Rattus: the main pest of crops
Brown rat (Rattus norvegicus)
Black rat (Rattus rattus)
Classification
(Globally
there are more than 1,000 species of rats and mice).
Family: Muridae
The Murids are
classified in 5 subfamilies,
around 150 genera and approximately 710 species.
Subfamilies:
1-Deomyinae (spiny mice, brush furred mice, link
rat).
2-Gerbillinae (gerbils, jirds and sand rats).
3-Leimacomyinae (Togo
4-Lophiomyinae (Crested Rat).
5-Murinae (Old World rats and
mice including the vlei rats)
Subfamily: Murinae
Including
10 genera: (Apodemus,
Bandicota,
Dacnomys,
Golunda,
Leopoldamys,
Millardia,
Mus,
Niviventer,
Rattus,
Vandeleuria).
Genus: Rattus
The genus Rattus proper
contains 66 extant species.
A subgeneric breakdown of the species has been proposed, but does not include
all species.
The list of 66 species of Genus Rattus (Typical rats)
+Incertae sedis
(uncertain
placement)
1-Annandale's Rat (Rattus
annandalei) – Indonesia, Malaysia, and Singapore.
2-Enggano Rat (Rattus
enganus) – Indonesia
3-Philippine Forest Rat (Rattus everetti) –
the Philippines.
4-Polynesian Rat (Rattus
exulans) –Fiji and most
Polynesian islands, New
Zealand, Easter
Island, and Hawaii.
5-Hainald's Rat (Rattus
hainaldi) – Indonesia
6-Hoogerwerf's Rat (Rattus
hoogerwerfi) – Indonesia
7-Korinch's Rat (Rattus
korinchi) – Indonesia
8-died Maclear's Rat (Rattus
macleari) – Christmas
Island.
10-Molaccan Prehensile-tailed Rat(Rattus
morotaiensis) – Indonesia
11-died Bulldog Rat (Rattus
nativitatis) – Christmas Island .
12-Kerala Rat (Rattus
ranjiniae) – India.
13-New Ireland Forest Rat (Rattus sanila).
14-Andaman Rat (Rattus
stoicus) –Andaman
Islands.
+Rattus norvegicus group
16-Himalayan Field Rat (Rattus nitidus) – Bangladesh, Bhutan, China, India, Indonesia, Laos, Myanmar, Nepal,Palau, the Philippines, Thailand, and Vietnam
18-Turkestan
Rat (Rattus pyctoris; obs.Rattus turkestanicus) –Afghanistan, China,
India, Iran, Kyrgyzstan, Nepal, and Pakistan.
+Rattus rattus group
19-Sunburned Rat (Rattus
adustus) –Enggano
Island, Indonesia .
20-Sikkim
Rat (Rattus andamanensis) – Bhutan, Cambodia, China , India ,
Laos , Myanmar , Nepal ,
Thailand , and Vietnam
21-Rice-field Rat (Rattus
argentiventer) – Southeast
Asia
22-Summit Rat (Rattus
baluensis) –Malaysia.
23-Aceh Rat (Rattus blangorum).
24-Nonsense Rat (Rattus
burrus) – India
25-Hoffmann's Rat (Rattus
hoffmanni) – Indonesia
26-Koopman's Rat (Rattus
koopmani) – Indonesia
27-Lesser Rice-field Rat (Rattus losea) – China , Laos , Taiwan, Thailand ,
and Vietnam
28-Mentawai Rat (Rattus
lugens) – Indonesia
29-Mindoro Black Rat (Rattus
mindorensis) – the Philippines
30-Little Soft-furred Rat (Rattus mollicomulus) – Indonesia
31-Osgood's Rat (Rattus
osgoodi) – Vietnam
32-Palm Rat (Rattus
palmarum) – India
35-Simalur Rat (Rattus
simalurensis) – Indonesia
36-Tanezumi Rat (Rattus
tanezumi) – Afghanistan, Bangladesh,
Cambodia, China, Cocos (Keeling) Islands, Fiji, India, Indonesia, Japan, North Korea, South Korea, Laos,
Malaysia, Myanmar, Nepal, Pakistan, the Philippines, Taiwan, Thailand, and
Vietnam.
37-Tawi-Tawi Forest Rat (Rattus tawitawiensis) – the Philippines
38-Malayan Field Rat (Rattus
tiomanicus) – Indonesia ,
Malaysia , the Philippines , and Thailand
+Rattus xanthurus group
39-Bonthain Rat (Rattus
bontanus; obs. Rattus foramineus) – Indonesia
40-Opossum Rat (Rattus
marmosurus) – Indonesia
41-Peleng Rat (Rattus
pelurus) – Indonesia
42-Southeastern
Xanthurus Rat (Rattus salocco) – Indonesia
43-Yellow-tailed Rat (Rattus xanthurus) – Indonesia
+Rattus leucopus group
(New Guinean group).
44-Arfak
Rat (Vogelkop Mountain Rat) (Rattus arfakiensis).
45-Western
New Guinea
46-Sula Rat (Rattus
elaphinus) – Indonesia
47-Spiny Ceram Rat (Rattus
feliceus) – Indonesia
48-Giluwe Rat (Rattus
giluwensis) –Papua
New Guinea.
49-Japen Rat (Rattus
jobiensis) – Indonesia
50-Cape
York Rat (Rattus leucopus) –Australia, Indonesia , and Papua New Guinea
51-Eastern Rat (Rattus
mordax) – Papua New
Guinea
52-Moss-forest Rat (Rattus
niobe) – Papua New Guinea ,
Indonesia
53-New Guinean Rat (Rattus
novaeguineae) – Papua
New Guinea
54-Arianus's Rat (Rattus omichlodes).
55-Pocock’s Highland Rat (Rattus
pococki).
56-Spiny Rat (Rattus
praetor) – Indonesia , Papua New Guinea , andSolomon Islands
57-Glacier Rat (Rattus
richardsoni) – Indonesia
58-Stein's Rat (Rattus
steini) – Indonesia and Papua New Guinea
59-Van Deusen's Rat (Rattus
vandeuseni) – Papua New
Guinea
60-Slender Rat (Rattus
verecundus) – Indonesia
and Papua New Guinea
+Rattus fuscipes group
(Australian group).
61-Dusky Rat (Rattus
colletti) – Australia
62-Bush Rat (Rattus
fuscipes) – Australia
63-Australian Swamp Rat (Rattus lutreolus) – Australia
64-Dusky Field Rat (Rattus
sordidus) – Australia , Indonesia , and Papua New Guinea
65-Pale Field Rat (Rattus
tunneyi) – Australia
66-Long-haired
Rat (Rattus villosissimus) – Australia
Two worldwide species
1-Brown Rat or Norway Rat: Rattus norvegicus (Berkenhout, 1769)
The brown rat, common rat, street rat, sewer rat, Hanover
One of the largest muroids,
it is a brown or grey rodent with a body up to 25 cm long,
and a similar tail length; the male weighs on average 350 g and the female 250 g. Thought to have originated in northern China ,
this rodent has now spread to all continents
except Antarctica, and is the dominant rat in Europe and much of North America
- making it the most successful mammal on the planet after humans. Indeed, with
rare exceptions, the brown rat lives wherever humans live, particularly in
urban areas.
Selective breeding of Rattus
norvegicus has produced the laboratory rat, an important model organism in biological research, as well as pet rats.
Naming and etymology
Originally called the "Hanover rat" by people
wishing to link problems in 18th century England with the House of Hanover, it is not known for
certain why the brown rat is named Rattus norvegicus (Norwegian rat), as it did not
originate from Norway. However,
the English naturalist John
Berkenhout, author of the 1769 book Outlines
of the Natural History of Great Britain, is most likely responsible for
popularizing the misnomer.
Berkenhout gave the brown rat the binomial
name Rattus norvegicus,
believing it had migrated to England
from Norwegian ships in 1728, although no brown rat had entered Norway
By the early to middle part of the 19th century, British
academics were aware that the brown rat was not native to Norway , hypothesizing (incorrectly) that it may
have come from Ireland , Gibraltar
or across the English
Channel with William the Conqueror. As early as
1850, however, a more correct understanding of the rat's origins was beginning
to develop. The British novelist Charles
Dickens acknowledged the misnomer
in the 2 June 1888 edition of his weekly journal, All the Year Round, writing:
"Now there is a mystery about the native country of the
best known species of rat, the common brown rat."
Academics began to understand the origins and corrected
etymology of the brown rat towards the end of the 19th century, as seen in the
1895 text Natural History by American scholar Alfred Henry
Miles:
"The brown rat is the species common in England Persia to England
Though the assumptions surrounding this species' origins
were not yet entirely accurate, by the 20th century, it was established among
naturalists that the brown rat did not originate in Norway ,
rather the species came from central Asia and (likely) China
Description
The fur is coarse and usually brown or dark
grey, while the underparts are lighter grey or brown. The length can be up to
25 cm, with the tail a further 25 cm , the same length as the body.
Adult body weight averages 550 g in males and about
350 g in females, but a very large individual can reach 900 g. Rats weighing
over 1 kg are exceptional, and stories of rats as big as cats are
exaggerations, or misidentifications of other rodents, such as the coypu and muskrat.
Brown rats have acute hearing,
are sensitive to ultrasound, and
possess a very highly developed olfactory
sense. Their average heart rate is 300 to 400 beats per minute, with a
respiratory rate of around 100 per minute. The vision of a pigmented rat is poor, around
20/600, while a nonpigmented (albino) with no melanin in its eyes has both
around 20/1200 vision and a terrible scattering of light within its vision.
Brown rats are dichromates which perceive colours rather like a
human with red-green
colorblindness, and their colour saturation may be quite faint. Their blue
perception, however, also has UV receptors, allowing them to see ultraviolet
lights that some species cannot.
Biology and behavior
The brown rat is usually active at night and is a good
swimmer, both on the surface and underwater, but unlike the related black rat (Rattus rattus), they are poor
climbers. Brown rats dig well, and often excavate extensive burrow systems. A
2007 study found brown rats to possess metacognition,
a mental ability previously only found in humans and some primates, but further
analysis suggested they may have been following simple operant conditioning
principles.
Communication
Brown rats are capable of producing ultrasonic
vocalizations. As pups, young rats use different types of ultrasonic cries to
elicit and direct maternal search behavior, as well as to regulate their
mother's movements in the nest. Although pups will produce ultrasounds around
any other rats at 7 days old, by 14 days old they significantly reduce
ultrasound production around male rats as a defensive response. Adult rats will
emit ultrasonic vocalizations in response to predators or perceived danger; the
frequency and duration of such cries depends on the sex and reproductive status
of the rat. The female rat will
also emit ultrasonic vocalizations during mating.
Chirping
Rats may also emit short, high frequency, ultrasonic,
socially induced vocalization during rough and tumble play, before receiving morphine, or mating, and when tickled.
The vocalization, described as a distinct "chirping", has been
likened to laughter, and is
interpreted as an expectation of something rewarding. Like most rat
vocalizations, the chirping is too high in pitch for humans to hear without
special equipment. Bat detectors are often used by pet owners for this purpose.
In clinical studies, the chirping is associated with
positive emotional feelings, and social bonding occurs with the tickler,
resulting in the rats becoming conditioned to seek the tickling. However, as
the rats age, the tendency to chirp appears to decline.
Rat chirp also can be used for mosquito control.
Other ultrasonic vocalisations, including a lower-frequency
'boom' or 'whoom' noise can be produced by bucks in a calm state, when grooming
or settling down to sleep.
Audible communication
Brown rats also produce communicative noises capable of
being heard by humans. The most commonly heard in domestic rats is bruxing, or
teeth-grinding, which is most usually triggered by happiness, but can also be
'self-comforting' in stressful situations, such as a visit to the vet. The
noise is best described as either a quick clicking or 'burring' sound, varying
from animal to animal.
In addition, they commonly squeak along a range of tones
from high, abrupt pain squeaks to soft, persistent 'singing' sounds during
confrontations.
Diet
The brown rat is a true omnivore and will consume almost
anything, but cereals form a substantial part of its diet.
Martin Schein, founder of the Animal Behavior Society in 1964, studied the diet of brown
rats and came to the conclusion that the most-liked foods of brown rats include scrambled eggs, macaroni and cheese,
and cooked corn kernels. According to Schein, the least-liked foods were raw
beets, peaches, and raw celery.
Foraging behavior is often population-specific, and varies
by environment and food source. Brown rats living near a hatchery in West
Virginia Italy island of Norderoog in the North Sea stalk and kill sparrows and ducks.
Reproduction and life cycle
The brown rat can breed throughout the year if conditions
are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and
litters can number up to 14, although seven is common. They reach sexual
maturity in about five weeks. The maximum
life span is up to three years,
although most barely manage one. A yearly mortality rate of 95% is estimated,
with predators and interspecies conflict as major causes.
When lactating, female rats display a 24 hour rhythm of
maternal behavior, and will usually spend more time attending to smaller
litters than large ones.
Brown rats live in large, hierarchical groups, either in
burrows or subsurface places, such as sewers and cellars. When food is in short
supply, the rats lower in social order are the first to die. If a large
fraction of a rat population is exterminated, the remaining rats will increase
their reproductive rate, and quickly restore the old population level.
Social behavior
Rats commonly groom
each other and sleep together. As
with dogs, rats create a social hierarchy, and each rat has its own
place in the pack. Rats are said to establish an order of hierarchy, so one rat
will be dominant over another one. Groups of rats tend to "play
fight", which can involve any combination of jumping, chasing, tumbling,
and boxing. Play fighting
involves rats going for each other's necks, while serious fighting involves
strikes at the others' back ends. If living space become limited, rats may turn
to aggressive behavior, which may result in the death of some animals, reducing
the burden over the living space.
Burrowing
Rats are known to burrow extensively, both in the wild and
in captivity, if given access to a suitable substrate.
Rats generally begin a new burrow adjacent to an object or structure, as this
provides a sturdy "roof" for the section of the burrow nearest to the
ground's surface. Burrows usually develop to eventually include multiple levels
of tunnels, as well as a secondary entrance. Older male rats will generally not
burrow, while young males and females will burrow vigorously.
Burrows provide rats with shelter and food storage, as well
as safe, thermoregulated nest sites. Rats use their burrows to escape from
perceived threats in the surrounding environment; for example, rats will
retreat to their burrows following a sudden, loud noise or while fleeing an
intruder. Burrowing can therefore be described as a "pre-encounter
defensive behavior", as opposed to a "postencounter defensive
behavior", such as flight, freezing, or avoidance of a threatening
stimulus.
Distribution and habitat
Likely originating from the plains of Asia, northern China and Mongolia
The brown rat may have been present in Europe as early as 1553 (from an illustration
and description by Swiss naturalist Conrad
Gesner in his book Historiae animalium, published
1551-1558).
Reliable reports dating to the 18th century document the
presence of the brown rat in Ireland
in 1722, England in 1730, France in 1735, Germany
in 1750, and Spain North America
until around 1750-1755.
The only brown rat-free zones in the world are the Arctic, the Antarctic, some especially isolated
islands, the province of Alberta in Canada ,
and certain conservation areas in New Zealand
In New
Zealand , first
arriving before 1800 (perhaps on James Cook's vessels),brown rats have posed a
serious threat to many of New Zealand Campbell Island
Diseases
Similar to other rodents,
brown rats may carry a number of pathogens,which can result in disease,
including Weil's disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever and hantavirus
pulmonary syndrome. In the United
Kingdom
This species can also serve as a reservoir for Toxoplasma gondii, the parasite
that causes toxoplasmosis, though
the disease usually spreads from rats to humans when domestic cats feed on
infected brown rats.The parasite has a long history with the brown rat, and
there are indications that the parasite has evolved to alter an infected rat's
perception to cat predation, making it more susceptible to predation and
increasing the likelihood of transmission.
Surveys and specimens of brown rat populations throughout
the world have shown this species is often associated with outbreaks of trichinosis.
but the extent to which the brown rat is responsible in transmitting Trichinella larvae to humans and other
synanthropic animals is at least somewhat debatable. Trichinella
pseudospiralis, a parasite previously not considered to be a potential
pathogen in humans or domestic animals, has been found to be pathogenic in
humans and carried by brown rats.
Brown rats are sometimes mistakenly thought to be a major
reservoir of bubonic plague, a
possible cause of the Black
Death. However, the bacterium responsible, Yersinia
pestis, is commonly endemic in only a few rodent species and is usually
transmitted zoonotically by rat fleas-common carrier rodents today
include ground squirrels and wood
rats.
Control
Control
Some of the common methods used to control the number of Norway
1-Trapping
Using traditional break-back traps, glue traps, live cage
traps and other humane traps
2-Poisoning
There are many types of poison available for the purpose of
controlling the Brown rat. The use of poison is controlled in most of the countries
in the world so its important to check your state legislations.
3-Proofing
Prevention is the best cure, There are ways to stop the
infestation occurring at the first place e.g. Blocking access points, better
waste management, better sewage design etc.
2-Black rat (Rattus rattus)
The black
rat (Rattus rattus) is
a common long-tailed rodent of the genus Rattus (rats) in the subfamily Murinae (murine rodents). The species
originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.
Taxonomy
The black rat was one of the many species originally
described by Linnaeus in his 18th century work, Systema Naturae, and it still
bears its original name of Rattus
rattus. It is the type
species of the genus Rattus. Alternate names include ship rat, roof rat, house rat, Alexandrine rat, and old English rat.
Description
A typical adult black rat is 32.4-46.4 cm long, including a 17-25
cm tail, and weighs 110-340 g. Despite its name, the black rat exhibits several
colour forms. It is usually black to light brown in colour with a lighter
underside.
Origin of Rattus rattus
Rattus rattus bone remains
that date back to the Norman Period have been discovered in Britain Levant (eastern Europe)
during post-glacial periods.The specific origin of the black rat is uncertain
due to the rat's disappearance and reintroduction. Evidence such as DNA and
bone fragments also suggests that rats did not originally come from Europe , but migrated from southeast Asia.
Diseases
Diseases
Rats are resilient vectors for many diseases because of
their ability to hold so many infectious bacteria in their blood. Rats played a
primary role in spreading bacteria, such as Yersinia
pestis, which is responsible for the Justinianic
plague and Bubonic plague. According to
epidemiological models, Yersinia
pestis originated outside of
Europe which indicates that Western and central Europe
have never had any natural rodent plagues.
Diet
Black rats eat a wide range of foods, including seeds,
fruit, stems, leaves, fungi, and a variety of invertebrates and vertebrates.
They are generalists, and thus
not very specific in their food preferences, which is indicated by their
tendency to feed on any meal provided for cows, swine, chickens, cats, and dogs.They
are similar to the tree squirrel in their preference of fruits and nuts. They
eat about 15 grams per day and drink about 15 ml per day.Their diet is
high in water content.They are a threat to many natural habitats because they
feed on native birds and insects. They are also a threat to many farmers since
they feed on a variety of agricultural-based crops, such as cereals, sugar
cane, coconuts, cocoa, oranges, and coffee beans.
Distribution and habitat
The black rat originated in India
and Southeast Asia, and spread to
the Near East and Egypt , and then throughout the Roman Empire, reaching England
Black rat populations can explode under certain
circumstances, perhaps having to do with the timing of the fruiting of the bamboo plant, and cause devastation to
the plantings of subsistence farmers; this phenomenon, Mautam, is happening currently
in parts of India
Black rats are thought to have arrived in Australia
In New Zealand
Throughout most of the world, black rats are found only in disturbed habitats near people, mainly near the coast. Black rats are the most frequent predator of small forest birds, invertebrates, and perhaps lizards inNew Zealand
Throughout most of the world, black rats are found only in disturbed habitats near people, mainly near the coast. Black rats are the most frequent predator of small forest birds, invertebrates, and perhaps lizards in
Black rats adapt to a wide range of habitats. In urban areas
they are found around warehouses residential, buildings, and other human
settlements. They are also found in agricultural areas, such as in barns and
crop fields. In urban areas they prefer to live in dry upper levels of
buildings, so they are commonly found in wall cavities and false ceilings. In
the wild, black rats live in cliffs, rocks, the ground, and trees.They are
great climbers and prefer to live in trees, such as pines and palm trees. Their
nests are typically spherical and made of shredded material, including sticks,
leaves, other vegetation, and cloth. In the absence of trees, they can burrow
into the ground. Black rats are also found around fences, ponds, riverbanks,
streams, and reservoirs.
The black rat, along with the brown rat, is one of the most
widespread rats and animal species in the world.
Home range
Home range refers
to the area in which an animal travels and spends most of its time. It is
thought that male and female rats have similar sized home ranges during the
winter, but male rats increase the size of their home range during the breeding
season. Along with differing between rats of different gender, home range also
differs depending on the type of forest in which the black rat inhabits.
Behavior
It is nocturnal and omnivorous,
with a preference for grains and fruit. Compared to the brown rat, it is a poor
swimmer, but more agile and a better climber, tending even to flee upwards. In
a suitable environment it will breed throughout the year, with a female
producing three to six litters of up to ten young. Females may regulate their
production of offspring during times when food is scarce, producing as few as
only one litter a year. R.
rattus lives for about 2–3
years. Social groups of up to sixty can be formed.
Nesting behavior
Through the usage of tracking devices such as radio
transmitters, rats have been found to occupy dens located in trees, as well as
on the ground.
Rats appear to den and forage in separate areas in their
home range depending on the availability of food resources.
All other habitat variables showed little to no correlation.While
this specie's relative, the Brown (Norway
Foraging behavior
As generalists, Black rats express great flexibility in
their foraging behavior. They are predatory animals and adapt to different
micro-habitats. They often meet and forage together in close proximity within
and between sexes.Rats tend to forage after sunset. If the food cannot be eaten
quickly, they will search for a place to carry and hoard to eat at a later time.
Although black rats eat a broad range of foods, they are highly selective
feeders; only a restricted number of the foods they eat are dominant foods.When
black rat populations are presented with a wide diversity of foods, they eat
only a small sample of each of the available foods. This allows them to monitor
the quality of foods that are present year round, such as leaves, as well as
seasonal foods, such as herbs and insects. This method of operating on a set of
foraging standards ultimately determines the final composition of their meals.
Also, by sampling the available food in an area, the rats maintain a dynamic
food supply, balance their nutrient intake, and avoid intoxication by secondary
compounds.
Ecology
Black rats (or their ectoparasites)
are able to carry a number of pathogens, of which bubonic plague (via the rat flea), typhus, Weil's disease, toxoplasmosis and
trichinosis are the best
known. It has been hypothesized that the displacement of black rats by brown
rats led to the decline of the Black
Death. This theory has, however, been deprecated, as the dates of these
displacements do not match the increases and decreases in plague outbreaks.[19]
Predators and parasites
The black rat serves as prey to cats and owls in domestic
settings. In less urban settings, rats are preyed upon by weasels, foxes, and
coyotes. These predators have little effect on the control of the black rat
population because black rats are agile and fast climbers. In addition to
agility, the black rat also makes use of its keen sense of hearing to detect
danger and quickly evade mammalian and avian predators. Rats serve as
outstanding vectors for transmittance of diseases because they have the ability
to carry bacteria and viruses in their systems. A few parasites that are common
to rats include Streptococcus
pnuemoniae, Corynebacterium
kutsheri,Bacillus piliformis, Pasteurella
pneumotropica, and Streptobacillus
moniliformis, to name a few. All of these bacterium are disease causing
agents in humans. In some cases, these diseases are incurable.
Complex pest
The black rat has been considered a complex pest, which is a
pest that influences the environment in both harmful and beneficial ways. In many
cases, after the black rat is introduced into a new area, the population size
of some native species declines or goes extinct altogether. This is often due
to the fact that the black rat is a good generalist with a wide dietary niche and a preference for complex habitats;
this causes strong competition for resources among small animals. This has led
to the black rat completely displacing many native species in Madagascar , the Galapagos, and the Florida Keys .
Control methods
Large-scale rat control programs have been taken to maintain
a steady level of the invasive predators in order to conserve the native
species in New Zealand
Pesticides, such as pindone and 1080 (sodium fluoroacetate), are
commonly distributed via aerial spray by helicopter as a method of mass control
on islands infested with invasive rat populations.
Bait, such as brodifacoum, is also used along with coloured
dyes in order to kill and identify rats for experimental and tracking purposes.
Another method to track rats is the use of wired cage traps, which are used
along with bait, such as rolled oats and peanut butter, to tag and track rats
to determine population sizes through methods like mark-recapture and
radio-tracking.
Poison control methods are effective in reducing rat
populations to nonthreatening sizes, but rat populations often rebound to
normal size within months. Besides their highly adaptive foraging behavior and
fast reproduction, the exact mechanisms for their rebound is unclear and are still
being studied.
References
1-From Wikipedia, the free encyclopedia
