Biology subjects

Tayasu, I.

Publications and source records attributed to Tayasu, I..

6 recordsLinked to original sources

bioRxiv2026

Radiocarbon dating of bird and mammal eye lenses reveals long-term chronology

Time-series isotope analysis using eye lenses has recently gained attention as a low-cost method for reconstructing long-term information from a single sampling event, with potential applications across diverse taxonomic groups. However, information from eye lenses in birds and mammals remains limited, particularly during early growth stages, and the eye-lens time axis over extended periods should be reconstructed. This study measured the radiocarbon (14C) isotope ratios of eye lenses to examine the temporal framework recorded in the eye lenses of birds and mammals. The calibrated 14C ages of the central lens sections closely matched the known birth years or years preceding their introduction to Japan. 14C ages increased progressively from the center toward the periphery of the eye lens. The Siberian tiger allowed information reconstruction spanning from 2002.4 {+/-} 1.6 to 2017.8 {+/-} 2.0, whereas the ring-tailed lemur yielded reconstructed ages from 2000.2 {+/-} 1.6 to 2019.2 {+/-} 0.0. Calendar ages for all individuals after approximately 2019 could not be resolved using the radiocarbon calibration approach applied in this study. The sulphur-crested cockatoo demonstrated the longest reconstructed time span, covering 27.4 years from 1968.7 {+/-} 1.3 to 1996.1 {+/-} 1.4. Our results indicate that eye lenses can reconstruct long-term information spanning much of the lifespan in mammals examined, whereas avian lenses can preserve information spanning several years to decades after birth. Lens growth patterns may differ according to life history; thus, future studies should examine a wider range of species and taxonomic groups with diverse life histories.

zoology↗

bioRxiv2025

Freshwater degradation as a potential extinction factor of the Japanese river otters

Exploring the causes of species extirpation can provide insights into mitigating the ongoing extinction. The Japanese otter Lutra lutra whiteleyi had extincted around 1970s, but their extinction causes remain unknown. To elucidate the ecological relationship among their foraging ecology, habitat degradation, and the extinction, we assessed dependency of Japanese otters on marine- or freshwater-derived resources using sulfur stable isotopes ({delta}34S: 34S/32S) of the museum specimens which were collected on the brink of extinction. Our results clearly showed that the otters strongly depended on marine-derived resources, indicating anthropogenic activities led to depletion of freshwater prey and a potential extinction factor of the last population of otters in Japan. Protecting freshwater habitats was crucial for preventing otter extinction in Japan, and for safeguarding the extant otter species.

ecology↗

bioRxiv2024

Isotope analysis of bird eye lens provides early-life information

Early-life environment has a long-lasting effect on later life, though its estimation is often prevented in the wild because of a lack of available methods. Recently, isotope analysis of eye lenses has attracted considerable interest as a means to reconstruct the environmental conditions experienced by animals during the developmental period. This analysis has mostly been confined to fish for practical reasons and remains to be resolved for application to other animals. In this study, we broadened its applicability by developing a novel approach and verifying its usability for the reconstruction of early-life environments. We performed a feeding experiment using Japanese quail (Coturnix japonica), in which we administered two diets: one composed mainly of C3 plants (low{delta} 13C and high{delta} 15N) and the other of C4 plants (high{delta} 13C and low{delta} 15N). Quails in the control group were continuously fed a C3-based diet from hatching until 200 days old, whereas those in the treatment groups (T10, T15, T20, and T40) were switched from the C3 to the C4-based diet at 10, 15, 20, and 40 days after hatching, respectively. We found that the{delta} 13C in the eye lenses of the treatment groups decreased from the center layer to the middle layer of the lens and then increased toward the outer layer, thus reflecting the diet change. In contrast, those of the control group exhibited a decreasing trend and equilibrated at the middle layer of the eye lens, with no increase thereafter. This novel approach revealed the postnatal feeding histories of the diet-shift experiment. The high{delta} 13C values observed in the center of the eye lenses would reflect the prenatal feeding environment, i.e., the C4-based diet consumed by their mothers, which is further reinforced by higher{delta} 15N values at this position due to the consumption of egg yolk-derived nutrition. These results indicate that the avian eye lens can be used as an "isotopic chronicle," which is a useful tool for reconstructing chronological isotopic information about their early-life history.

ecology↗

bioRxiv2024

Changes in plant physiology during cultivation periods influence the hydrogen isotope ratio of the water-insect relationship

O_LIThe hydrogen stable isotope ratios ({delta}2H) in tissues of terrestrial insects are widely applied to estimate natal origins in field populations. The hydrogen isotopes of insect tissues incorporate those of environmental waters through the insects metabolic processes. Water sources and abiotic environmental factors reflect changes in plant physiology, as indicated by the {delta}2H values of plants. However, the influence of plant physiology on the assimilation of hydrogen in insect tissues derived from water through feeding diets is still unknown. C_LIO_LIWe experimentally examined the influence of water on the {delta}2H values of maize leaf and of forewings of Mythimna separata (Lepidoptera: Noctuidae) and Spodoptera frugiperda (Noctuidae). We prepared five specific water samples for cultivating maize, aiming to replicate the gradient of {delta}2H values observed in environmental waters across the Japanese archipelago. C_LIO_LIThe mean-percentage contribution of water to hydrogen in maize leaves was 17.6% (July-August) and 25.1% (September-October). Linear analyses indicated that 17.4% and 32.7% of hydrogen in M. separata and S. frugiperda forewings were derived from water through the consumption of maize leaves. The slope values of linear regression between the insect forewings and the maize leaves supplied in the final instar were closest to 1.0. These results indicated that the {delta}2H values of maize leaves and insect forewings reflected those of water resources. C_LIO_LIThis study presented the seasonal changes in climate conditions that affected the {delta}2H values of host plants and insect tissues. Changes in plant physiology with seasonal variations may influence the interpretation of the linear relationship between water and insect tissues on the estimation of natal origins. C_LI

ecology↗

bioRxiv2024

A novel method for fine-scale retrospective isotope analysis in mammals using eye lenses

1. Investigating individual behavioural variations in mammals is essential for understanding their ecology and evolution, and plays a critical role in conservation and management practices. However, the reconstruction of long-term individual behaviour, such as via bio-logging, remains challenging owing to cost constraints and limitations of battery life and the impact of device size for smaller animals. This study proposed and tested a novel and cost-effective method for retrospective isotope analysis using mammalian eye lenses, specifically focusing on brown bears (Ursus arctos). 2. Seven pairs of bear eye lenses were collected from southwestern Hokkaido, Japan. One or both lenses of each bear were segregated into small fragments from the outermost to the core tissues, and the nitrogen and carbon stable isotope ratios ({delta}15N and {delta}13C) of each eye lens fragment were measured. These isotope ratios in the eye lenses were compared with the {delta}15N and {delta}13C of potential brown bear diets in the study region (C3 herbs, C3 fruits, terrestrial animals, and corn). Additionally, we compared the isotopic patterns in both the right and left lenses of the same bear in two individuals to evaluate the consistency of our preparation protocol. 3. In all eye lenses, high {delta}15N values were identified near the core, which gradually decreased towards the outer tissues, indicating ontogenetic dietary shifts related to lactation and weaning in the early life stage. Bears from study areas with high corn availability exhibited substantial increases in {delta}13C and {delta}15N in the outer lens tissues, suggesting a dietary shift toward corn consumption after weaning. Isotopic patterns between lens pairs from the same individual were similar, although discrepancies increased in tissues located 1.00 to 2.25 mm from the core, highlighting the need for standardisation in sample processing. 4. This study demonstrates a novel and simple technique for retrospective isotope analysis in wild mammals using eye lenses, effectively reconstructing the feeding histories of brown bears. Our findings provide a new avenue for studying individual time-series behavioural patterns, with important implications for the fields of mammalian ecology, evolution, conservation, and management.

ecology↗

bioRxiv2020

Timing of fruiting rather than topography determines the direction of vertical seed dispersal by mammals and birds

Vertical seed dispersal toward higher or lower altitudes has been recognized as one of the critical processes for plants to escape from climate change. Studies exploring vertical seed dispersal are scarce, preventing the prediction of future vegetation dynamics. In the present study, we show that the timing of fruiting, rather than topography, determines the direction of vertical seed dispersal by mammals and birds across mountains in central Japan. We found strong uphill seed dispersal of summer fruiting cherry and weak downhill seed dispersal of summer-to-autumn fruiting cherry, irrespective of mountains and animals. The ascent or descent of animals, following the altitudinal gradients in food plant phenology in the temperate zone, was considered to be a driver of the biased seed dispersal. We found that megafauna (i.e., bears) intensively dispersed seeds vertically. The results suggest that the timing of fruiting and megafauna strongly affect whether animal-dispersed temperate plants can maintain their populations under climate change.

ecology↗