Shaped by Their Environment: Variation in Blue Whale Morphology across Three Productive Coastal Ecosystems

Abstract

Species ecology and life history patterns are often reflected in animal morphology. Blue whales are globally distributed, with distinct populations that feed in different productive coastal regions worldwide. Thus, they provide an opportunity to investigate how regional ecosystem characteristics may drive morphological differences within a species. Here, we compare physical and biological oceanography of three different blue whale foraging grounds: (1) Monterey Bay, California, USA; (2) the South Taranaki Bight (STB), Aotearoa New Zealand; and (3) the Corcovado Gulf, Chile. Additionally, we compare the morphology of blue whales from these regions using unoccupied aircraft imagery. Monterey Bay and the Corcovado Gulf are seasonally productive and support the migratory life history strategy of the Eastern North Pacific (ENP) and Chilean blue whale populations, respectively. In contrast, the New Zealand blue whale population remains in the less productive STB year-round. All three populations were indistinguishable in total body length. However, New Zealand blue whales were in significantly higher body condition despite lower regional productivity, potentially attributable to their non-migratory strategy that facilitates lower risk of spatiotemporal misalignment with more consistently available foraging opportunities. Alternatively, the migratory strategy of the ENP and Chilean populations may be successful when their presence on the foraging grounds temporally aligns with abundant prey availability. We document differences in skull and fluke morphology between populations, which may relate to different feeding behaviors adapted to region-specific prey and habitat characteristics. These morphological features may represent a trade-off between maneuverability for prey capture and efficient long-distance migration. As oceanographic patterns shift relative to long-term means under climate change, these blue whale populations may show different vulnerabilities due to differences in migratory phenology and feeding behavior between regions. Spanish abstract La ecología y patrones de historia de vida de las especies a menudo se reflejan en la morfología animal. Las ballenas azules están distribuidas globalmente, con poblaciones separadas que se alimentan en diferentes regiones costeras productivas de todo el mundo. Por lo tanto, brindan la oportunidad de investigar cómo las características regionales de los ecosistemas pueden impulsar diferencias morfológicas dentro de una especie. Aquí, comparamos la oceanografía física y biológica de tres zonas de alimentación diferentes de la ballena azul: (1) Bahía de Monterey, California, EE. UU., (2) Bahía del sur de Taranaki (BST), Nueva Zelanda, y (3) Golfo de Corcovado, Chile. Adicionalmente, comparamos la morfología de las ballenas azules de estas regiones utilizando imágenes de aeronaves no tripuladas. La Bahía de Monterey y el Golfo de Corcovado son estacionalmente productivos y apoyan la estrategia migratoria de la historia de vida de las poblaciones de ballena azul chilena y del Pacífico Norte Oriental (PNO), respectivamente. Por el contrario, la población de ballena azul de Nueva Zelanda permanece en la menos productiva BST durante todo el año. Las tres poblaciones eran indistinguibles en cuanto a la longitud corporal total. Sin embargo, las ballenas azules de Nueva Zelanda tenían una condición corporal significativamente mayor a pesar de una menor productividad regional, potencialmente atribuible a su estrategia no migratoria que facilita un menor riesgo de desalineación espaciotemporal con oportunidades de alimentación disponibles de manera más consistente. Alternativamente, la estrategia migratoria de las poblaciones de ballenas PNO y chilena puede tener éxito cuando su presencia en las zonas de alimentación se alinea temporalmente con la abundante disponibilidad de presas. Documentamos diferencias en la morfología del cráneo y la aleta caudal entre poblaciones, que pueden estar relacionadas con diferentes comportamientos de alimentación adaptados a las características de hábitat y presas específicas para cada región. Estas características morfológicas pueden representar una compensación entre la maniobrabilidad para la captura de presas y una migración eficiente a larga distancia. A medida que los patrones oceanográficos cambian en términos de mediano a largo plazo debido al cambio climático, estas poblaciones de ballenas azules pueden mostrar diferentes vulnerabilidades debido a diferencias en la fenología migratoria y el comportamiento de alimentación entre regiones.

Fig. 1

Left column: cumulative physical forcing metric for each region (coastal upwelling transport index, upwelling index, and zonal wind stress, respectively), with shading denoting the 50% (dark gray) and 90% (light gray) annual accumulation windows. Center column: biological productivity in each system. Each dotted line represents a different year. The dark black line represents the mean. Colored lines correspond to years morphological data were collected; corresponding sampling periods are denoted by colored ribbons along the x-axis. Right column: box plots show group median, interquartile range (IQR, box), maximum and minimum 1.5*IQR (vertical lines). Black bars above box plots with asterisks indicate statistically significant differences based on Monte Carlo ANOVA results.

Fig. 2

(A) Example UAS image of a blue whale, with morphological measurements depicted in white. (B–G) Each panel compares a specific morphological measurement between the ENP, New Zealand, and Chilean blue whale populations. Box plots show group median, interquartile range (IQR, box), maximum and minimum 1.5*IQR (vertical lines). Each point represents a measurement for an individual whale, with vertical lines showing the 95% HPDI. All measurements are standardized by total body length. Black horizontal bars with asterisks indicate statistically significant differences between groups based on Monte Carlo ANOVA results.

Fig. 3

Scaled schematic comparing morphology between the ENP, New Zealand, and Chilean blue whale populations. Separated lines indicate statistically significant differences; overlapping dashed lines indicate no significant differences.

Fig. 4

Scaling of each morphometric measurement compared to body length, on a log-log scale. Dotted black lines represent a theoretical isometric relationship, where slope = 1. Each point represents an individual blue whale. Light bars around each point represent the 95% HPDI associated with each measurement. Solid, colored lines show fitted linear relationships by population (resulting coefficients in Supplementary Table S2).