Linking hunting weaponry to attack strategies in sailfish and striped marlin.

Linking morphological differences in foraging adaptations to prey choice and feeding strategies has provided major evolutionary insights across taxa. Here, we combine behavioural and morphological approaches to explore and compare the role of the rostrum (bill) and micro-teeth in the feeding behaviour of sailfish (Istiophorus platypterus) and striped marlin (Kajikia audax) when attacking schooling sardine prey. Behavioural results from high-speed videos showed that sailfish and striped marlin both regularly made rostrum contact with prey but displayed distinct strategies. Marlin used high-speed dashes, breaking schools apart, often contacting prey incidentally or tapping at isolated prey with their rostra; while sailfish used their rostra more frequently and tended to use a slower, less disruptive approach with more horizontal rostral slashes on cohesive prey schools. Capture success per attack was similar between species, but striped marlin had higher capture rates per minute. The rostra of both species are covered with micro-teeth, and micro-CT imaging showed that species did not differ in average micro-tooth length, but sailfish had a higher density of micro-teeth on the dorsal and ventral sides of their rostra and a higher amount of micro-teeth regrowth, suggesting a greater amount of rostrum use is associated with more investment in micro-teeth. Our analysis shows that the rostra of billfish are used in distinct ways and we discuss our results in the broader context of relationships between morphological and behavioural feeding adaptations across species.

Injury-mediated decrease in locomotor performance increases predation risk in schooling fish.

The costs and benefits of group living often depend on the spatial position of individuals within groups and the ability of individuals to occupy preferred positions. For example, models of predation events for moving prey groups predict higher mortality risk for individuals at the periphery and front of groups. We investigated these predictions in sardine (Sardinella aurita) schools under attack from group hunting sailfish (Istiophorus platypterus) in the open ocean. Sailfish approached sardine schools about equally often from the front and rear, but prior to attack there was a chasing period in which sardines attempted to swim away from the predator. Consequently, all sailfish attacks were directed at the rear and peripheral positions of the school, resulting in higher predation risk for individuals at these positions. During attacks, sailfish slash at sardines with their bill causing prey injury including scale removal and tissue damage. Sardines injured in previous attacks were more often found in the rear half of the school than in the front half. Moreover, injured fish had lower tail-beat frequencies and lagged behind uninjured fish. Injuries inflicted by sailfish bills may, therefore, hinder prey swimming speed and drive spatial sorting in prey schools through passive self-assortment. We found only partial support for the theoretical predictions from current predator-prey models, highlighting the importance of incorporating more realistic predator-prey dynamics into these models.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Utility of biological sensor tags in animal conservation.

Electronic tags (both biotelemetry and biologging platforms) have informed conservation and resource management policy and practice by providing vital information on the spatial ecology of animals and their environments. However, the extent of the contribution of biological sensors (within electronic tags) that measure an animal's state (e.g., heart rate, body temperature, and details of locomotion and energetics) is less clear. A literature review revealed that, despite a growing number of commercially available state sensor tags and enormous application potential for such devices in animal biology, there are relatively few examples of their application to conservation. Existing applications fell under 4 main themes: quantifying disturbance (e.g., ecotourism, vehicular and aircraft traffic), examining the effects of environmental change (e.g., climate change), understanding the consequences of habitat use and selection, and estimating energy expenditure. We also identified several other ways in which sensor tags could benefit conservation, such as determining the potential efficacy of management interventions. With increasing sensor diversity of commercially available platforms, less invasive attachment techniques, smaller device sizes, and more researchers embracing such technology, we suggest that biological sensor tags be considered a part of the necessary toolbox for conservation. This approach can measure (in real time) the state of free-ranging animals and thus provide managers with objective, timely, relevant, and accurate data to inform policy and decision making.

El Uso de Etiquetas de Sensor Biológico en la Conservación de Animales Resumen Las etiquetas electrónicas (plataformas tanto de bio-telemetría como de bio-registro) han informado a la conservación y a la política y práctica del manejo de recursos al proporcionar información vital sobre la ecología espacial de los animales y su ambiente. Sin embargo, la extensión de la contribución de los sensores biológicos (dentro de las etiquetas electrónicas) que miden el estado de un animal (p. ej.: ritmo cardíaco, temperatura corporal y detalles sobre el movimiento y la energética) es menos evidente. Una revisión de la literatura reveló que, a pesar de un número creciente de etiquetas sensoriales de estado disponibles comercialmente y un enorme potencial de aplicación de dichos dispositivos en la biología animal, hay pocos ejemplos de su aplicación en la conservación. Las aplicaciones existentes se rigieron por cuatro temas principales: cuantificar la perturbación (p. ej.: vehicular, de tráfico aéreo o de ecoturismo), examinar los efectos del cambio ambiental (p. ej.: cambio climático), entender las consecuencias de la selección y uso de hábitat, y estimar el gasto energético. También identificamos muchas otras maneras en que las etiquetas sensoriales podrían beneficiar a la conservación, como determinar la efectividad potencial de las intervenciones de manejo. Con el incremento en la diversidad de sensores en plataformas disponibles comercialmente, técnicas menos invasivas de etiquetado, tamaños más pequeños de los dispositivos, y más investigadores adoptando dicha tecnología, sugerimos que las etiquetas de sensor biológico se consideren como una parte de la caja de herramientas necesaria para la conservación. Esta estrategia puede medir (en tiempo real) el estado de animales libres y así proporcionar a los manejadores datos objetivos, oportunos, relevantes y precisos para informar la toma de decisiones y la política.

How sailfish use their bills to capture schooling prey.

The istiophorid family of billfishes is characterized by an extended rostrum or 'bill'. While various functions (e.g. foraging and hydrodynamic benefits) have been proposed for this structure, until now no study has directly investigated the mechanisms by which billfishes use their rostrum to feed on prey. Here, we present the first unequivocal evidence of how the bill is used by Atlantic sailfish (Istiophorus albicans) to attack schooling sardines in the open ocean. Using high-speed video-analysis, we show that (i) sailfish manage to insert their bill into sardine schools without eliciting an evasive response and (ii) subsequently use their bill to either tap on individual prey targets or to slash through the school with powerful lateral motions characterized by one of the highest accelerations ever recorded in an aquatic vertebrate. Our results demonstrate that the combination of stealth and rapid motion make the sailfish bill an extremely effective feeding adaptation for capturing schooling prey.