INTRODUCTION. 1.1 Reproduction in Reptiles and Snakes. 1.5 General Description and Classification of Montane Trinket Snake

Transcription

1 INTRODUCTION 1.1 Reproduction in Reptiles and Snakes 1.2 Modes of Development in Snakes 1.3 Ecdysis in Snakes 1.4 Food Consumption in Snakes 1.5 General Description and Classification of Montane Trinket Snake 1.6 Scope of the Present Study

2 Reptiles are the group of air-breathing vertebrates that have internal fertilization, amniotic development, and epidermal scales covering part or all of their body. The major groups of living reptiles include the turtles (Order: Testudines), lizards and snakes (Order: Squamata), and crocodiles (Order: Crocodelia) account for over 8,700 species. Among these snakes are elongate legless carnivorous reptiles of the Suborder Serpentes that can be distinguished from lizards by their lack of limbs, eyelids and external ears. Like all squamates, snakes are ectothermic vertebrates covered with overlapping scales. Like lizards, from which they evolved, they have loosely articulated skulls, and most can dislocate their lower jaw in order to swallow prey much larger than their own head. In order to accommodate their narrow bodies, snakes paired organs (such as kidneys) appear one in front of the other instead of side by side and they have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca (Jordan & Verma, 1989). All living creatures are motivated by two overwhelming primal instincts. These are survival of the individual and survival of the species as a whole. In the realm of the first instinct falls the daily routine of foraging for food, finding shelter against the hostile elements and defense against, or avoidance of predation. In the realm of the second falls the annual rituals of seeking a mate, mating and producing offspring. So reproduction is one of the most important attributes of life (Tinkle & Whitfield, 1977). 1.1 Reproduction in Reptiles and Snakes: Reptiles are sexually reproducing organisms where the fertilization always takes place inside the reproductive tract of female (Daniel, 1983). Most snakes have a specific mating season each year (some time two seasons). It may vary from region to region depending upon the climatic conditions. In colder regions, spring is the mating season; whereas, snakes can 1

3 undergo mating in any season in case of tropical regions. Towards Southern India, more species have two mating seasons. Some snakes which have two breeding seasons are wolf snakes, striped keel back, trinket snakes and saw scaled viper (Whitaker, 1978). The reproductive cycle or breeding responses of many snakes is prompted by the average temperatures they experience and the photoperiod, which is the length of the day verses the length of the night. Snakes sexually become active after emerging from winter dormancy or at least a period of comparative inactivity as the temperature and photoperiod increases in the spring. So snakes undergo winter cooling then warm for their reproductive activity (Applegate, 1987). Sexual reproduction in snakes is initiated by recognition of opposite sex by tactile stimulation using pheromones. Use of pheromones ensures that, courtship and copulation occur between members of the same species. Both male and female have musk glands at the base of the tail and the secretion from these glands has a typical foul smell which is species specific. The odour of the female is an attractant for a male or many males. Using the vomeronasal organ, the male will determine chemically sex and species of the approached female using the pheromones emitted from its skin. If it is not a sexually receptive female of the right species, the male will quickly move off in search of a new mate (Sharma, 1998). Mostly during warming period, sexually active males will approach any other snakes they see and the reaction of the approached will then determine the outcome of the encounter. Male snake will begin the preliminaries to the sexual act if female snake fails to react in any particular manner. If the female is a receptive one then male will attempt to mate with her. At this time snakes may engage in a ritual fight called the combat dance (Whitaker & Captain, 2008). This battle may vary from species to species. Elapids and Vipers tend to indulge in a form of ritualized wrestling and no biting takes place. Colubrids on the other 2

4 hand can get very violent and bite each other severely. Certain species indulge in no combat but follow the female in amicable groups. Usually mating starts with placing head on back of the female, coiling tail and attempts to join their cloaca together. However, the female usually moves off while male is trying to mate. The male usually only succeeds after many hours - or sometimes days of trying. Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization. The male has paired sex organs called hemipenes which are often forked and stored inverted in the male s tail. It is inserted into the cloaca of the female and copulation lasts for hours (Whitaker & Captain, 2008). After mating, the female snake stores the sperms in the oviduct for about 1-2 months. The female then produces large eggs, which after releasing from the ovary are fertilized by the sperms from the oviduct. Fertilization takes place high up in the oviduct. The fertilized egg then moves down into the oviduct where the uterine glands secrete substances to surround it. The nature of these substances depends on whether the egg is to be laid or gestated within the body. The length of embryonic development depends on species, and within the species on temperature. The variations are great with a range of 2 to 5 months encompassing most species. Females of many snakes have the ability to store sperm and fertile eggs may be laid even three or four years after separation from male (Daniel, 1983). This is called Psuedopregnancy. According to Rivas and Gordon (2005) in natural habitat psuedopregnancy is because of low encounter rate between males and female snakes. Pregnancy can be difficult to determine. The snake usually swells in the mid- to hindsection of her body, but in the case of overweight animals, the swelling can be obscured by fat deposits. The snake may bask more and her appetite may decrease. Her body position may change; she may lie on her side or may coil more loosely than normal. Close to the time she 3

5 will lay, she may be more restless. Pregnant snakes typically shed about two weeks before they lay the eggs. 1.2 Modes of Development in Snakes Based on mode of bearing young or development of young, snakes are classified into three groups namely oviparous, viviparous and ovoviviparous. About 60 species or 25% of Indian snakes are ovoviviparous (Whitaker & Captain, 2008). In these snakes, eggs are retained within the body of the female and develop internally until they are almost ready to hatch. Examples: sand boas, sea snakes, vine snakes, vipers etc,. Recently, it has been confirmed that several species of snakes are fully viviparous, such as the boa constrictors and green anaconda, nourishing their hatchlings through placenta as well as yolk sac which is highly unusual among reptiles. In viviparous snakes female does not develop enclosed eggs and gives birth to live young. On the other hand, oviparous snakes produce egg and lay eggs that develop outside the body of the female and hatch after laying. Rest of the Indian snakes like King Cobra, Cobra, Krait, Python, Rat snake, Trinket snakes, Checkered Keelback, etc., are egg layers. Oviparous organisms do not face physiological stress during gestation. Aside from the energy required to build and defend nests, there is an advantage of a decreased metabolic cost because once the eggs are laid, there is no more physiological investment from the mother. Oviparous organisms also have the potential to produce a great number of young per reproduction because of the reduced costs in producing eggs verses gestation. Gestation period in oviparous snakes lasts between days and the clutch of eggs produced by the female varies between 2 and 50. The clutch size (number of eggs laid by female per reproduction) produced by the female depends upon the body size, age, breeding season and frequency of reproduction. As female age and size increases the clutch size also 4

6 increases. The size of a clutch of eggs/babies may vary from 2 to 6 in case of smaller snakes like Green Snake, Snail Eater, Wolf Snake, Banded Trinket Snake, Banded Racer, Kukri Snake etc., to 40 or more in large snakes like Pythons, Rat Snakes, Com Snake, Grass Snake, Cobras, Vipers etc. However the usual range is between 4 and 16 eggs which is observed in Flying snakes, Trinket snakes, Red Spotted Royal Snake, Common Sand Boa etc,. Most of the Colubridae members lay 15 to 30 eggs in a clutch (Nande & Sawan, 2007). Many oviparous terrestrial species deposit flexible- shelled eggs into the soil. Humidity and temperature plays crucial role in the development of egg (Bass, 2010). These eggs are sensitive to the hydration level of the environment. Water exchange of eggs during incubation may be affected by the soil water potential. The availability of water during egg incubation influences egg mass and hatchling size. Eggs incubated in dry soils absorb less water and give rise to small hatchlings (Marco et. al. 2004). In oviparous snakes eggs are protected by shells that are thin and leathery. Reptile eggs are more vulnerable to drying out than bird eggs. So humidity plays a crucial role in the development of snake eggs. The female lays the fertilized eggs in shallow holes or under the rocks. Humidity affects the ease with which oxygen enters the egg through the shell and the rate of carbon dioxide diffusion (Bass, 2010). Warmer temperatures cause reptile embryos to develop faster, while cooler temperatures result in slower embryonic development. The juvenile snake comes out of the egg, by biting the egg cover with the help of egg tooth. Snakes have a single median egg tooth which allows the hatchling to escape from the egg (Bandali et. al. 2007). Incubation temperature has been shown to affect emergence behaviour. Hatchlings from eggs that were incubated at medium temperatures require less time to emerge and stronger protective responses than snakes incubated at other temperatures. The hatchlings incubated at medium temperatures are generally less vulnerable to predators 5

7 than hatchlings incubated at higher or lower temperatures (Burger, 1998). Eggs incubated at low temperature will have more morphological abnormalities and even some fails to hatch (Burger et al. 1987). A newborn hatchling is a perfect miniature replica of the snake which gave birth to it - except for its head which is proportionately larger. It has to fend for itself from the moment it is bom. Young snakes start feeding within a month. Most hatchlings of rat snakes will consume pink mice seven to ten days after their first shed (Griswold, 1995). But they can survive without feeding for several months depending upon the amount of yolk reserve that the snakes have at the time of hatching (Jazbec. 1995). The growth of a newborn snake is indeterminate and continuous. It depends on availability of nutrients and frequency of feeding. Another factor influencing the rate of growth is temperature. In the tropics snakes tend to grow much faster than in temperate climates. All these factors influence sexual maturity - even within a single species. Males tend to mature at a younger age than females. The juvenile snakes may attain maturity within two to four years depending on the species. Mortality among infant snakes is very high and predation is great. The hatchlings of most egg laying reptiles hatch long after the parents have abounded the eggs. Most species of snakes do not show parental care; especially snakes that lay eggs do not typically build nests in the manner of many other reptiles. They leave the eggs shortly after laying. However, some individual species such as King cobra and Pythons show some degree of parental care. King Cobra (Ophiphagous hannah) during breeding season construct nest and incubate the egg until they hatch (Sharma, 1998). They stay in close vicinity even after hatching of hatchlings and there by guard the offspring. In all python species females brood their eggs until hatching but they do not care for their neonates. Aubret et al., (2003) working on Ball Python (Python regins) reported that female python coil around their eggs 6

8 until hatching and they do not feed during this time. The female python will not leave the eggs except to occasionally bask in the sun or drink water. The coiling around eggs will generate heat to incubate eggs. The pythons stay with their eggs for the two months until they hatch which provides safety from predators. They also warm the eggs in order to speed up the hatching process by twitching their muscles (Lillywhite et al., 2009). Some species select a crevice to lay eggs like Keelback, Racers, Rat snakes etc,. Grass snakes choose to lay their eggs in decaying vegetation which produces necessary heat as they decompose. However, in most of the snakes (egg layers and live bearers) parent snakes do not look after their hatchlings. Baby snakes have to find their food and watch out for predators all on their own. Some viper species will stay with their hatchlings until they shed their skin (Tinkle & Whitfield, 1977). 1.3 Ecdysis in snakes: The skin of a snake is covered in scales. Snake skin has a smooth and dry texture. Most snakes use specialized belly scales to travel and gripping surfaces. The body scales may be smooth, keeled, or granular. Snakes lack eyelids and instead have a clear transparent scale called spectacle to cover and protect the eye. The spectacle remains permanently closed (also known as brille). The shedding of scales is called ecdysis or moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer. Snake scales are not discrete but extensions of the epidermis hence they are not shed separately, but are ejected as a complete contiguous outer layer of skin during each moult, akin to a sock being turned inside out (Davies & Valerie, 1997). Moulting is repeated periodically throughout a snake's life. Before a moult, the snake stops eating and often hides or moves to a safe place. The skin becomes dull and dry looking and the eyes become cloudy or blue-coloured (Plate 8.2). A milky appearance is seen on the 7

9 belly and the spectacles become milky white, obscuring vision. A skin in this condition is said to be opaque or pre shed snake. During this time the inner surface of the old outer skin become fragile and liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake crawl out of its old skin. The old skin breaks near the mouth and the process of ecdysis is initiated by rubbing the jaws against stones or any rough surfaces until skin become loosened to enable the snake literally crawl out of it (Sharma, 1998). In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. During ecdysis the dead outer layer of the spectacle and the forked tongue is also shed and a new larger and brighter layer of skin appears underneath (Plate 8.3). The frequency of shedding depends on many factors like species, age, nutritional and reproductive status, the presence of skin parasites or bacteria, and ambient temperature and humidity (Davies & Valerie, 1997). An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times or even more for a year. In newly hatched snake postnatal ecdysis establishes a transepidermal barrier to prevent loss of water from the body (Tu et al. 2002). The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact. This periodic renewal has led to the snake being a symbol of healing and medicine (Davies & Valerie, 1997). Most coloubrids will shed skin before egg laying (Applegate, 1987). This is called pre-laying shedding. Rat snakes tend to lay eggs nine days after their pre-laying ecdysis (Griswold, 1995). Moulting serves a number of functions firstly, the old and worn skin is replaced and secondly it helps to get rid of parasites such as mites and ticks. 8

10 1.4 Food Consumption in snakes: Snakes use smell to track their prey. It smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's organ or the vomeronasal organ in the mouth for examination. The fork in the tongue gives the snake a sort of directional sense of smell and taste simultaneously. The snake keeps its tongue constantly in motion, sampling particles from the air, ground, and water analyzing the chemicals found and determining the presence of prey or predators in its local environment (Daniel, 1983). All snakes are strictly carnivorous and feed on a wide variety of animals like lizards, other snakes, small mammals, birds, eggs, fish, snails or insects (Olivier et al. 2005). As snakes cannot bite or tear their food to pieces, a snake must swallow its prey whole. The body size of a snake has a major influence on its eating habits. Smaller snakes eat smaller prey. In snakes consumption of single prey item can lead to abrupt increase in body mass. An abrupt change in body mass, concentrated in a particular area of the body may promote corresponding ecological and behavioural change (Mehta, 2006). Juvenile pythons might start out feeding on lizards or mice and graduate to small deer or antelope as an adult. During food consumption snakes always swallow head first and then the remaining body parts are simply drawn in (Plate 7). Most snakes have small backwardly directed teeth that prevent chances of escaping of prey during swallowing (Milne & Mergery, 1959). Food consumption of snakes varies in different season. Female snakes will continue to feed for several weeks into pregnancy but stop feeding during the last three weeks before egg laying, while males will continuously feed during breeding season (Griswold, 1995). The snake's jaw is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, snakes have a very flexible lower jaw, the two halves of which are not 9

11 rigidly attached, and numerous other joints in their skull, allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself, as snakes do not chew. For example, the African Egg-eating Snake has flexible jaws adapted for eating eggs much larger than the diameter of its head. This snake has no teeth, but does have bony protrusions on the inside edge of its spine which are used to aid in breaking the shells of the eggs it eats. While the majority of snakes eat a variety of prey animals, there is some specialization by some species. King cobras and the Australian Bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating Colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise. After eating, snakes become dormant while the process of digestion takes place. Digestion is an intense activity, especially after consumption of very large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy, and the digestive system is up-regulated to full capacity within 48 hours of prey consumption. Being cold-blooded (ectothermic), the surrounding temperature plays a large role in a snake's digestion. 30 degrees Celsius is the ideal temperature for snakes to digest their food. So much metabolic energy is involved in a snake's digestion that results in an increased body temperature above the surrounding environment. Because of this, a snake disturbed after having eaten recently will often regurgitate its prey in order to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient, with the snake's digestive enzymes dissolving and absorbing everything except prey's hair and claws, which are excreted along with waste. Longevity in snakes: No authentic records are available with regard to the longevity of Indian snakes. When compared to other animals the reptiles in general are known to have the longest 10

12 longevity record due to their cold blooded nature. The life span of Colubridae members vary from two years to fifty years (Das et. al., 2007). 1.5 General description and Classification of Montane Trinket snake Phylum: Group: Subphylum: Superclass: Class: Order: Suborder: Family: Genus: Species: Chordata Craniata vertebrata Tetrapoda Reptilia Squamata Serpentes Colubridae Coelognathus helena It has two subspecies namely Coelognathus helena monticollaris (Shulz, 1992) (Montane trinket snake) and Coelognathus helena helena (Daudin, 1803) (common trinket snake) Distribution: Trinket snake is a non venomous, semi arboreal, constrictor snake of native to South Central Asia. In India, Montane trinket snake is endemic to Western Ghats found in Tamil Nadu, Kerala, Karnataka, Maharashtra (Whitaker & Captain 2008). Though it was thought to be endemic to Western Ghats, it was also reported outside the Western Ghats like Melghat and The Mollem National Park, Goa (Nande & Sawan 2007). Snake has also been documented from Patigala Rain Forests of Southern Sri Lanka (Gamage et al., 2006). 11

13 Average length: Females are always larger than males while males have proportionately longer tail. The average length of female ranges from 120cm to 150cm and male 90cm to 120cm. The smallest known specimen measured around 35cm in length. Description: The Montane Trinket snake is a tan and chocolate brown snake with two prominent dark stripes on the later part of the body and light bands and checks on the fore part (Plate 1.1). The long forehead is unmarked. The eyes are prominent with round pupils. The two short dark lines on either side of the neck may join medially to form an inverted V (Plate 1.2). The underside or belly of the body is pearly white in colour. The scales are smooth and glossy. The scales in front of the eye are scattered with pores, which may be sensory. The midbody scales are smooth and those of the hind body and tail sometimes weakly keeled. The bands consist of several rows of large pale yellow or oval or round spots encircled with black (Plate 1.1). Hind body has black longitudinal stripes (Plate 1.3). Body colour above is brown of varying degree of intensity, yellowish or whitish on lip. There is an oblique streak from eye to lip (Plate 1.1). Sutures on both lips dark streaked. Fore body for available length beautifully ornamented with oscillated cross bars. Usually on nape a conspicuous V- shaped black mark or two lines either parallel or occasionally meeting to for two or three cross bars on the back. A broad dark stripe extending to tip of tail gradually replaces cross bars on sides of the body. Habitat: Trinket is a semi arboreal snake. During the hot weather it lives deep in termite moulds, rock piles and crevices. In the cool season they emerge and are found in leafy trees, bushes and braches. They are common near the scrub zones of rain forest edges, rice field, plantation and meadow near water. It prefers forests but may frequently venture towards human habitation and occasionally enter human dwellings. 12

14 1.1 Adult Montane Trinket Snake 1.2 V shaped Markings on neck oblique streak from eye to lip and prominent cross bands 1.3 Prominent lateral dark stripes on posterior side of the body 1.4 Characteristic S shaped coiling by adult snake when excited PLATE 1: Identification Characters of Adult Montane Trinket Snake X 13

15 Habits: They are active both during day and night. Though generally well-mannered and calm when handled, some will swell their necks, open the mouth and make quick strikes at an aggressor. Sometimes they vibrate their tail in defense like rattle snake. The neck is compressed, spine-arched and throat distinctly pouched. The anterior one-third of the body becomes erect and throws the forebody into broad sigmoid curves or S shaped coiling which are straightened in the act of striking (Plate 1.4). When tamed it repeatedly strikes on. the object. When excited, the scales of the body expand and separate which reveals the pinkish body part of the intervening skin. Scalation: Identification'. The following characteristics are to be noted. Nine pairs of Supralabials of which the fifth and sixth pairs touch the eye. 19 to 21 rows of Costals two head lengths before the vent. Entire Anal shield. Nearly equally sized Frontal and Parietal shields. Lepidosis: Rostral: Compressed and touches six shields. Frontal: Relatively thin and elongated. Supraoculars: Slender and elongated being nearly as long as the Frontal. Parietals: Each individual shield is slightly larger than the Fronta.l Prefrontals: Highly broad as well as long. Intemasals: Touch the nostrils. Nasals: Divided along a vertical line on which the nostril is situated. Loreals: Entire. Preoculars: Entire. Postoculars: Divided into two equally sized shields. Temporals: Divided into two equally sized elongated shields. Supralabials: Nine pairs of which the fifth and sixth pairs touch, the eye. Mental: Stereotype in form. «Infralabials: Six pairs of which the sixth is the largest. 14

16 Sublinguals: Anterioir and Posterior pairs equal in size and the Posterior pair is separated by smaller scales. Costals: Only the Ultimate row is enlarged. Ventrals: 217 to 265. Anal Shield: Entire. Subcaudals: 74 to 97, divided. Dentition: Maxillary: 19 to 25. Palatine: 10 to 14. Pterygoid: 15 to 30. Mandibular: 22 to 30. Longevity>: It lives up to 12 to 15 years. Long term captive data is not available. However Bronikowski (2008) reported that the trinket snakes can live upto 15 years. Food: Trinket snake feeds on a variety of prey (Daniel, 1983). It feeds mainly on small mammals especially rodents. It uses is camouflage to stalk its prey and initially disorients its victim with a blitz strike. It then surrounds its prey with its coils and weakens it by repeated biting. The victim is finally killed by making constrictions and then swallowed (Plate 7). It may prey upon birds, frogs, small lizards and other snakes as well but shows a high preference towards the mammals. It is notorious for its voracious appetite. Captive specimens prefer rats and mice and tend to lose interest in lizards especially if they remain motionless. Breeding: Very little is known. Breeding season varies from January to February and July to August. Female is oviparous and capable of producing large clutch in captivity. It generally lays 8 to 12 eggs. The young resemble the adults. MP <59 Pu/TSH 15

17 1.6 Scope of the present investigation Snakes constitute a group of limbless slithering reptiles and are highly fascinating in their awe-inspiring ability. Living snakes are found on every continent except Antarctica. World population of snakes is comprised of about 2920 species belonging to eighteen families and 456 genera. Of this vast number 375 are venomous. The Family Coloubridae includes well over half of all snake species on the earth. There are 1938 species under 304 genera in this family throughout the world. Indian subcontinent boasts of housing approximately 10 percent of the total snake species found in the world. Of the vast number of the faunal spectrum of snakes, 270 species were reported from India. The Western Ghats, also known as the Sahyadri Hills, are well known for their rich and unique assemblage of flora and fauna. Being one of the biodiversity hot spots among the 34 biodiversity hot spots of world, the region harbours approximately 260 reptilian species (Daniel, 2002; Sawant et al 2010). Of these, about 157 species of reptiles are known to be from Western Ghats. Majority of the reptiles are snakes representing 87 species, of which 61 species are endemic and eight are poisonous. Whitaker (2008) confirmed the occurrence of 25 species of snakes from the silent valley, one of the least disturbed areas of Western Ghats. In India, there are two subspecies of Coelognat/ms Helena is found. The Montane trinket snake (Coelognathus Helena monticollaris) which is endemic to Western Ghats and Common Trinket Snake (Coelognathus helena Helena) is found throughout India. The Montane trinket snake is a rodent killer; hence it serves as a natural check on the population of various rodent species. The Montane Trinket snake prefers mixed moist deciduous and evergreen forest habitats to others and has on some occasions been observed to take residence in caves as well as abandoned manmade structures in search of rats. Hence they are commonly seen near the human settlements. This snake is wrongly considered as 16

18 venomous snake because of the stripes marking in their body and people kill the snake. Even though it is an endemic snake of Western Ghats, very little information is available on the breeding, reproductive biology and captive management of Coelognathus Helena. Whitaker (1978) mentioned the clutch size of Coelognathus Helena without mentioning the subspecies. Mehta (2006) observed effect of prey size on the feeding of young Coelognathus helena. Hence the present study was envisaged to look into the breeding biology of these snakes. Following aspects were looked into during the study period of two years i.e. December 2008 to December to understand the mating season and behavior during this period. 2. to observe gestation period and incubation period of snake. 3. to study the egg development pattern and effect of environmental factors like temperature and moisture content on egg development. 4. to observe hatchability of eggs and growth pattern of hatchlings. 5. to study the parental care and cannibalism. 6. to study the food consumption and food preference of both adults and young snakes. 7. to study the ecdysis in male and female in different season. 17