disentangling the processes that drive the evolution of complex biological structures is a major aim of evolutionary biology. Multivariate data in comparative studies can reflect various signals (e.g., ecological, allometric, phylogenetic, or combinations thereof; [32]). In this study, we employed 3D landmark configurations, a functional subdivision of the vertebral column in 24 dolphin species (Family Delphinidae), and phylogenetic comparative methods to test the effects of phylogenetic, ecological and allometric signals along the vertebral column of the most diverse cetacean family.
Our results indicate that vertebral morphology in dolphins is shaped by distinct but overlapping signals: strong ecological effects on the thorax-torso boundary (ThTo), the mid torso (Tm), and the synclinal point (SP); vertebral size effects, particularly thto and SP; and phylogenetic constraints on Tm, SP, and the tailstock (TS). These findings suggest that different regions of the column evolve under varying combinations of ecological, allometric, and phylogenetic influences.
Ecological signal
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Previous studies on cetacean vertebral morphology have mainly focused on ecological signal [8, 9, 10, 14, 15, 25].Based on vertebral morphology, it has been proposed that the most recent common ancestor (MRCA) of crown delphinids, as well as those of each subfamily (Lissodelphininae, Globicephalinae, and Delphininae), likely inhabited offshore environments, showing morphologies associated with oceanic non-fast-swimming species, with subsequent shape diversification linked to the biomechanical demands of different habitats and habits [10, 11]. Our results revealed ecological signal in three regions critical for swimming efficiency: ThTo, Tm, and SP.
Both the anterior and middle torso (thto and Tm) are especially crucial regions, as they are where the longissimus muscle generates the greatest forces that are transferred to the flukes [42]. In these regions,habitat explained more than 37% of total variation,with relatively high Z values,suggesting that habitat has a greater effect on vertebral morphology than that expected by chance. The synclinal point marks the transition between the stable torso and the flexible tailstock, representing the area where muscle forces that affect the fluke’s angle of att
## Phylogenetic signal
Studies on phylogenetic signals in cetaceans remain scarce.Vicari et al. [23] reported phylogenetic signals for both skull size and shape, which were stronger for size (Kmult = 0.653) than for shape (Kmult = 0.565), with size linked to ecological traits but shape showing no association. Conversely, Galatius et al. [21] reported strong phylogenetic signals in delphinid skull shapes, with subfamilies displaying distinct morphologies. For vertebrae, Viglino et al. [16] detected phylogenetic signals in traits related to swimming muscle architecture across delphinids and Pontoporia blainvillei, but this signal disappeared when Pontoporia was excluded, suggesting a strong effect of this distantly related species and highlighting the importance of considering several species in this type of study. Marchesi et al. [50] reported no significant phylogenetic signal within porpoises, likely due to parallel convergence, but strong signals were recovered across delphinoids, particularly in Tm, suggesting supra-family constraints during delphinoid diversification (~ 20 Mya [50]).
Our results parallel these findings: a phylogenetic signal was detected along the dolphin vertebral column in centroid size (CS), shape (procrustes coordinates), and ordination methods maximising ecological (PhyPCA) or phylogenetic (PACA) signals. Size exhibited a weak but significant phylogenetic signal (Kmult < 0.62, Z < 2.07) in the three most posterior regions, which are considered sub regions within the caudal region [14], suggesting that partial phylogenetic constraints on vertebral size in these areas are responsible for differences in swimming performance. With respect to shape (Procrustes coordinates, PhyPCA, and PACA), the phylogenetic signal was evident to varying degrees in all regions except ThTo (in phypca). Disregarding the dataset,, with the lowest *K*mult values occurred in
How Ecology Shapes Dolphin Evolution: A Look at Vertebral Morphology
Dolphins are incredibly diverse, and understanding why they’ve evolved so many different forms is a major question in evolutionary biology. Recent research points to a strong connection between a dolphin’s surroundings and the shape of its vertebrae – the bones that make up its spine. This isn’t just about random changes; it’s about natural selection favoring specific traits in specific habitats.
Why Vertebrae Matter
Vertebral shape isn’t arbitrary. It directly impacts how a dolphin moves through the water. Different shapes provide advantages for different lifestyles. For example, dolphins that need to be highly maneuverable in coastal waters will have different vertebral structures than those built for long-distance, open-ocean travel. Think of it like comparing a sports car to a semi-truck – both get you there, but they’re designed for very different purposes.
What the Research Shows
Scientists have been using advanced geometric morphometrics – essentially, detailed measurements of shape – to analyze dolphin vertebrae. They’re finding clear correlations between vertebral morphology and ecological factors. these factors include water depth, temperature, and even the type of prey available. It’s not just where a dolphin lives, but how it lives that influences its evolution.
Specifically, studies reveal that dolphins in shallower, more variable environments tend to exhibit greater vertebral flexibility. This allows for quicker turns and better navigation in complex habitats. Conversely, dolphins in deeper, more stable waters often have more rigid vertebral columns, optimized for efficient, long-distance swimming. This makes intuitive sense – you wouldn’t need a highly flexible spine if you’re primarily cruising in open water.
Fine-Scale Ecology is Key
What’s particularly interesting is that these ecological influences operate at a relatively small scale. It’s not just “coastal” versus “offshore.” Subtle differences in habitat – a particular bay, a specific current, or the presence of certain prey – can all contribute to vertebral diversification. This highlights the importance of considering ecological factors at a very detailed level when studying evolution. Researchers are using sophisticated evolutionary models to pinpoint the exact selection pressures at play.
Future Research Directions
More work is needed to fully understand the evolutionary scenarios driving these changes.Researchers plan to use models that test whether different parts of the vertebral column are evolving towards specific, optimal shapes. They’ll also explore whether dolphins in similar environments independently evolve similar vertebral forms – a phenomenon known as convergent evolution.
However, analyzing complex shape data presents challenges. current methods have limitations when dealing with high-dimensional data. Overcoming these hurdles will be crucial for unlocking a more complete picture of dolphin evolution. Ultimately, understanding how ecology shapes dolphin vertebrae provides valuable insights into the broader processes of adaptation and diversification in marine mammals.
References
57 beaulieu JM, Jhwueng DC, Boettiger C, O’Meara BC. Modelling stabilizing selection: expanding the Ornstein-Uhlenbeck model of adaptive evolution. Evolution. 2012;66:2369-83.