The Mikura Dolphins

Mikura Island is one of the most well studied areas for wild dolphin research in Japan. About a seven and a half hour ferry ride from Tokyo, this small subtropical island has a human population of around 300 people and a dolphin population of 160 Indo-Pacific bottlenose dolphins that inhabit the surrounding waters. The island is a dormant volcano with a boulder-strewn seafloor, covered in parts by seaweed that the dolphins enjoy rubbing up against. Mikura Island Tourist Information Centre has been respectfully guiding snorkel swim tours for around 28 years so that people may observe these marine mammals in their natural underwater environment. From mid April to October, when the sea conditions are just right, visitors can slip beneath the waves and catch a glimpse into dolphin life. The tourist centre sticks to a carefully created code of conduct to ensure the dolphins are not harassed or disturbed by the swimmers' presence, with strict rules not to touch, feed or chase the dolphins. Since 1994 research surveys have been conducted with students and researchers coming from both within Japan and internationally to conduct underwater video-identification studies. The Mikura dolphins can be individually recognised by their bodily scars, scratches and unique markings. A dolphin’s age can even be estimated by how spotty its belly is, with dolphins over the age of 35 having the most speckles.

Photo credit: Mai Sakai

The dolphins spend their days mostly resting and socialising under the hazy glow of the warm sun, often engaging in object carrying behaviours like playing seaweed tag or, more bleakly, with plastic bags floating in the sea. Feeding is thought to mainly occur at night on cephalopod and fish species found in the top oceanic layers as well as the deeper mesopelagic layer. Day foraging has also been observed, most commonly in females who may be using the sunlit hours to teach their calves a few hunting techniques as well as topping up on some extra nutrients needed for the high-energy requirements of being a mum or pregnant female. Mum's choice of staying in the surface waters rather than diving to feed also helps keep babies protected from predators. Females tend to stick together in groups, usually those with calves will travel in nursery groups and those without calves associate together too. Around Mikura, Females usually first give birth from 7-13 years of age. Before this period however a female may develop her maternal skills by 'babysitting', swimming with and temporarily taking care of another female's calf. An additional reason females may stick together is to protect their calves and themselves from the harassment of males. Males become sexually mature around 7-8 years old and tend to get pretty rowdy. Groups of male dolphins, especially younger males, can frequently be spotted engaging in socio-sexual behaviour where individuals will excitedly cluster together touching, rubbing and biting, many with erect penises. This kind of rambunctious behaviour is important for males to strengthen friendships that are often long-lasting. Some male relationships around Mikura have been known to last over 9 years and it is these friendships that will help them to perfect their mating skills. Socio-sexual behaviour is an important component of dolphin life and starts from an early age, acquiring social skills and enabling practice for future courtship, with male calves even practicing with their mothers.

Male socio-sexual behaviour - photo credit: Mai Sakai

For a dolphin, maintaining social bonds is essential to their communication, reproduction and survival. Most dolphin species live in a fission-fusion society, allowing for fluidity in group composition with regular changes between members. With dolphins frequently mixing up associations throughout the day, social contact behaviours, like flipper rubbing, help to keep track of everyone. Many of the social behaviour studies around Mikura have focused on flipper rubbing, where a dolphin rubs another individual using its pectoral fin, or 'flipper'. Similar to when primates groom each other, flipper rubbing has social benefits, strengthening bonds between rubbing pairs. Just as a cuddle can calm you down, a flipper rub may also relieve any tension between pairs and help the dolphins to ‘de-stress’. Usually dolphins tend to rub other dolphins of the same age and sex but it is especially common between mothers and their calves as a bonding tool and provides opportunities for calf social learning. Dolphins also have other ways of working on their relationships through touch. Contact swimming, in which dolphin pairs will swim close together whilst resting their fin on the other, is a bonding tool seen more frequently in females. Even synchrony can act as a social behaviour, surfacing to breathe or diving together at exactly the same time demonstrates how in sync a pair is. Synchronous swimming as well as improving social affiliations also has hydrodynamic advantages, saving a little energy by gliding in the other's slipstream.

Flipper rubbing - photo credit: Mai Sakai

From this extraordinary insight into the dolphin's world has come an excellent array of research delving into their lives and behaviour as well as some unique findings. A rare baby adoption was observed around Mikura when a female dolphin, who had never been a mum before, nursed and adopted a calf whose mother had died entangled in a fishing net. This foster mother was not related to the biological mother nor had she shared any social affiliation with her. Although the calf ultimately couldn't be sustained this way, it still shows empathy in these marine mammals and an instinct to help other dolphins in need. Another unusual encounter, of a very different nature yet still oddly human-like, was recorded in these waters. This time whilst observing a resting group of mixed sex and ages, a 16 year old male spontaneously ejaculated. As the group was resting and there were no signs of sexual behaviour it was suggested that the male had been in a drowsy state and his neural control system relaxed.

Mikura Island is a wonderful example of human and animal coexistence. By maintaining their respect for the animals, each year swimmers are treated to magical encounters with these intelligent creatures. These encounters provide researchers with invaluable knowledge to help understand this species even better, allowing us a privileged peek into the dolphin world.

Learn more about the Mikura Dolphins here:

References:

Connor et al. (2019) The Indo-Pacific bottlenose dolphin (Tursiops aduncus). In B. Würsig (Ed.), Ethology and behavioral ecology of odontocetes. (See references in this chapter for more information on Mikura Dolphins)

Dudzinski et al (2008), (2010) Pectoral fin comparison studies

Genfu et al. (2021) Age‐related changes to the speckle patterns on wild Indo‐Pacific bottlenose dolphins. Marine Mammal Science.

Kogi et al. (2004) Demographic parameters of Indo-Pacific bottlenose dolphins (Tursiops aduncus) around Mikura Island, Japan. Marine Mammal Science.

Mikurashima Tourist Information

Morisaka et al. (2013) Spontaneous ejaculation in a wild Indo-Pacific bottlenose dolphin (Tursiops aduncus). PLoS One

Sakai et al. (2006) Flipper rubbing behaviors in wild bottlenose

dolphins (Tursiops aduncus). Marine Mammal Science.

Sakai et al. (2016) A wild Indo-Pacific bottlenose dolphin adopts a socially and genetically distant neonate, Nature

Takahashi et al. (2020) Prey species and foraging behaviour of Indo-Pacific bottlenose dolphins (Tursiops aduncus) around Mikura Island in Japan, Aquatic Mammals.