Joseph C. Gaspard

Audiogram and auditory critical ratios of two Florida manatees ( Trichechus manatus latirostris )

SUMMARY Manatees inhabit turbid, shallow-water environments and have been shown to have poor visual acuity. Previous studies on hearing have demonstrated that manatees possess good hearing and sound localization abilities. The goals of this research were to determine the hearing abilities of two captive subjects and measure critical ratios to understand the capacity of manatees to detect tonal signals, such as manatee vocalizations, in the presence of noise. This study was also undertaken to better understand individual variability, which has been encountered during behavioral research with manatees. Two Florida manatees (Trichechus manatus latirostris) were tested in a go/no-go paradigm using a modified staircase method, with incorporated ‘catch’ trials at a 1:1 ratio, to assess their ability to detect single-frequency tonal stimuli. The behavioral audiograms indicated that the manatees’ auditory frequency detection for tonal stimuli ranged from 0.25 to 90.5 kHz, with peak sensitivity extending from 8 to 32 kHz. Critical ratios, thresholds for tone detection in the presence of background masking noise, were determined with one-octave wide noise bands, 7–12 dB (spectrum level) above the thresholds determined for the audiogram under quiet conditions. Manatees appear to have quite low critical ratios, especially at 8 kHz, where the ratio was 18.3 dB for one manatee. This suggests that manatee hearing is sensitive in the presence of background noise and that they may have relatively narrow filters in the tested frequency range.

Evaluation of Adrenocortical Function in Florida Manatees (Trichechus manatus latirostris)

The study objectives were to determine the predominant manatee glucocorticoid; validate assays to measure this glucocorticoid and adrenocorticotropic hormone (ACTH); determine diagnostic thresholds to distinguish physiological vs. pathological concentrations; identify differences associated with sex, age class, female reproductive status, capture time, and lactate; and determine the best methods for manatee biologists and clinicians to diagnose stress. Cortisol is the predominant manatee glucocorticoid. IMMULITE 1000 assays for cortisol and ACTH were validated. Precision yielded intra- and inter-assay coefficients of variation for serum cortisol: ≤23.5 and ≤16.7%; and ACTH: ≤6.9 and ≤8.5%. Accuracy resulted in a mean adjusted R(2)≥0.87 for serum cortisol and ≥0.96 for ACTH. Assay analytical sensitivities for cortisol (0.1 µg/dl) and ACTH (10.0 pg/ml) were verified. Methods were highly correlated with another IMMULITE 1000 for serum cortisol (r=0.97) and ACTH (r=0.98). There was no significant variation in cortisol or ACTH with sex or age class and no correlation with female progesterone concentrations. Cortisol concentrations were highest in unhealthy manatees, chronically stressed by disease or injury. ACTH was greatest in healthy free-ranging or short-term rehabilitating individuals, peracutely stressed by capture and handling. Cortisol concentrations ≥1.0 µg/dl were diagnostic of chronic stress; ACTH concentrations ≥87.5 pg/ml were diagnostic of peracute stress. In healthy long-term captive manatees, cortisol (0.4±0.2 µg/dl) and ACTH (47.7±15.9 pg/ml) concentrations were lower than healthy free-ranging, short-term rehabilitated or unhealthy manatees. Capture time was not significantly correlated with cortisol; ACTH correlation was borderline significant. Cortisol and ACTH were positively correlated with lactate.

Four-choice sound localization abilities of two Florida manatees,Trichechus manatus latirostris

SUMMARY The absolute sound localization abilities of two Florida manatees (Trichechus manatus latirostris) were measured using a four-choice discrimination paradigm, with test locations positioned at 45 deg., 90 deg., 270 deg. and 315 deg. angles relative to subjects facing 0 deg. Three broadband signals were tested at four durations (200, 500, 1000, 3000 ms), including a stimulus that spanned a wide range of frequencies (0.2–20 kHz), one stimulus that was restricted to frequencies with wavelengths shorter than their interaural time distances (6–20 kHz) and one that was limited to those with wavelengths longer than their interaural time distances (0.2–2 kHz). Two 3000 ms tonal signals were tested, including a 4 kHz stimulus, which is the midpoint of the 2.5–5.9 kHz fundamental frequency range of manatee vocalizations and a 16 kHz stimulus, which is in the range of manatee best-hearing sensitivity. Percentage correct within the broadband conditions ranged from 79% to 93% for Subject 1 and from 51% to 93% for Subject 2. Both performed above chance with the tonal signals but had much lower accuracy than with broadband signals, with Subject 1 at 44% and 33% and Subject 2 at 49% and 32% at the 4 kHz and 16 kHz conditions, respectively. These results demonstrate that manatees are able to localize frequency bands with wavelengths that are both shorter and longer than their interaural time distances and suggest that they have the ability to localize both manatee vocalizations and recreational boat engine noises.

Manatee vibrissae: evidence for a “lateral line” function

Aquatic mammals use vibrissae to detect hydrodynamic stimuli over a range from 5 to 150 Hz, similar to the range detected by lateral line systems in fishes and amphibians. Manatees possess ∼5,300 vibrissae distributed over the body, innervated by ∼209,000 axons. This extensive innervation devoted to vibrissae follicles is reflected in enlarged, elaborate somatosensory regions of the gracile, cuneate, and Bischoffs brain‐stem nuclei, ventrobasal thalamus, and presumptive somatosensory cortex. Our preliminary psychophysical testing indicates that in Florida and Antillean manatees the Weber fraction for detection thresholds for grating textures ranges from 0.025 to 0.14. At the lower end of this range, sensitivity is comparable to human index finger thresholds. For hydrodynamic stimuli of 5–150 Hz, detection threshold levels for manatees using facial or postfacial vibrissae were substantially lower than those reported for harbor seals and similar to reports of sensitivity for the lateral line systems of some fish. Our findings suggest that the facial and postfacial vibrissae are used to detect hydrodynamic stimuli, whereas only the facial vibrissae are used for direct contact investigation.

Ancient convergent losses of Paraoxonase 1 yield potential risks for modern marine mammals

Adaptive conflicts with the modern world Mammals evolved in terrestrial environments. Those that now live in the marine environment have had to adapt to the particular selective pressures that this environment imposes. Meyer et al. surveyed the genomes of several marine mammal species to identify regions of convergent change. Multiple losses of the Paraoxonase 1 gene are evident in marine mammals, likely resulting from remodeling of lipid metabolism or antioxidant networks. The multiple occurrences of this loss of function across taxa indicate an evolutionary benefit. However, Paraoxonase 1 is the primary mammalian defense against organophosphorus toxicity. Marine mammals may be at a great disadvantage in the Anthropocene if run-off of this agricultural product into the marine environment continues. Science, this issue p. 591 Convergent loss of Paraoxonase 1 may leave marine mammals unable to metabolize organophosphates. Mammals diversified by colonizing drastically different environments, with each transition yielding numerous molecular changes, including losses of protein function. Though not initially deleterious, these losses could subsequently carry deleterious pleiotropic consequences. We have used phylogenetic methods to identify convergent functional losses across independent marine mammal lineages. In one extreme case, Paraoxonase 1 (PON1) accrued lesions in all marine lineages, while remaining intact in all terrestrial mammals. These lesions coincide with PON1 enzymatic activity loss in marine species’ blood plasma. This convergent loss is likely explained by parallel shifts in marine ancestors’ lipid metabolism and/or bloodstream oxidative environment affecting PON1’s role in fatty acid oxidation. PON1 loss also eliminates marine mammals’ main defense against neurotoxicity from specific man-made organophosphorus compounds, implying potential risks in modern environments.

Evoked potential measurements of the West Indian manatee modulation rate transfer function

Evoked potential measurements of two Florida manatees (Trichechus manatus latirostris) suggest that these herbivores have evolved an auditory system with high temporal resolution. The manatee modulation rate transfer function (MRTF) is maximally sensitive to a 600‐Hz amplitude modulation (AM) rate. This modulation rate is midway between the AM sensitivities of terrestrial mammals (chinchillas, gerbils, and humans) (80–150 Hz) and dolphins (1000–1200 Hz). We also demonstrate evoked potential responses to carrier frequencies up to 40 kHz. These results suggest that manatees may have reasonable underwater localization abilities despite the high speed of sound underwater, which could be important in enabling them to localize oncoming boats.