Metabolic trade-offs in tropical and subtropical marine mammals: unique maintenance and locomotion costs in West Indian manatees and Hawaiian monk seals.

Laura Denum,Beau P. Richter,Nicole M. Thometz,Jason S. John,Katharine Boerner,Traci L. Kendall,Terrie M. Williams,Joseph C. Gaspard

doi:10.1242/jeb.237628

Abstract

ABSTRACTUnlike the majority of marine mammal species, Hawaiian monk seals (Neomonachus schauinslandi) and West Indian manatees (Trichechus manatus latirostris) reside exclusively in tropical or subtropical waters. Although potentially providing an energetic benefit through reduced maintenance and thermal costs, little is known about the cascading effects that may alter energy expenditure during activity, dive responses and overall energy budgets for these warm-water species. To examine this, we used open-flow respirometry to measure the energy expended during resting and swimming in both species. We found that the average resting metabolic rates (RMRs) for both the adult monk seal (753.8±26.1 kJ h−1, mean±s.e.m.) and manatees (887.7±19.5 kJ h−1) were lower than predicted for cold-water marine mammal species of similar body mass. Despite these relatively low RMRs, both total cost per stroke and total cost of transport (COTTOT) during submerged swimming were similar to predictions for comparably sized marine mammals (adult monk seal: cost per stroke=5.0±0.2 J kg−1 stroke−1, COTTOT=1.7±0.1 J kg−1 m−1; manatees: cost per stroke=2.0±0.4 J kg−1 stroke−1, COTTOT=0.87±0.17 J kg−1 m−1). These lower maintenance costs result in less variability in adjustable metabolic costs that occur during submergence for warm-water species. However, these reduced maintenance costs do not appear to confer an advantage in overall energetic costs during activity, potentially limiting the capacity of warm-water species to respond to anthropogenic or environmental threats that require increased energy expenditure.

Highlights

Thermoregulation and locomotion represent two of the most energetically costly physiological demands for marine mammals (Davis, 2014; Gallivan et al, 1983; Noren et al, 1999; Whittow, 1987; Williams et al, 1999)
We evaluated whether the energetic costs for maintenance and locomotion are altered with tropical or subtropical living by marine mammals, by measuring the metabolic responses of two warm-water species representing distinct evolutionary lineages; Hawaiian monk seals (Neomonachus schauinslandi) and West Indian manatees (Trichechus manatus latirostris)
We evaluated the effect of the dive response on energy expenditure, by comparing the energetics of continuous surface swimming to submerged swimming in West Indian manatees

Summary

Introduction

Thermoregulation and locomotion represent two of the most energetically costly physiological demands for marine mammals (Davis, 2014; Gallivan et al, 1983; Noren et al, 1999; Whittow, 1987; Williams et al, 1999). Many marine mammals exhibit higher maintenance metabolic rates than terrestrial mammals of similar body mass (Williams et al, 2001). Living in warm water should reduce these maintenance costs due to a decrease in the thermal gradient for heat transfer compared to cold-water marine mammal species. Of the 129 extant marine mammal species, less than 15% are found exclusively in subtropical or tropical waters (IUCN 2020) These distributions are driven in part by the higher primary productivity, and food resource availability, of colder marine regions. The majority of marine mammal species appear to maintain energetic balance by taking advantage of this increased prey availability to compensate for the elevated maintenance demands associated with living in cold water. Superior insulation in the form of thick blubber layers, novel fur structures and densities, and modified dermal perfusion provide an additional thermal advantage, and have allowed marine mammals to radiate into some of the most thermally challenging habitats on earth (Castellini, 2009; Whittow, 1987)

Methods

Results

Discussion

Conclusion