THE GECKOS

 

Geckos are small lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 to 60 cm (0.64 to 24 inches). Most geckos cannot blink, but they often lick their eyes to keep them clean and moist. They have a fixed lens within each iris that enlarges in darkness to let in more light.  

 

 Gold dust day geckolicking its cornea to clear it of dust Geckos are unique among lizards for their vocalizations, which differs from species to species. Most geckos in the family Gekkonidae use chirping or clicking sounds in their social interactions, tokay geckos (Gekko gecko) are known for their loud mating 

 

calls, and some other species are capable of making hissing noises when alarmed or threatened. They are the most species-rich group of lizards, with about 1,500 different species worldwide.^[2] The New Latin  

gekko and English "gecko" stem from the Indonesian-Malay gēkoq, which is imitative of sounds that some species make.^[3]
All geckos except species in the family Eublepharidae lack eyelids; instead, the outer surface of the eyeball has a transparent membrane, the cornea. Species without eyelids generally lick their own corneas 

 

when they need to clear them of dust and dirt.^[4]
Nocturnal species have excellent night vision; their color vision in low light is 350 times more sensitive than human color vision.^[5] The nocturnal geckos evolved from diurnal species, which had lost the eye 

 

 

rods. The gecko eye, therefore, modified its cones that increased in size into different types, both single and double. Three different photopigments have been retained and are sensitive to UV, blue, and green. They also use a multifocal optical system that allows them to generate a sharp image for at least two different depths.^[6][7]
 

 

Like most lizards, geckos can lose their tails in defense, a process called autotomy.^[8] Many species are well known for their specialised toe pads that enable them to climb smooth and vertical surfaces, and even cross indoor ceilings with ease. Geckos are well known to people who live in warm regions of the world, 

 

where several species make their home inside human habitations. These (for example the house gecko) become part of the indoor menagerie and are often welcomed, as they feed on insects, including moths and mosquitoes. Unlike most lizards, geckos are usually nocturnal.
The largest species, the kawekaweau, is only known from a single, stuffed specimen found in the basement of a museum in Marseille, France. This gecko was 60 cm (24 in) long and it was likely endemic 

 

 

to New Zealand, where it lived in native forests. It was probably wiped out along with much of the native fauna of these islands in the late 19th century, when new invasive species such as rats and stoats were introduced to the country during European colonization. The smallest gecko, the Jaragua sphaero, is a mere 1.6 cm long and was discovered in 2001 on a small island off the coast of the Dominican Republic.^[9]

 

Like other reptiles, geckos are ectothermic,^[10] producing very little metabolic heat. Essentially, a gecko's body temperature is 

 

dependent on its environment. Also, to accomplish their main functions—such as locomotion, feeding, reproduction, etc.—geckos must have a relatively elevated temperature.^[10]

 

All geckos shed their skin at fairly regular intervals, with species differing in timing and method. Leopard geckos shed at about two- to four-week intervals. The presence of moisture aids in the shedding. When shedding begins, the gecko speeds the process 

 

by detaching the loose skin from its body and eating it.^[11] For young geckos, shedding occurs more frequently, once a week, but when they are fully grown, they shed once every one to two months.^[12]

^]

 

Adhesion ability

 
See also: Synthetic setae and Gecko feet

About 60% of gecko species have adhesive toe pads that allow them to adhere to most surfaces without the use of liquids or surface tension. Such pads have been gained and lost repeatedly over the course of gecko evolution.^[13] Adhesive toepads evolved independently in about 11 different gecko lineages and were lost in at least 9 lineages.^[13]
 

 The spatula-shaped setae arranged in lamellae on gecko footpads enable attractive van der Waals' forces (the weakest of the weak chemical forces) between the β-keratin lamellae/setae/spatulae structures and the surface.^[14][15] These van der Waals interactions involve no fluids; in theory, a boot made of synthetic setae would adhere as easily to the surface of the International Space Station as it would to a living-room 

 

 A recent study has however shown that gecko adhesion is in fact mainly determined by electrostatic interaction (caused by contact electrification), not van der Waals or capillary forces.^[18]
The setae on the feet of geckos are also self-cleaning and usually remove any clogging dirt within a few steps.^{[19][20] [21]} Teflon, which has very low surface energy,^[22] is more difficult for geckos to adhere to than many other surfaces.
 

 Gecko adhesion is typically improved by higher humidity,^{[16][17][23][24][25]} even on hydrophobic surfaces, yet is reduced under conditions of complete immersion in water. The role of water in that system 

 

 

is under discussion, yet recent experiments agree that the presence of molecular water layers (water molecules carry a very large dipole moment) on the setae, as well as on the surface, increase the surface energy of both, therefore the energy gain in getting these surfaces in contact is enlarged, which results in an increased gecko adhesion force.^{[16][17][23][24][25]} Moreover, the elastic properties of the b-keratin change with water uptake.^[16][17][23]
 

 

Gecko toes seem to be "double jointed", but this is a misnomer and is properly called digital hyperextension.^[26] Gecko toes can hyperextend in the opposite direction from human fingers and toes. This allows them to overcome the van der Waals force by peeling their toes off surfaces from the tips inward. In essence, by this peeling action, the gecko separates spatula by spatula from the surface, so for 

 

each spatula separation, only some force necessary. (The process is similar to removing Scotch Tape from a surface.)
Geckos' toes operate well below their full attractive capabilities most of the time because the margin for error is great depending upon the surface roughness, and therefore the number of setae in contact with that surface.

 Use of small van der Waals force requires very large surface areas; every square millimeter of a gecko's footpad contains about 14,000 hair-like setae. Each seta has a diameter of 5 μm. Human hair varies from 18 to 180 μm, so the cross-sectional area of a human hair is equivalent to 12 to 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae.^[19] Each spatula is 0.2 μm long^[19] (one five-millionth of a meter), or just below the wavelength of visible light.^[27]
 

 The setae of a typical mature 70 g (2.5 oz) gecko would be capable of supporting a weight of 133 kg (293 lb):^[28][29] each spatula can exert an adhesive force of 5 to 25 nN.^[23][30] The exact value of the 

 

adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies ^[25][31] have moreover shown that the component of the surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers (up to 100 nm beneath the surface); taking that into account, the adhesive strength can be inferred.

 

 

Apart from the setae, phospholipids—fatty substances produced naturally in their bodies—also come into play.^[32] These lipids lubricate the setae and allow the gecko to detach its foot before the next step.
The origin of gecko adhesion likely started as simple modifications to the epidermis on the underside of 

 

the toes. This was recently discovered in the genus Gonatodes from South America.^[33] Simple elaborations of the epidermal spinules into setae have enabled Gonatodes humeralis to climb smooth surfaces and sleep on smooth leaves.
Biomimetic technologies designed to mimic gecko adhesion could produce reusable self-cleaning dry adhesives with many applications. Development effort is being put into these technologies, but manufacturing synthetic setae is not a trivial material design task.

 

Gecko skin does not generally bear scales, but appears at a macro scale as a papillose surface which is made from hair-like protuberances developed across the entire body. These confer superhydrophobicity, and the unique design of the hair confers a 

 

profound antimicrobial action. These protuberances are very small, up to 4 microns in length, and tapering to a point.^[34] Gecko skin has been observed to have an anti-bacterial property, killing gram-negative bacteria when it comes in contact with the skin.^[35]

 

Geckos are polyphyodonts and able to replace each of their 100 teeth every 3 to 4 months.^[36] Next to the full grown tooth there is a small replacement tooth developing from the odontogenicstem cell 

 

 

in the dental lamina.^[37] The formation of the teeth is pleurodont; they are fused (ankylosed) by their sides to the inner surface of the jaw bones. This formation is common in all species in the order Squamata.

 

 The infraorder Gekkota is divided into seven families, containing about 125 genera of geckos, including the snake-like (legless) pygopods.^{[13][38][39][40][41]}

 

 

 

 

 

 

 

 

THE HIMALAYAN GRAY LANGURS

 

Gray langurs, also called Hanuman langurs and Hanuman monkeys, are Old World monkeys native to the Indian subcontinent constituting the genus Semnopithecus.^[1] Traditionally only one species Semnopithecus entellus was recognized, but since about 2001, additional species have been recognized. The taxonomy has been in flux, but currently eight species are recognized.

 

 

Gray langurs are terrestrial, inhabiting forest, open lightly wooded habitats, and urban areas on the Indian subcontinent. Most species are found at low to moderate altitudes, but the Nepal gray langur and Kashmir gray langur occur up to 4,000 m (13,000 ft) in the Himalayas.^[2][3]

 

These langurs are largely gray (some more yellowish), with a black face and ears. Externally, the various species mainly differ in the 

 

darkness of the hands and feet, the overall color and the presence or absence of a crest.

Langurs mostly walk quadrupedally and spend half of their time on the ground and the other half in trees. They will also make bipedal hops, climbing and descending supports with the body upright, and leaps. Langurs can leap 3.6–4.7 m (12–15 ft) horizontally and 10.7–12.2 m (35–40 ft) in descending.^[8]

 

Traditionally, only Semnopithecus entellus was recognized as a species, the remainder all being treated as subspecies. In 2001, it was proposed that seven species should be recognized.^[4] This was 

 followed in Mammal Species of the World in 2005,^[1] though several of the seven species intergrade, and alternative treatments exist where only two species (a northern and a southern) are 

 recognized.^[5]Phylogenetic evidence supports at least three species: a north Indian, a south Indian and a Sri Lankan one.^[9][10]

 

It has been suggested that the Semnopithecus priam thersites is worthy of treatment as a species rather than a subspecies, but at present this is based on limited evidence.^[11] During a study based on external morphology and ecological niche modelling in Peninsular India six main types were found, but continued to label all as subspecies. Coat color is highly variable, possible due to phenotypic plasticity and therefore of questionable value in species delimitation.^[12][13]

 

It has been suggested that Trachypithecus should be considered only a subgenus of Semnopithecus.^[5] If maintaining the two as separate monophyletic genera, the purple-faced langur and Nilgiri langur belong in Semnopithecus instead of their former genus Trachypithecus. At present it is unclear where the T. pileatus species group (consisting of the capped langur, Shortridge's langur and Gee's golden langur) belongs, as 

 

available mtDNA data place it in Semnopithecus, while Y chromosome data place it in Trachypithecus.^[9] A possible explanation for this is that the T. pileatus species group is the result of fairly recent hybridization between Semnopithecus and Trachypithecus.^[10]

Results of analysis of mitochondrial cytochrome b gene and two nuclear DNA-encoded genes of several colobine species revealed that Nilgiri and purple-faced langurs cluster with gray langur, while  

Trachypithecus species form a distinct clade.^[14] Since then, two other species have been moved from Trachypithecus to Semnopithecus:^[15][16]

 

 In addition, Semnopithecus dussumieri has been determined to be invalid.^{[12][16][17][18]} Most of the range that had been considered S. dussumieri is now considered S. entellus.^[15][16]

 

 

A 2013 genetic study indicated that while S. entellus, S. hypoleucos, S. priam and S. johnii are all valid taxa, there has been hybridization between S. priam and S. johnii.^[19] It also indicated that there has been 

 

some hybridization between S. entellus and S. hypoleucos where their ranges overlap, and a small amount of hybridization between S. hypoleucos and S. priam.^[19] It also suggested that S. priam and S. johnii diverged from each other fairly recently.^[19]

 

The entire distribution of all gray langur species stretches from the Himalayas in the north to Sri Lanka in the south, and from Bangladesh in the east to Pakistan in the west.^[20] They possibly 

 

occur in Afghanistan.^[5] The bulk of the gray langur distribution is within India, and all seven currently recognized species have at least a part of their range in this country.^[4

 

 Gray langurs can adapt to a variety of habitats.^[21] They inhabit arid habitats like deserts, tropical habitats like tropical rainforests and temperate habitats like coniferous forests, deciduous habitats and mountains habitats. They are found at sea level to altitudes up to 4,000 m (13,000 ft).^[2][3] They can adapt well to human settlements, and are found in villages, towns and areas with housing or agriculture.^[22] They live in densely populated cities like Jodhpur, which has a population numbering up to a million.^[23]

 

Gray langurs are diurnal. They sleep during the night in trees but also on man-made structures like towers and electric poles when in human settlements.^[24] When resting in trees, they generally prefer the highest branches.^[25]

 

 

Ungulates like bovine and deer will eat food dropped by foraging langurs.^[26] Langurs are preyed upon by leopards, dholes and tigers.^[27] Wolves, jackals, Asian black bears and pythons may also prey on langurs.

 

Gray langurs are primarily herbivores. However, unlike some other colobines they do not depend on leaves and leaf buds of herbs, but will also eat coniferous needles and cones, fruits and fruit buds, evergreen petioles, shoots and roots, seeds, grass, bamboo, fern rhizomes, mosses, and lichens. Leaves of trees and shrubs rank at the top of preferred food, followed by herbs and grasses. Non-plant material consumed include spider webs, termite mounds and insect larvae.^[28] They forage on agricultural crops and other human foods, and even accept handouts.^[29] Although they occasionally drink, langurs get most of their water from the moisture in their food.^[30]

 All-male groups tend to be the smallest of the groups and can consist of adults, subadults, and juveniles. Some populations have only multiple-male groups as mixed sex groups, while others have only one-male groups as mixed sexed groups.^[31]

 

 Some evidence suggests multiple-male groups are temporary and exist only after a takeover, and subsequently split into one-male and all-male groups.^[33]

 

 

Social hierarchies exist for all group types.^[34][35] In all-male groups, dominance is attained through aggression and mating success.^[36] With sexually mature females, rank is based on physical condition and age.^[37][38] The younger the female, the higher the rank. Dominance rituals are most common among 

 

high-ranking langurs.^[35] Most changes in social rank in males take place during changes in group members. An adult male may remain in a one-male group for 45 months.^[39] The rate of male replacement can occur quickly or slowly depending on the group.^[40]

 

 

Females within a group are matrilineally related. Female memberships are also stable, but less so in larger groups.^[32][37] Relationships between the females tend to be friendly. They will do various activities with each together, such as foraging, traveling and resting. They will also groom each other regardless of their 

 

rank. However, higher-ranking females give out and receive grooming the most.^[41] In addition, females groom males more often than the other way around.^[42] Male and female relationships are usually positive. Relationships between males can range from peaceful to violent. While females remain in their natal groups, males will leave when they reach adulthood.^[32] Relationships between groups tend to be hostile. High-ranking males from different groups will display, vocalize, and fight among themselves.

 

 In one-male groups, the resident male is usually the sole breeder of the females and sires all the young. In multiple-male groups, the highest-ranking male fathers most of the offspring, followed by the next-ranking males and even outside males will father young.

 

 

Female gray langurs do not make it obvious that they are in estrous. However, males are still somehow able to deduce the reproduction state of females.^[44] Females signal that they are ready to mate by shuddering the head, lowering the tail, and presenting their anogenital regions.^[45] Such solicitations do not always lead to copulation. When langurs mate, they are sometimes disrupted by other group members.^[39] Females have even been recorded mounting other females.^[46]

 

 

The gestation period of gray langur lasts around 200 days, at least at Jodhpur, India. In some areas, reproduction is year-around.^[45] Year-round reproduction appears to occur in populations that capitalize on human-made foods. Other populations have seasonal reproduction.^[39] Infanticide is common among gray 

langurs. Most infanticidal langurs are males that have recently immigrated to a group and driven out the prior male. These males only kill infants that are not their own.^[47] Infanticide is more commonly reported in one-male groups, perhaps because one male monopolizing matings drives the evolution of this trait. In multiple-male groups, the costs for infanticidal males are likely to be high as the other males may protect the infants and they can't ensure that they'll sire young with other males around. Nevertheless, infanticide does occur in these groups, and is suggested that such practices serve to return a female to estrous and gain the opportunity to mate.^[48]

 

 

Females usually give birth to a single infant, although twins do occur. Most births occur during the night.^[49] Infants are born with thin, dark brown or black hair and pale skin. Infants spend their first week attached to their mothers' chests and mostly just suckle or sleep.^[50] They do not move much in terms of 

 

 

locomotion for the first two weeks of their life. As they approach their sixth week of life, infants vocalize more.^[51] They use squeaks and shrieks to communicate stress. In the following months, the infants are capable of quadrupedal locomotion and can walk, run and jump by the second and third months. Alloparenting occurs among langurs, starting when the infants reach two years of age. The infant will be given to the other females of the group. However, if the mother dies, the infant usually follows.^[50] Langurs are weaned by 13 months.

 

 Gray langurs are recorded to make a number of vocalizations:

 

 Gray langurs have stable populations in some areas and declining ones in others.^[54] Both the black-footed gray langur and Kashmir gray langur are considered threatened.^[3][55] The latter is the rarest species of gray langur, with less than 250 mature individuals remaining.^[3]

 

In India, gray langurs number at around 300,000.^[56] India has laws prohibiting the capturing or killing of langurs, but they are still hunted in some parts of the country.^[12] Enforcement of these laws has proven to be difficult and it seems most people are unaware of their protection.^[57] Populations are also threatened by mining, forest fires and deforestation for wood.^[58]

Langurs can be found near roads and can become victims of automobile accidents. This happens even in protected areas, with deaths by automobile collisions making nearly a quarter of mortality in Kumbhalgarh Wildlife Sanctuary in Rajasthan, India.^[59] Langurs are considered sacred in the Hindu religion and are 

 

sometimes kept for religious purposes by Hindu priests and for roadside performances. However, some religious groups use langurs as food and medicine, and parts of gray langurs are sometimes kept as amulets for good luck.^[60]

 

 

Because of their sacred status and their less aggressive behavior compared to other primates, langurs are generally not considered pests in many parts of India. Despite this, research in some areas show high levels of support for the removal of langurs from villages, their sacred status no longer important.^[61] 

Langurs will raid crops and steal food from houses, and this causes people to persecute them.^[61] While people may feed them in temples, they do not extend such care to monkeys at their homes.^[62] Langurs stealing and biting people to get food in urban areas may also contribute to more persecutions.^[63]