Comparative analyses of animal-tracking data reveal ecological significance of endothermy in fishes

Abstract

Despite long evolutionary separations, several sharks and tunas share the ability to maintain slow-twitch, aerobic red muscle (RM) warmer than ambient water. Proximate causes of RM endothermy are well understood, but ultimate causes are unclear. Two advantages often proposed are thermal niche expansion and elevated cruising speeds. The thermal niche hypothesis is generally supported, because fishes with RM endothermy often exhibit greater tolerance to broad temperature ranges. In contrast, whether fishes with RM endothermy cruise faster, and achieve any ecological benefits from doing so, remains unclear. Here, we compiled data recorded by modern animal-tracking tools for a variety of free-swimming marine vertebrates. Using phylogenetically informed allometry, we show that both cruising speeds and maximum annual migration ranges of fishes with RM endothermy are 2-3 times greater than fishes without it, and comparable to nonfish endotherms (i.e., penguins and marine mammals). The estimated cost of transport of fishes with RM endothermy is twice that of fishes without it. We suggest that the high energetic cost of RM endothermy in fishes is offset by the benefit of elevated cruising speeds, which not only increase prey encounter rates, but also enable larger-scale annual migrations and potentially greater access to seasonally available resources.

Keywords: body temperature; marine predator; migration; swim speed.

Fig. 1.

Cruising speed and the cost of transport as a function of body mass. (A) Cruising speed of fishes with RM endothermy (pink) and fishes without it (light blue) recorded in the wild, with other vertebrates swimmers [seabirds, marine mammals (gray), and sea turtles (black)] for comparison. (B) Cost of transport (i.e., the energy needed to move a unit body mass over a unit distance) estimated for each fish species with each cruising speed. See main text for the equations of phylogenetically informed regression lines shown in the figure, except for seabirds (speed = 1.60 × mass^0.04) and marine mammals (speed = 0.78 × mass^0.10) (21) in A.

Fig. 2.

The maximum annual migration ranges for various vertebrate swimmers. The range for a species is measured from long-term animal-tracking studies as the maximum value (among individuals) of linear distances along the Earth’s surface between the two furthest points on an individual’s annual migration loop.

Fig. 3.

The relationship between cruising speed and maximum annual migration range in fishes. Among the species shown in Figs. 1A (cruising speed) and 2 (maximum annual migration range), only 13 species for which both parameters are available are used in this figure. The least-squares regression line is as follows: range = 3,095 × speed – 117.