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Something about otters
Otter Classification
(GENERAL otters)
Carnivora
Typically meat-eaters with sharp canine teeth for killing and holding prey and keen
slicing cheek teeth for cutting up flesh.
Mustelid
Mustelids have long, thin bodies and short legs - ideal for pushing through dense
undergrowth or hunting in tunnels. They can walk on the soles of their feet and
have specialised skulls with elongated cranium often topped with a narrow crest.
Lutrine Species
Eurasian otter
Lutra lutra
Asian short-clawed otter
Amblonyx cinerea
Indian smooth-coated otter
Lutrogale perspicillata
Hairy-nosed otter
Lutra sumatrana
Cape clawless otter
Aonyx capensis
Congo clawless otter
Aonyx congica
Spotted-necked otter
Hydrictis maculicollis
North American river otter
Lutra canadensis
Neotropical otter
Lutra longicaudis
South American river otter
(various)
Brasilian giant otter
Pteronura brasiliensis
Marine otter
(sea cat) Lutra felina
Sea otter
Enhydra lutris
Sensory Perceptions
(GENERAL otters)
Scent is used
for hunting on land, for communication and for detecting danger. Otterine sense
of smell is likely to be similar in sensitivity to dogs.
Otters have small eyes and are probably short-sighted on land. But they do have
the ability to modify the shape of the lens in the eye to make it more spherical,
and hence overcome the refraction of water. In clear water and good light, otters
can hunt fish by sight.
The otter's eyes and nostrils are placed
high on its head so that it can see and breathe even when the rest of the body is
submerged.
Whisker-feel
Long whiskers (vibrissae) growing
around the muzzle are used to detect the presence of fish. They detect regular vibrations
caused by the beat of the fish's tail as it swims away. This allows otters to hunt
even in very murky water.
Physically, otter ears are tiny for streamlining, but they still have very sensitive
hearing. However, while water transmits sound better than air, it is almost impossible
to determine which way the sounds are coming from. Dolphins have special modification
to their ears to overcome this problem, but otters don't so it is unlikely that
otters use their hearing to hunt prey. Otter ears are protected by valves which
close them against water pressure.
Aquatic Adaptations
(GENERAL otter)
Aquadynamic
All mustelids have long sinuous bodies,
which makes them ideally adapted to semi-aquatic life (minks are the only other
semi-aquatic mustelids). Underwater, the otter holds its legs against the body,
except for steering, and the hind end of the body is flexed in a series of vertical
undulations.
River otters have webbing which extends for much of the length of each digit, though
not to the very end. Giant otters and sea otters have even more prominent webs,
while the Asian short-clawed otter has no webbing - they hunt for shrimps in ditches
and paddy fields so they don't need the swimming speed.
An otter's tail (or rudder, or stern) is stout at the base and tapers towards the
tip where it flattens. This forms part of the propulsion unit when swimming fast
under water.
Otter fur consists of two types of hair: stout guard hairs (up to 20mm long) which
form a waterproof outer covering, and underfur which is dense and fine, equivalent
to an otter's thermal underwear!
The fur must be kept in good condition
by grooming. Sea water reduces the waterproofing and insulating qualities of otter
fur when salt crystals form in the fur. This is why freshwater pools are important
to otters living on the coast. After swimming, they wash the salts off in the pools
and then squirm on the ground to rub dry against vegetation.
High metabolic rate
Otter metabolism is 20% higher than other similarly-sized animals to compensate
for losing heat in the water. Despite this, they get chilled if they stay in the
water too long. Coastal otters will only fish for 15 minutes at a time.
The otter's very large lungs allow
it to remain underwater for up to 4 minutes if need be.
Otter Spotting
(Lutra Lutra)
Otters are elusive creatures by nature and are only usually active at night, especially
around dawn and dusk. This makes the chances of a chance encounter extremely slim.
It is also important to be in the right
part of the country - south-west England, central and west Wales and northern Scotland
remain the only places with thriving otter populations, although they are expanding
slowly back into their previous range from these centres and from Norfolk.
Even setting out at the crack of dawn
on an otter river, it may take weeks of watching before catching the tiniest glimpse
of a wake in the water or a hunched back disappearing into the bushes.
The most common sign of otters, used
to build up the valuable otter surveys which have been used to track the range of
the otter in Britain, is the spraint. Otters typically spraint in certain places,
often quite visible. Sometimes they even make little mounds of mud or grass to spraint
upon (sign heaps). This is to mark their territory, not to provide a consolation
prize to frustrated otter watchers!
A perfect spraint is 6-8cm long about
1cm thick and dark in colour, usually full of tiny fishbones. A fresh one easily
distinguished from a mink scat by the scent - otter spraint smells pleasantly musky
or fishy. Honestly.
Other signs an otter might leave are
partly-eaten fish and paw prints (called seals). Otters have five-toed paws,
whereas dogs only have four. Even if the fifth toe does not leave a mark, the print
will appear lop-sided instead of symmetrical.
Breeding Habits
(Lutra Lutra)
Eurasian otters will breed any time where food is readily available. In places where
condition is more severe, Sweden for example where the lakes are frozen for much
of winter, cubs are born in Spring. This ensures that they are well grown before
severe weather returns. In the Shetlands, cubs are born in summer when fish is more
abundant.
Though otters can breed every year, some do not. Again, this depends on food availability.
Other factors such as food range and quality of the female may have an effect.
Gestation for Eurasian otter is 63 days, with the exception of Lutra canadensis
whose embryos may undergo delayed implantation. Otters normally give birth in more
secure dens to avoid disturbances. Nests are lined with bedding (reeds, waterside
plants, grass) to keep the cubs warm while mummy is away feeding.
Litter Size varies
between 1 and 5 (2 or 3 being the most common). For some unknown reason, coastal
otters tend to produce smaller litters.
At five weeks they open their eyes - a tiny cub of 700g. At seven weeks they're
weaned onto solid food. At ten weeks they leave the nest, blinking into daylight
for the first time. After three months they finally meet the water and learn to
swim. After eight months they are hunting, though the mother still provides a lot
of food herself. Finally, after nine months she can chase them all away with a clear
conscience, and relax - until the next fella shows up.
At Home
(Lutra Lutra)
Otters are found on rivers and lakes, around estuaries and along coastlines. Without
the influence of man, they would probably be found on every river, lake, estuary
and coast in Britain!
A number of constraints and preferences
limit suitable habitats for otters. Water is a must and the rivers must be large
enough to support a healthy population of fish. Being such shy and wary creatures,
they will prefer territories where man's activities do not impinge greatly. Of course,
there must also be no other otter already in residence - this has only become significant
again recently as populations start to recover.
A typical range for a male river otter might be 25km of river, a female's range
less than half this. However, the productivity of the river affects this hugely
and one study found male ranges between 12 and 80km. Coastal otters have a much
more abundant food supply and ranges for males and females may be just a few kms
of coastline. Because male ranges are usually larger a male otter may find his range
overlaps with two or three females - not bad!
Otter homes can be called Holts, Couches, Hovers, Resting Site or Den. Otters typically
use a lot of different resting places throughout their home territory, ranging from
holts beneath the roots of riverbank trees to flattened couches in dense vegetation
like redbuds. It seems that the less man's presence intrudes the more comfortable
otters are with "lying rough" in the open.
Although the majority of these resting
places are near water, they don't all have to be right along the riverbank. Breeding
dens especially can be found more than 100m from the closest stream - probably to
make sure there is no chance of flooding.
Characteristics
(Lutra Lutra)
The Eurasian otter is brown furred,
with a lighter patch beneath its chin and throat. It has the classic long and sleek
mustelid body with partially webs paws for swimming. An adult male may be up to
4 feet long and 30lbs. Females are smaller, around 16lbs typically. The Eurasian
otter's nose is about the smallest among the otter species and has a characteristic
shape described as a shallow 'W'.
About
Food
(Lutra Lutra)
Studies of otter spraints have given us valuable information of otter diet. Freshwater
otters generally take whatever fish are most readily available - sticklebacks, trout,
roach, perch or eels. When there is a choice, otters will tend to concentrate on
the ones easiest to catch, particularly eels. Any sizes will do though very tiny
fish are probably ignored.
Pike are taken infrequently, probably
because of their habit of lying in wait for their prey amongst the water weeds rather
than swimming around after it.Coastal
otters commonly eat eelpout, rocklings, sea scorpion and butterfish, all slow-moving,
bottom-dwelling fish.
An otter's diet will vary through the year as it concentrates on the best things
to catch. Frogs can become an important part of the diet during the breeding and
hibernating seasons. When the eels burrow into the mud in winter, the more active
river fish like roach and trout become easier to catch as the cold water slows them
down.
Otters will eat anything that they can get hold of - there are records of sparrows
and snakes and slugs being gobbled. Apart from fish the most common prey are crayfish,
crabs and water birds (grabbed from beneath like Jaws!). Small mammals are occasionally
taken, most commonly rabbits but sometimes even moles.
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