Universal metabolic constraints shape the evolutionary ecology of diving in animals

Diving as a lifestyle has evolved on multiple occasions when air-breathing terrestrial animals invaded the aquatic realm, and diving performance shapes the ecology and behaviour of all air-breathing aquatic taxa, from small insects to great whales. Using the largest dataset yet assembled, we show that maximum dive duration increases predictably with body mass in both ectotherms and endotherms. Compared to endotherms, ectotherms can remain submerged for longer, but the mass scaling relationship for dive duration is much steeper in endotherms than in ectotherms. These differences in diving allometry can be fully explained by inherent differences between the two groups in their metabolic rate and how metabolism scales with body mass and temperature. Therefore, we suggest that similar constraints on oxygen storage and usage have shaped the evolutionary ecology of diving in all air-breathing animals, irrespective of their evolutionary history and metabolic mode. The steeper scaling relationship between body mass and dive duration in endotherms not only helps explain why the largest extant vertebrate divers are endothermic rather than ectothermic, but also fits well with the emerging consensus that large extinct tetrapod divers (e.g. plesiosaurs, ichthyosaurs and mosasaurs) were endothermic.

Methods
Data on diving performance and body mass of ectotherms and endotherms were collated from the published literature. We have drawn on a previously published dataset, initially constructed by Schreer and Kovacs (1997) and subsequently updated by Halsey et al. (2006), Brischoux et al. (2008) and Hayward et al. (2016). We used the "penguiness book" to locate most of the studies used. We rechecked all records and added data from the primary literature to generate the most comprehensive dataset on dive duration to date, comprising 1,792 records for 286 species of which 62 were ectotherms and 224 endotherms.

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