Otter Ecology & Field Signs, Pt 2

One of my local Otters, a female that I’ve come to know as “Tasha”, due to her unusual lip markings.

As discussed in part 1 of this series, the Eurasian Otter population is now widely spread across the UK, and with a little detective work you can often find where Otters are living using feeding signs, prints, spraints and other field signs they leave behind. When you do find you have resident Otters, the next thing to do is to try and understand their behaviour and one good way of doing this is by looking at their social interaction with other Otters. Whilst it is true Otters are largely solitary (and for adult males this is absolutely true apart from coming together with females for a few days to mate) , adult Females spend a lot of time with their cubs, helping them to learn how to hunt and fend for themselves until they leave her side at around 10-16 months. Otters have a fascinating way of parenting and in this blog we will give an overview of the breeding habits and raising of young of the Eurasian Otter in the UK and also highlight the biggest causes of infant mortality.

Finding a mate

Eurasian Otters reach sexual maturity at somewhere between 18 months and 2 years of age, and because the female is continually polyoestrous (that is to say can be in heat all year round) breeding can occur at any time of the year, although from my observations mating between October and March seems quite common in Southern England. Eurasian Otters are usually solitary animals and typically only come together for mating. In order to find a mate, male otters will establish a territory that covers the range of several female territories. They will then defend this territory against other males and attract females with a variety of vocalizations and scent markings. The female similarly will let the dog otter know that she is ready for mating, communicating via pheromones in her spraint.

The average territory size of a male Eurasian Otter in the UK can range from 10-20 km of river or lake shoreline, while the average territory size of a female is usually much smaller, ranging from 1-5 km. The size of the territory will depend on the availability of food and the density of the otter population.

Once a male finds a female, there can be much following and play until mating occurs, usually in the water, and it can be quite vigorous, with the male often biting the female on the back or neck. Pregnancy lasts for 60-64 days typically and unlike some other Mustelids (E.g. European Badger) there is no delayed implantation of the egg.

From a photography and watching perspective, if you see 2 adult sized Otters together, but only for a handful of days, there is a fair chance that this is a male and female who have come together to mate. It’s far from guaranteed, but it makes sense to mark the date in your Wildlife diary or notebook, and keep a good look out for cubs above ground in about 5 months time.

Spraint is by Otters to used to communicate among other things, age, sex, identity, and reproductive status.

Birthing Dens and Holts

Once a female is pregnant, she will seek out a suitable birthing den or natal holt to give birth and raise her young. Unlike non-natal holts, these dens can be little further from the waters edge (but aren’t always) and are primarily chosen for the protection they provide. Typical sites are a rocky hollow, in extensive tree roots, old Badger setts, or in very dense vegetation. Natal holts can also be above ground (particularly in extensive reedbeds), where the mother otter will make a covered nest using vegetation and sticks. All birthing dens regardless of location are usually lined with soft materials such as grasses, mosses, and leaves to provide a warm and comfortable environment for the young. In addition, the female will often add a layer of her own fur to the nest to keep the youngsters warm. The prime purpose of the natal holt is to provide protection from predators and the elements, and to this end they are usually in tucked away locations, where most of us would not accidentally come across one.

As I wrote earlier, gestation is approximately 60-64 days and the litter is usually two or three young , but can be as many as five. Depending on where you are in the UK the young can be known as kits, pups, or cubs. When born, the kits are about 12 cm (4 inches) long and weight between 100 and 150g, which makes them about the size and weight of an average banana. The kits are born blind and remain so until about 4 weeks of age, and they then stay a further 4-6 weeks in the natal holt, making a total of 8-10 weeks. During this time the female Otter leaves them several times throughout the day and night to hunt, and returns to feed and clean them. The Dog Otter plays no part in raising the young.

Otter natal holts can be remarkably small from the outside, and other than the level of protection they offer don’t differ significantly from other lying up places that Otters might use at certain times of the year.

Energy Expenditure and Milk Production

Raising young is an energy-intensive process for Eurasian Otters, and the female will spend a significant amount of energy producing milk for her kits. The high fat content of otter milk, which can reach up to 60% in some cases, requires a significant amount of energy to produce. In addition, the female will also need to expend energy diving for food to support her own energy needs.

It is estimated that a female otter will expend approximately 20% of her daily energy intake producing milk for her kits. The energy expenditure required to produce milk and wean a typical litter can be substantial, and it is not uncommon for female otters to lose a significant amount of weight during the weaning process. It is thought that this massive investment of energy may be one reason why many female Otters only breed every other year. Of course it could also simply be that most kits don’t leave their mother until they are over 12 months old, and as such she would not advertise herself as being in heat to the dog Otter.

Weaning

Eurasian Otter kits rely entirely on their mother for warmth and nourishment during the first few weeks of their lives. As they grow, they will begin to explore their surroundings and develop the skills they need to hunt and survive on their own. At around 7 or 8 weeks the first solid food is introduced to them by the mother and the weaning process begins. They will be fully weaned by the time they are 6 months old, or so. During the weaning process, the mother will gradually reduce the amount of milk she provides to the kits and encourage them to start eating solids. By the time they are fully weaned, the kits will be able to catch their own food, and depending on the individual (and their mother) the youngsters will leave and establish their own territory by the time they are 10-16 months old.

Surprisingly Otter kits are not natural swimmers as their fur is very buoyant. As a result the mother has to take the cubs to the water when they are 3-4 months old and may need to encourage them to swim.

Mum and older cub fishing together

Infant Mortality

Unfortunately, the mortality rate for Eurasian Otter kits is relatively high, and it is not uncommon for only 50% of a litter to survive their first year. The biggest causes of infant mortality include poisoning, disease, habitat degradation, and simple accidents. In the UK predation is not a significant threat to young otters, although in Northern Scotland the kits can be preyed upon by Golden Eagles, and there have been reports of Badgers trying to dig out natal holts. Like all many animals, Otters are vulnerable to disease such as botulism and distemper, but by far the biggest killer is interaction with man, whether that is through accidental poisoning, getting caught in fishing nets (or mink traps), being run over, or by a pet dog finding a young Otter or the natal holt.

Female Otters are known to be highly protective of their young and will fiercely defend them against any perceived threat. However, in rare cases, they have also been known to attack and kill their own young. Infanticide can occur in animals for a variety of reasons, including resource competition, genetic incompatibility, or a lack of maternal care and bonding. In some cases, female Otters may kill their own young if a kit is sick or injured. This behaviour may be an adaptation to conserve resources for the surviving offspring and improve their chances of survival. In other cases, male Otters may kill young offspring if they are not their own. This behaviour is known as filial infanticide and may occur when a male takes over a territory and wants to mate with the resident female. By killing the young, the male can reduce competition for resources and increase the chances of mating and producing his own offspring.

It is important to note that these cases of infanticide are relatively rare and occur in only a small proportion of otter populations. The vast majority of Eurasian Otters are highly protective of their young and will go to great lengths to ensure their survival.

Habitat degradation is a major cause of infant mortality in Eurasian Otters. Human activities such as illegal dumping of waste, oil spills, dam construction, and deforestation can have a negative impact on the quality of the otter's habitat and reduce the availability of food, shelter, and breeding sites. In addition, roads and deaths from car accidents have a significant impact to the Otter population, particularly for young Otters that are leaving their mother to establish new territories. The dispersed young may have to cross roads to avoid other dominant Otters, or simply in their search of their own territory. In times of flood, traditional routes may be impassable, or food sources more widely dispersed, and again they are forced to cross roads.

A secluded watercourse like this burn running through ancient woodland is an ideal location for an Otter to give birth to her cubs, with a wide choice of potential natal holt sites, abundant food, and very little human disturbance.

Field Signs of Breeding

The fist thing to say here is that UNDER NO CIRCUMSATNCES IS IT OK TO INTERFERE WITH OR GET CLOSE TO AN OTTER HOLT (or day bed / couch), regardless of whether it is a breeding den or not. As well as being illegal, its also completely unacceptable to put any animal at risk just to get a photo or a close up look. If at any point you wonder if you are too close to a holt, you absolutely are, and you should quietly back away. Similalrly, if you stumble across a potential holt by accident , quietly move away, and conduct several days observation from at least 40 metres (100-120ft). Do not set up camera traps, sand traps or any other paraphernalia close to the holt. It’s illegal and not needed…….go old school (get your thermals on, grab your binos and camo jacket, and watch from a distance until you know if its in use or not). It should also be noted that the legal protection for holts and places of shelter is not just whilst an Otter is present, it is at all times, SO STAY AWAY.

Unfortunately (for wildlife watchers and photographers) Otters do not advertise the presence of the young, thereby minimising the risk of predation. However, if you watch an Otter regularly and you get to know it’s habits, you may well start to see a change in behaviour when the female is making herself available for a dog Otter, or is looking for a natal holt. These changes can be as simple as a change in sprainting behaviour, both in terms of frequency and site selection, to more obvious signs such as starting to visit a hollow under trees, or area or reedbed, and collecting nesting materials. As I mentioned above, if you see 2 adult Otters together, where normally you have only observed an adult female, and it is just for a few days, it is possible that this is a mating pair, particularly if you see the male following the female, or them rolling and playing “chase” together. However, remember that kits remain with their mothers until they are almost fully grown, so watch closely to be sure you are not watching a mother with her young.

One way to try and identify potential natal den sites which has been successful for me, is to look at a known females territory and then attempt to identify potential breeding den sites by study of maps and aerial photographs. These days it is possible to do this very easily using Google Earth or any other similar online mapping site, and has the benefit of being completely non-intrusive to the animal as its conducted away from the bank. I look for reed beds, lakes, rivers, canals and ponds that are lined with deciduous woodland or areas of dense scrub that are not accessible to the public, or similarly desolate buildings or structures (or areas of boulders) that are immediately adjacent to watercourses. Once you have identified these sites, you then need to try and establish if these areas are prone to flooding, and if they are remove them from your short list, as no experienced mother Otter will risk denning in a place where the kits could be washed out. The environment agency map flood plains for a variety of uses and this information can be found online for free. I use UK Postcode Checker. Find a postcode on the map or by address. (postcode-checker.co.uk) and then the Environment Agency website Where do you want to check? - GOV.UK (check-long-term-flood-risk.service.gov.uk) to understand the risk of flooding in the area. These flood maps are also useful to understand the smaller local watercourses as even in the case of natal dens that are some distance from the main watercourse they will normally be linked to the water by a ditch, area of wetland or some other semi aquatic feature.

An example of the online flood mapping available from the Environment agency.

Once you have whittled your study area down to likely (or at least potential) den sites, the next step is to conduct a bankside survey (but not near the holt) looking for signs of Otter activity and a distant watch of any potential holt (100m+), looking for comings and goings of the mother Otter, and signs that may be visible from a distance using binoculars such as heavily used paths from the water to the potential natal holt site. Again please remember UNDER NO CIRCUMSATNCES IS IT OK TO GET CLOSE TO AN OTTER HOLT and I strongly recommend contacting your Local Wildlife Trust or Local Otter Group to understand when is the best time of year to conduct this survey and to get suitable training before heading out to the bank.

Vocalisations

The other very useful indicator that kits are around is the vocalisations that can be heard. Like many baby animals Otter kits can be very vocal both when playing with their siblings and when communicating with mum. Otters produce a variety of whistles and chirps, which sound more bird like than you might expect. A common call you may hear on the bank when the mother is out with her young is the contact call, which is a single whistle lasting maybe half a second that is repeated every couple of seconds. A good example of this sound has been caught on video by Steve Waite and can be found here. Certainly if you hear whistles or chattering noises coming from reeds, scrub/dense brush, bolder piles or tree roots, it would be worth retiring to a safe distance and watching for a few hours to see if you can confirm your suspicions that an Otter is present.

Having caught a Perch, the mother Otter calls her almost fully grown kit and passes them the catch.

In the third and final part of this mini series all about Otters (which will be coming after my trip to Mull in May), we look at some practical tips for photographing Otters both on lowland watercourses and coastal areas. Until then “Keep clicking”.