruction. Deaths from dwarf rattlesnake bites are unknown.
Threat and protection
Little rattlesnakes are heavily persecuted in parts of their habitat, especially in the USA, as are rattlesnakes. There are also wild-caught animals for terrarium keeping . In addition to active hunting, the destruction of the habitat plays a major role, as a result of which these species are pushed back. However, none of the species is included in the IUCN's Red List of Threatened Species.
In 1758, in his Systema naturae , Carl von Linné described the rattlesnakes and the forest rattlesnake and the shower rattlesnake , which had already been mentioned in various publications. The dwarf rattlesnake was not described as Crotalus miliarius until 1766 . The Massassauga recognized Constantine Samuel Rafinesque as a species of its own in 1818 , the Mexican dwarf rattlesnake was discovered in 1865 by Edward Drinker Cope . It was not until 1884 that Samuel Garman described the dwarf rattlesnakes as a separate genus Sistrurus .
Mythology and Cultural History
In mythology and cultural history, there is usually no distinction between rattlesnakes and dwarf rattlesnakes, and the latter, as very small species, are given much less attention than the particularly large or particularly venomous rattlesnake species. The aspects of cultural history that apply to both genres are treated accordingly in the mythology and cultural history of rattlesnakes .
Sources and further information
Most of the information in this article has been taken from the sources given under literature; the following sources are also cited:
- Richard A. Seigel: Ecology and conservation of an endangered rattlesnake, Sistrurus catenatus, in Missouri, USA Biological Conservation 35, 1986; Pages 333 to 346.
- Christopher L. Parkinson, Scott M. Moody, Jon E. Alquist: Phylogenetic relationships of the 'Agkistrodon complex' based on mitochondrial DNA sequence data. In Symp. Zool. Soc. London 70, 1997; Pages 63-78
- e.g. in Christopher L. Parkinson: Molecular Systematics and Biogeographical History of Pitvipers as Determined by Mitochondrial Ribosomal DNA Sequences. Copeia, Vol. 1999, No. 3 (Aug. 2, 1999); Pages 576-586 ( abstract )
- R.W. Murphy, J. Fu, A. Lathrop, JV Feltham and V. Kovac: Phylogeny of the rattlesnakes (Crotalus and Sistrurus) inferred from sequences of five mitochondrial DNA genes. and CL Parkinson, JA Campbell, and P. Chippindale: Multigene phylogenetic analysis of pitvipers, with comments on their biogeography. Both in: GW Schuett, M. Höggren, ME Douglas and HW Greene (eds.): Biology of the vipers. Eagle Mountain Publishing, Eagle Mountain, Utah, 2002.
- For example in Alec Knight, David Styer, Stephan Pelikan, Jonathan A. Campbell, Llewellyn D. Densmore III, David P. Mindell: Choosing Among Hypotheses of Rattlesnake Phylogeny: A Best-Fit Rate Test for DNA Sequence Data. Systematic Biology 42, No. 3 (Sep. 1993); Pages 356-367 ( abstract )
- List of species according to Mattison 1996 and ITIS
- Chris Mattison: Rattlers! - A natural history of rattlesnakes . Blandford, London 1996, ISBN 0-7137-2534-6 .
- Dieter Schmidt: snakes. Biology, species, terraristics . bede-Verlag, Ruhmannsfelden 2006, ISBN 3-89860-115-3 .
- Jonathan A. Campbell, Edmund D. Brodie (Eds.): The Biology of the Pit Vipers . Selva, Tyler, Texas 1992.
- Rattlesnake pit (Klapperschlangen.de) - Comprehensive information page on rattlesnakes, with a focus on terraristics
- Rattlesnakes on Gifte.de - Poison information
- Sistrurus in The Reptile Database