Biology Letters最新文献

For studies of the evolution of vertebrate brain anatomy and potentially associated behaviours, reconstructions of digital brain endocasts from computed tomography scans have revolutionized our capacity to collect neuroanatomical data. However, measurements from digital endocasts must be validated as reflecting actual brain anatomy, which is difficult because the collection of soft tissue information through histology is laborious and time-consuming. In birds, the reliability of digital endocast measurements as volume proxies for the two largest brain regions-the telencephalon and cerebellum-remains to be validated despite their use as proxies, e.g. of cognitive performance or flight ability. We here use the largest dataset of histology and digital endocasts to date, including 136 species from 25 avian orders, to compare digital endocast surface area measurements with actual brain volumes of the telencephalon, cerebellum and whole-brain endocast. Using linear and phylogenetically informed regression analyses, we demonstrate that endocast surfaces are strongly correlated with their brain volume counterparts for both absolute and relative size. This provides empirical support for using endocast-derived cerebellar and telencephalic surface areas in existing and future studies of living and extinct birds, with potential to expand to the dinosaur-bird transition in the future.

Animals commonly form dominance relationships that determine the priority of access to resources and influence fitness. Dominance relationships based on age, immigration order or nepotism (alliances with kin) conventions are usually more stable than those based on intrinsic characteristics such as physical strength. Unlike most mammals, female gorillas disperse from their groups, typically more than once in their lifetimes, disrupting their group tenures and/or any alliances. Thus, we predicted that they form unstable dominance relationships that are not based on conventions. Contrarily, using a 24-year dataset on five groups of both gorilla species, we found that females form strikingly stable dominance relationships, maintained over their whole co-residence in a group (mean dyadic co-residence = 4.8, max = 17.3 years). Specifically, we observed rank reversals in only two out of 92 female dyads, and all other rank improvements resulted from emigration or death of higher-ranking females (passive mobility). These results mirror observations in chimpanzees, suggesting that dominance dynamics might have deep roots in hominid evolution. Our study challenges a hypothesized link between hierarchy instability and dispersal, particularly among animals in which fitness consequences of rank improvement may not be great enough to counterbalance the potentially high costs of challenging higher-ranking individuals.

Bacterial strains that inhabit the gastrointestinal tracts of hominids have diversified in parallel (co-diversified) with their host species. The extent to which co-diversification has been mediated by partner fidelity between strains and hosts or by geographical distance between hosts is not clear due to a lack of strain-level data from clades of hosts with unconfounded phylogenetic relationships and geographical distributions. Here, I tested these competing hypotheses through meta-analyses of 7121 gut bacterial genomes assembled from wild-living ape species and subspecies sampled throughout their ranges in equatorial Africa. Across the gut bacterial phylogeny, strain diversification was more strongly associated with host phylogeny than with geography. In total, approximately 14% of the branch length of the gut bacterial phylogeny showed significant evidence of co-diversification independent of geography, whereas only approximately 4% showed significant evidence of diversification associated with geography independent of host phylogeny. Geographically co-occurring heterospecific hosts (Pan and Gorilla) universally maintained distinct co-diversified bacterial strains. Strains whose diversification was associated with geography independent of host phylogeny included clades of Proteobacteria known to adopt free-living lifestyles (e.g. Escherichia). These results show that co-diversification of gut bacterial strains with hominids has been driven primarily by fidelity of strains to host lineages rather than geography.

Strict maternal inheritance of mitochondria is known to be the rule in animals, but over 100 species across six orders of bivalves possess doubly uniparental inheritance (DUI) of mitochondria. Under DUI, two distinctive sex-specific mitogenomes coexist. In marine and freshwater mussels, each mitogenome has an additional protein-coding gene, called female- and male-specific open reading frame or forf and morf, respectively. The function(s) of the associated FORF and MORF proteins remain unknown. Herein, we show that these proteins present similar tissue expression patterns in two distantly related DUI species: MORF was only expressed in male gonads, whereas FORF was expressed in all tissues of both sexes in the marine mussel Mytilus edulis and the freshwater mussel Venustaconcha ellipsiformis. Moreover, MORF was only expressed during the reproductive season, while FORF presented no clear seasonality pattern in M. edulis. Immunocytochemistry revealed the presence of both proteins in mitochondria and acrosomes of late spermatids and mature sperm. We hypothesize that MORF has a key function in spermatogenesis, while FORF has a more general function in both sexes. We also propose that both proteins may be involved in the fertilization process. The involvement of MORF in paternal mitochondrial transmission is also discussed.

It is unclear how habitat features alter animal responses to social instability. Only by uncovering such interactions can we fully understand the evolutionary drivers and fitness consequences of sociality. We capitalize on a management-induced manipulation of social stability in an island population of free-ranging feral horses (Equus caballus), living across three distinct habitat types. We tested whether female group-changing behaviour (a reliable measure of social instability) affected (i) female-female aggression, (ii) rank within female dominance hierarchies, (iii) stability of female hierarchies (in the groups they joined and/or left), and (iv) how habitat characteristics shaped these responses. Female group-changing behaviour positively predicted aggression from other females, but only when habitat features such as visibility and freshwater distribution were considered. We found no strong association among female group-changing behaviour and the aggression initiated, female rank or the stability of female dominance hierarchies. Our work reveals that animal responses to social instability are nuanced and impacted by the surrounding habitat. A better understanding of these impacts can help elucidate evolutionary drivers of sociality and mitigate unintended effects of management practices.

Despite numerous studies on the rise and fall of terrestrial megafauna in the late Quaternary, knowledge about marine megafauna from this period remains limited. In this study, we performed radiocarbon dating and partial mitochondrial DNA sequencing from the skeletal remains of three species of small odontocetes (Pacific white-sided dolphins, Dall's porpoises and harbour porpoises) excavated from prehistoric archaeological sites around the Japanese shore dating back to 8500-1000 years ago (ya). Pacific white-sided dolphins that habituated the eastern coast of Hokkaido around 2000 ya belonged to different maternal groups than those from over 5000 ya and today. Furthermore, the species composition excavated from eastern Hokkaido sites varies between 5000 and 2000 ya. These findings suggest two significant population turnovers of small odontocetes on the east coast of Hokkaido, a transitional zone between the coastal area of East Asia and the offshore North Pacific. Notably, the first turnover, occurring between 5000 and 2000 ya, represents the oldest evidence of local population turnovers of marine megafauna during the late Quaternary.

Assessment of species' vulnerability to climate change has been limited by mismatch between coarse macroclimate data and the fine scales at which species select habitat. Habitat mediates climate conditions, and fine-scale habitat features may permit species to exploit favourable microclimates, but habitat preferences can also constrain their ability to do so. We leveraged fine-resolution models of near-surface temperature and humidity in grasslands to understand how microclimates affect climatic exposure and demographics in a grassland bird community. We asked: (i) Do species select favourable nest-site microclimates? (ii) Do habitat preferences limit the ability of species to access microclimates? (iii) What are the demographic consequences of microclimatic exposure? We found limited evidence that grassland birds select cooler microclimates, which may buffer eggs and nestlings from extreme heat. Instead, many species appeared constrained by vegetation preferences. While facultative generalists displayed flexibility to use denser vegetation that provided buffering from high temperatures (>39°C), most obligate species nested in more exposed microclimates. Nesting success in facultative species was not well explained by microclimate, but success in specialized grassland obligates declined with increasing microclimate temperatures. These findings illustrate how microclimate and habitat use can interact to influence the potential vulnerability of species to future climate change.

Mycovores (animals that consume fungi) are important for fungal spore dispersal, including ectomycorrhizal (ECM) fungi symbiotic with forest-forming trees. As such, fungi and their symbionts may be impacted by mycovore extinction. New Zealand (NZ) has a diversity of unusual, colourful, endemic sequestrate (truffle-like) fungi, most of which are ECM. As NZ lacks native land mammals (except bats), and sequestrate fungi are typically drab and mammal-dispersed, NZ's sequestrate fungi are hypothesized to be adapted for bird dispersal. However, there is little direct evidence for this hypothesis, as 41% of NZ's native land bird species became extinct since initial human settlement in the thirteenth century. Here, we report ancient DNA and spores from the inside of two coprolites of NZ's extinct, endemic upland moa (Megalapteryx didinus) that reveal consumption and likely dispersal of ECM fungi, including at least one colourful sequestrate species. Contemporary data from NZ show that birds rarely consume fungi and that the introduced mammals preferentially consume exotic fungi. NZ's endemic sequestrate fungi could therefore be dispersal limited compared with fungi that co-evolved with mammalian dispersers. NZ's fungal communities may thus be undergoing a gradual species turnover following avian mycovore extinction and the establishment of mammalian mycovores, potentially affecting forest resilience and facilitating invasion by exotic tree taxa.

Butterfly wing patterns exhibit notable differences between the dorsal and ventral surfaces, and morphological analyses of them have provided insights into the ecological and behavioural characteristics of wing patterns. Conventional methods for dorsoventral comparisons are constrained by the need for homologous patches or shared features between two surfaces, limiting their applicability across species. We used a convolutional neural network (CNN)-based analysis, which can compare images of the two surfaces without focusing on homologous patches or features, to detect dorsoventral bias in two types of intraspecific variation: sexual dimorphism and mimetic polymorphism. Using specimen images of 29 species, we first showed that the level of sexual dimorphism calculated by CNN-based analysis corresponded well with traditional assessments of sexual dissimilarity, demonstrating the validity of the method. Dorsal biases were widely detected in sexual dimorphism, suggesting that the conventional hypothesis of dorsally biased sexual selection can be supported in a broader range of species. In contrast, mimetic polymorphism showed no such bias, indicating the importance of both surfaces in mimicry. Our study demonstrates the potential versatility of CNN in comparing wing patterns between the two surfaces, while elucidating the relationship between dorsoventrally different selections and dorsoventral biases in intraspecific variations.

This study employs an integrated approach, combining three-dimensional flow visualization and two-dimensional flow measurement to investigate the underlying unsteady aerodynamic mechanisms of hovering hawkmoths. Using a single vortex ring model, three aerodynamic force components, such as aerodynamic force induced by unsteady circulation, vortex loop size variation and added mass, are estimated within a dimensionless time (normalized by one wing beat cycle) range of 0.418 < T^* < 0.455, where both the vortex loop circulation and loop size data are available. The force analysis reveals that the unsteady circulation-induced aerodynamic force dominates the overall force production and contributes 67% of the total force while the vortex loop size variation and added mass effect-induced aerodynamic forces only count for 25% and 8%, respectively. These findings suggest the hawkmoth primarily relies on unsteady circulation to generate aerodynamic forces.