Life history constrains biochemical development in the highly specialized odontocete echolocation system

How an animal receives sound may influence its use of sound. While 'jaw hearing' is well supported for odontocetes, work examining how sound is received across the head has been limited to a few representative species. The substantial variation in jaw and head morphology among odontocetes suggests variation in sound reception. Here, we address how a divergent subspecies, the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) hears low-, mid-and high-frequency tones, as well as broadband clicks, comparing sounds presented at different locations across the head. Hearing was measured using auditory evoked potentials (AEPs). Click and tone stimuli (8, 54 and 120 kHz) were presented at nine locations on the head and body using a suctioncup transducer. Threshold differences were compared between frequencies and locations, and referenced to the underlying anatomy using computed tomography (CT) imaging of deceased animals of the same subspecies. The best hearing locations with minimum thresholds were found adjacent to a mandibular fat pad and overlaying the auditory bulla. Mean thresholds were not substantially different at locations from the rostrum tip to the ear (11.6 dB). This contrasts with tests with bottlenose dolphins and beluga whales, in which 30-40 dB threshold differences were found across the animals' heads. Response latencies increased with decreasing response amplitudes, which suggests that latency and sensitivity are interrelated when considering sound reception across the odontocete head. The results suggest that there are differences among odontocetes in the anatomy related to receiving sound, and porpoises may have relatively less acoustic 'shadowing'.

Section: Introductionmentioning

confidence: 99%

Hearing pathways in the Yangtze finless porpoise,Neophocaena asiaeorientalis asiaeorientalis

Mooney

Ketten

et al. 2013

“…N 2 solubility in toothed whale blubber (melon) and reception (mandibular fat bodies) of high frequency sound. One of the characteristic features of the acoustic fats is that they all contain some amount of WE (varying from ~5-10% in porpoises to >90% in beaked whales) Koopman and Zahorodny, 2008), even if the blubber of the species in question does not. Necropsies of stranded whales have suggested that the acoustic fat bodies might be vulnerable to the formation of gas bubbles under certain conditions (Fernández et al, 2005;Jepson et al, 2003).…”

Section: Physiological and Biological Implications Of The Datamentioning

confidence: 99%

“…The following temperature program was used: 65°C for 2min, hold at 165°C for 0.40min after ramping at 20°Cmin -1 , hold at 215°C for 6.6min after ramping at 2°Cmin -1 , and hold at 250°C for 5min after ramping at 5°Cmin -1 . Up to 80 different fatty acids/alcohols were identified (see Koopman, 2007;Koopman and Zahorodny, 2008). Peak identification was based on comparisons of retention time to standards (Nu Chek Prep) and known samples.…”

Section: Lipid Composition Analysismentioning

confidence: 99%

Solubility of nitrogen in marine mammal blubber depends on its lipid composition

Koopman

Westgate

2012

SUMMARYUnderstanding the solubility of nitrogen gas in tissues is a crucial aspect of diving physiology, especially for air-breathing tetrapods. Adipose tissue is of particular interest because of the high solubility of nitrogen in lipids. Surprisingly, nothing is known about nitrogen solubility in the blubber of any marine mammal. We tested the hypothesis that N 2 solubility is dependent on the lipid composition of blubber; most blubber is composed of triacylglycerols, but some toothed whales deposit large amounts of waxes in blubber instead. The solubility of N 2 in the blubber of 13 toothed whale species ranged from 0.062 to 0.107ml N 2 ml -1 oil. Blubber with high wax ester content had higher N 2 solubility, observed in the beaked (Ziphiidae) and small sperm (Kogiidae) whales, animals that routinely make long, deep dives. We also measured nitrogen solubility in the specialized cranial acoustic fat bodies associated with echolocation in a Rissoʼs dolphin; values (0.087ml N 2 ml -1 oil) were 16% higher here than in its blubber (0.074ml N 2 ml -1 oil). As the acoustic fats of all Odontocetes contain waxes, even if the blubber does not, these tissues may experience greater interaction with N 2 . These data have implications for our understanding and future modeling of diving physiology in Odontocetes, as our empirically derived values for nitrogen solubility in toothed whale adipose were up to 40% higher than the numbers traditionally assumed in marine mammal diving models.

“…Studies on species with different life-history timelines indicate that the attainment of an 'adult-like' composition of acoustic lipids coincides with the timing of nutritional independence (i.e. the duration of maternal care; Koopman and Zahorodny, 2008).…”

Section: %mentioning

confidence: 99%

Function and evolution of specialized endogenous lipids in toothed whales

Koopman

2018

The Odontocetes (toothed whales) possess two types of specialized fat and, therefore, represent an interesting group when considering the evolution and function of adipose tissue. All whales have a layer of superficial blubber, which insulates and streamlines, provides buoyancy and acts as an energy reserve. Some toothed whales deposit large amounts of wax esters, rather than triacylglycerols, in blubber, which is unusual. Waxes have very different physical and physiological properties, which may impact blubber function. The cranial acoustic fat depots serve to focus sound during echolocation and hearing. The acoustic fats have unique morphologies; however, they are even more specialized biochemically because they are composed of a mix of endogenous waxes and triacylglycerols with unusual branched elements (derived from amino acids) that are not present in other mammals. Both waxes and branched elements alter how sound travels through a fat body; they are arranged in a 3D topographical pattern to focus sound. Furthermore, the specific branched-chain acid/alcohol synthesis mechanisms and products vary phylogenetically (e.g. dolphins synthesize lipids from leucine whereas beaked whales use valine). I propose that these specialized lipids evolved first in the head: wax synthesis first emerged to serve an acoustic function in toothed whales, with branched-chain synthesis adding additional acoustic focusing power, and some species secondarily retained wax synthesis pathways for blubber. Further research is necessary to elucidate specific molecular mechanisms controlling the synthesis and deposition of wax esters and branched-chain fatty acids, as well as their spatial deposition within tissues and within adipocytes.