Mikura Island Research Articles

Observations and Detailed Descriptions of Sociosexual Behavior in Wild Indo-Pacific Bottlenose Dolphins (Tursiops aduncus)

Sociosexual behavior is not directly related to reproduction. It is observed between mature and immature individuals of both sexes and interactions can be between mixed- or same-sex individuals. This behavior may have important social functions. We observed and recorded the sociosexual behavior of wild Indo-Pacific bottlenose dolphins (Tursiops aduncus), underwater off Mikura Island, Japan, with regards to erection and mounting. This behavior was observed mainly between mothers and sons and in groups of subadult males (age 1–13 years). In subadult males, mounting behavior toward one passive individual from two or more individuals playing an active role, occurred in most cases. This behavior involving three or more participants has rarely been observed in other animal species, except for bowhead whales (Balaena mysticetus), common bottlenose dolphins (T. truncatus), and beluga whales (Delphinapterus leucas). Sociosexual behavior in the Mikura Island dolphins may not be agonistic because individuals in the passive role did not try to escape from the individuals playing the active role. Further studies on sociosexual behavior T. aduncus could reveal the social functions of this behavior, such as younger animals practicing mating behaviors required as adults and developing their relationships for when they are adults.

Noninvasive age estimation for wild Indo‐Pacific bottlenose dolphins (Tursiops aduncus) using speckle appearance based on quantification‐theory model I analysis

AbstractAge is an important parameter for the study of ecology and population dynamics. Current methodologies for age estimation in cetaceans generally involve invasive or postmortem techniques. They are not suitable for wild marine mammals with small populations. Our previous study shows that in Indo‐Pacific bottlenose dolphins, both the number and size of body speckles increase with age. We observed speckles of 41 known‐age individuals (28 females and 13 males) between the ages of 7 and 25 years using underwater video data. The ages of individuals were obtained from a long‐term identification survey since 1994. Here, we created an age‐estimation model using quantification theory type I analysis. The model estimated the age accurately from speckle observation (R2 = 0.77, standard deviation of 2.58 years). We clarified the age of 89 individuals (64% of 139 unknown age individuals) near Mikura Island. This is the first study to use a noninvasive age estimation method to statistically verify its accuracy and revealed the age of the majority of the population.

Body length and growth pattern of free-ranging Indo-Pacific bottlenose dolphins off Mikura Island estimated using an underwater 3D camera

Body length and growth pattern of free-ranging Indo-Pacific bottlenose dolphins off Mikura Island estimated using an underwater 3D camera

Prey Species and Foraging Behaviour of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) Around Mikura Island in Japan

Prey Species and Foraging Behaviour of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) Around Mikura Island in Japan

Seasonal and spatial shifts in feral cat predation on native seabirds vs. non-native rats on Mikura Island, Japan

Understanding how invasive predators impact native species is essential for the development of effective control strategies, especially in insular environments where alternative non-native prey species exist. We examined seasonal and spatial shifts in diet of feral cat Felis silvestris catus focusing on the predation on native streaked shearwaters, Calonectris leucomelas, and introduced rats, Rattus rattus and R. norvegicus, which are alternative prey to shearwaters, on Mikura Island, Japan. Streaked shearwaters breed at low elevations on the island from spring to autumn, whereas rats inhabit the island throughout the year, which makes them an alternative prey when native shearwaters are absent. Fecal analysis revealed that feral cats dramatically shifted their diets from introduced rats in winter to streaked shearwaters in seabird-season in low elevation areas of the island, while cats preyed on rats throughout the year at high altitudes on the island. This finding suggests that feral cats selectively prey on shearwaters. This is probably because of their large body size and less cautious behavior, and because introduced rats sustain the cat population when shearwaters are absent. The number of streaked shearwaters killed was estimated to be 313 individuals per cat per year, which represents an indication of top-down effects of feral cats on streaked shearwaters. Further studies on the demographic parameters and interspecific interactions of the three species are required to enable effective cat management for the conservation of streaked shearwaters on this island.

Identification of turbidite and hemipelagic mud layers in deep-sea sediments and depositional ages of the hemieplagic mud layers collected at eastern Mikura Island

Identification of turbidite and hemipelagic mud layers in deep-sea sediments and depositional ages of the hemieplagic mud layers collected at eastern Mikura Island

Camera trap records a domestic cat catching an endangered Izu thrush on Mikura Island, Japan

Camera trap records a domestic cat catching an endangered Izu thrush on Mikura Island, Japan

Mitochondrial DNA and cuticular hydrocarbon analyses of Lethe diana (Lepidoptera, Nymphalidae, Satyrinae) inhabiting Mikura Island, Japan

Mitochondrial DNA and cuticular hydrocarbon analyses of Lethe diana (Lepidoptera, Nymphalidae, Satyrinae) inhabiting Mikura Island, Japan

Emigration of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) from Mikura Island, Japan

Emigration of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) from Mikura Island, Japan

Dual Foraging Strategy and Chick Growth of Streaked ShearwaterCalonectris leucomelasat Two Colonies in Different Oceanographic Environments

Parent Procellariiformes alternate between long-range foraging trips to feed in distant productive marine areas and short-range trips to feed in less productive areas around their breeding colony. Foraging trip duration, diet, energy expenditure, and chick growth of Streaked Shearwaters Calonectris leucomelas were compared between two colonies, one on Mikura Island located in the warm and less productive waters of the Kuroshio current, and the other on Sangan Island located in the cold and more productive waters of the Kuroshio/Oyashio transition zone. Parent shearwaters breeding on Mikura alternated between short (≤3 days) trips to Kuroshio waters in order to provision their chicks and long (>3 days) trips to the cold Oyashio region in order to accumulate energy reserves for themselves. Shearwaters breeding on Sangan mainly took short trips (≤2 days) to the nearby Kuroshio/Oyashio transition zone, but also took longer trips (>2 days) to the cold Oyashio region. Parent shearwaters breeding on Mikura, however, made more frequent long trips (24%) than those breeding on Sangan (9%). Parents at both colonies commonly brought Japanese Anchovy Engraulis japonica and Darkedged-wing Flyingfish Cypselurus hiraii for their young, while parents on Mikura also brought Pacific Saury Cololabis saira and stomach oil after long trips. Parents on Mikura delivered energy at a lower rate, resulting in lower chick growth rates and smaller fledging masses than on Sangan. Unlike other Procellariiformes seabirds, shearwater parents on Sangan accumulate their own body reserves even during the chick rearing period because of better foraging efficiency than on Mikura. This may indicate that chicks were satiated by the meals delivered to them allowing parents to utilize the remaining provisions for their own body reserves.

A wild Indo-Pacific bottlenose dolphin adopts a socially and genetically distant neonate.

Alloparental behaviour and adoption have been reported in many mammals and birds. Such behaviours are energetically costly, and their causes and functions remain unclear. We observed the adoption behaviour of a wild Indo-Pacific bottlenose dolphin (Tursiops aduncus) near Mikura Island, Japan. A calf was seen with its mother on six observation days. Following the mother’s death, the calf was observed with a sub-adult female on all 18 observation days from May to September 2012. On three days, the calf was observed swimming with this female in the suckling position and milk was seen leaking from the female’s mammary slit. A five-year dataset revealed no significant social or kin relationships between the biological mother and allomother, indicating that kinship and social relationships did not play an important role in the observed adoption.

Reduced vitamin A (retinol) levels indicate radionuclide exposure in Streaked Shearwaters (Calonectris leucomelas) following the 2011 Fukushima nuclear accident

The Fukushima nuclear accident in 2011 released significant amounts of radionuclides into the marine environment. Exposure to radiation reduces levels of antioxidants such as carotenoids and vitamins A and E within exposed individuals. Such reductions can cause teratogenic or mutagenetic effects leading to reduced reproductive viability and fitness. Reduced antioxidant levels therefore may be used as an indicator of radionuclide contamination and to infer individual or population level impacts; however, the taxa-specific responses of marine organisms, such as seabirds, are poorly understood. As top predators, seabirds are ideal bio-indicators of the prevalence of contaminants and pollutants in marine ecosystems. At-sea foraging distributions of Streaked Shearwaters (Calonectris leucomelas) from Mikura Island (MKR), Japan during the post egg-laying period coincide with the Fukushima nuclear plume while the breeding colony on Birou Island (BRU) lies outside the affected zone. We examined the physiological responses of Streaked Shearwater chicks at MKR and BRU to possible radiation exposure during the 2011 breeding season, four to seven months after the Fukushima nuclear accident. Fledging mass did not differ between islands but fledglings from MKR displayed significantly reduced vitamin A levels. Available information suggests these depletions most likely result from radiation exposure due to the Fukushima nuclear accident, implying that the risk of radionuclide contamination is considerably elevated for Streaked Shearwaters on MKR, where more than 60% of the world's population breeds. While additional negative impacts are expected due to delayed effects of radionuclide transport via biomagnification in the food chain, this study highlights the potential immediate and worrisome consequences of the Fukushima nuclear accident for marine wildlife.

Foraging behavior and Diet of Streaked ShearwatersCalonectris leucomelasRearing Chicks on Mikura Island

Abstract Foraging behavior and diet of Streaked Shearwaters Calonectris leucomelas rearing chicks on Mikura Island was studied using depth and temperature recording data-loggers and stomach contents. Water mass where birds were foraging was estimated using sea surface temperature experienced by birds. Birds spent 76–96% of their time at sea flying, 4–24% for landing on the water, and made a few (0.5–17.0 per day) shallow (<6 m) dives. Birds made many short (<2 days) and some long (4–10 days) trips. During short trips, birds stayed in the warmer Kuroshio and Kuroshio-Oyashio mixed regions, and fed on Japanese Anchovy Engraulis japonicus, Common Squid Todarodes pacificus and Flying Fish Cypselurus hiraii. During long trips, birds stayed in the colder Oyashio region and fed on anchovy and Pacific Saury Cololabis saira. Birds made more dives during short trips than they did during long trips. Streaked Shearwaters breeding on the island in low—productive Kuroshio water, therefore, adopted dual foraging strateg...

Tactile Contact Exchanges Between Dolphins: Self-rubbing versus Inter-individual Contact in Three Species from Three Geographies

Self-rubbing and social-rubbing (pectoral fin contact between dolphin pairs) were compared for observations conducted on three dolphin study groups: wild dolphin groups in The Bahamas and around Mikura Island, Japan, and a third group of captive dolphins at the Roatan Institute of Marine Sciences, Roatan, Honduras. A primary aim of this research was to determine whether self-rubbing and social pectoral fin rubbing served overlapping functions. Self-rubbing rates were nearly identical between the three study sites, suggesting that site-specific differences (e.g., environmental conditions, substrate, presence of rocks or coral, social grouping) do not affect the rates at which dolphins rub their bodies against non-dolphin objects. The function of self-rubbing is not entirely clear, and likely involves a combination of factors (e.g., play, pleasure), with functions such as hygiene possibly being shared by both self-rubbing and social-rubbing. Rubbing behavior in general (e.g., rates, body parts used) were similar at all three sites for all three species, suggesting that rubbing is an evolutionarily conserved behavior for delphinid species. Still, subtle and individually distinct differences were documented among our study groups with respect to how often and with whom dolphins exchanged pectoral fin contact or engaged in self-rubbing. Site-specific social pressures and predation risks, as well as individual personality might play a role with respect to the expression of an individual’s observed rubbing behavior.

A comparison of pectoral fin contact behaviour for three distinct dolphin populations

Tactile exchanges involving the pectoral fin have been documented in a variety of dolphin species. Several functions (e.g., social, hygienic) have been offered as possible explanations for when and why dolphins exchange pectoral fin contacts. In this study, we compared pectoral fin contact between dolphin dyads from three distinct dolphin populations: two groups of wild dolphins; Atlantic spotted dolphins (Stenella frontalis) from The Bahamas and Indo-Pacific bottlenose dolphins (Tursiops aduncus) from around Mikura Island, Japan; and one group of captive bottlenose dolphins (Tursiops truncatus) residing at the Roatan Institute for Marine Sciences, Anthony's Key Resort. A number of similarities were observed between the captive and wild groups, including; rates of pectoral fin contact, which dolphin initiated contact, posture preference, and same-sex rubbing partner preference. Unlike their wild counterparts, however, dolphins in the captive study group engaged in petting and rubbing at equal rates, females were more likely to contact males, males assumed the various rubbing roles more frequently than females, and calves and juveniles were more likely to be involved in pectoral fin contact exchanges. These results suggest that some aspects of pectoral fin contact behaviour might be common to many dolphin species, whereas other aspects could be species specific, or could be the result of differing environmental and social conditions.

Fine-scale analysis of synchronous breathing in wild Indo-Pacific bottlenose dolphins (Tursiops aduncus)

We quantitatively analysed synchronous breathing for dyads in Indo-Pacific bottlenose dolphins at Mikura Island, Tokyo, Japan. For most cases, we observed dyads swimming in the same direction (97%), in close proximity (i.e., less than 1.5m) and with their body axes parallel as they breathed synchronously. Moreover, the pairs engaged in identical behaviour before and after the synchronous breathing episodes. These results suggest that the dolphins synchronize their movements, and that synchronous breathing is a component of "pair-swimming", an affiliative social behaviour. Same sex pairs of the same age class frequently engaged in synchronous breathing for adults and subadults, as well as mother-calf and escort-calf pairs. The distance between individuals during synchronous breathing for mother-calf pairs was less than for other pairs. The distance observed between individuals for female pairs was less than for male pairs. The time differences between each exhale for each of the two dolphins involved in synchronous breathing episodes for female pairs were smaller than for male pairs, and time differences for adult pairs were smaller than subadult pairs. These results suggest that age and sex class influenced the characteristics of this behaviour.

Foraging trip decisions by the streaked shearwater Calonectris leucomelas depend on both parental and chick state

Parents of albatross and shearwater species employ a dual foraging strategy, feeding their chicks quickly in repeated short trips and then restoring their own fuel reserves during longer trips. A decline in parental body condition is believed to trigger longer trips, but chick body condition and age may also play a role. To investigate these factors in the little-studied streaked shearwater Calonectris leucomelas, we monitored the nest attendance of 17 pairs on Mikura Island in 2005 using an automated identification system. We also monitored body mass changes and meal masses of 5 of the 17 pairs using an automated weighing system. Although the birds did not show a clear dual foraging pattern, trip duration varied widely from 1 to 15 days. On average, the birds fed chicks 67.6 g during nighttime meals at 2.74-day intervals. Since meal mass did not depend on trip duration, feeding efficiency (meal mass delivered per unit trip duration) decreased as trip duration increased. Parents accumulated more energy reserves when they took longer trips. Parents appeared likely to initiate longer trips when their body condition declined or chick body condition recovered.

Phycisphaera mikurensis gen. nov., sp. nov., isolated from a marine alga, and proposal of Phycisphaeraceae fam. nov., Phycisphaerales ord. nov. and Phycisphaerae classis nov. in the phylum Planctomycetes

Three strains, FYK2301M01(T), FYK2301M18 and FYK2301M52, all being Gram-negative, spherical, motile and facultatively anaerobic, were isolated from a marine alga (Porphyra sp.) collected on Mikura Island, Japan. Colonies of the strains were circular and pink-pigmented on Marine Agar 2216 (Difco) at 25 degrees C. Cells of the strains reproduced by binary fission. The G+C content of the DNA was 73 mol%. The major isoprenoid quinone was MK-6. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the strains are the members of the WPS-1 group (Nogales et al., 2001) comprising no validly described taxa within the phylum Planctomycetes. The highest similarity value of the 16S rRNA gene sequences of the strains to those in the established bacterial taxa was only 78.7% to Planctomyces brasiliensis DSM 5305(T). From the taxonomic data obtained in this study, it is proposed that the new marine isolates be placed in a novel genus and species named Phycisphaera mikurensis gen. nov., sp. nov. within a new family, order and class Phycisphaeraceae fam. nov., Phycisphaerales ord. nov. and Phycisphaerae classis nov. in the phylum Planctomycetes. The type strain of Phycisphaera mikurensis is FYK2301M01(T) (= NBRC 102666(T) = KCTC 22515(T)).

A comparison of pectoral fin contact between two different wild dolphin populations

Contact behaviour involving the pectoral fin has been documented in a number of dolphin species, and various explanations about its function have been offered. Pectoral fin contact can take a variety of forms, and involves a number of body parts and movements, likely differing depending upon social or ecological context. For this study, we compare the pectoral fin contact behaviour of two species of wild dolphins: Indo-Pacific bottlenose dolphins (Tursiops aduncus) from around Mikura Island, Japan, and Atlantic spotted dolphins (Stenella frontalis) from The Bahamas. The two study populations exhibit surprising similarity in the ways in which pectoral fin contacts are used, despite differences in species and environmental conditions at the two sites. Differences in contact rates for calves between the two sites suggest that calf-focused aggression from adult dolphins is more prevalent at Mikura than in The Bahamas. Our results suggest that pectoral fin contact behaviour seems to be driven primarily by social pressures, and may be similar in function to allogrooming described in primates.

Post-breeding movement and activities of two Streaked Shearwaters in the north-western Pacific

ABSTRACT We used miniature light-level/immersion loggers (geolocators) to study the movements and behavior of two Streaked Shearwaters (Calonectris leucomelas) during their travel away from Japan, after the breeding season. During the period from late October until late December, the tracked shearwaters moved south over subtropical pelagic waters with low productivity. The birds traveled to the seas off northern New Guinea and the Gulf of Carpentaria, Australia, 3400–5200 km distant from Mikura Island, their breeding colony in Japan. During the wintering period (December–February), the birds were on the sea surface for 77–85% of their time on average. The migratory and wintering behavior of Streaked Shearwaters are discussed in relation to the physical and biological marine environment of the north-western Pacific.