Background noises mask the detection of sound throughout a limited frequency range termed the critical bandwidth. Critical bandwidths of a harbour seal (Phoca vitulina) were measured, using behavioural psychophysical techniques, by indirect (critical ratios) and direct (two-tone masking) methods underwater and in air. Underwater critical ratios were determined at 4, 8, 16, and 32 kHz, using white noise spectrum levels of 50, 56, 60, and (or) 70 dB re 1 μPa. The critical ratios (pooled data, threshold ±SD) were 19 ± 9, 22 ± 7, 25 ± 7, and 27 ± 5 dB for the respective frequencies. In-air critical ratios were determined at 2, 4, 8, and 16 kHz, using white noise spectrum levels ranging from 23 to 50 dB re 20 μPa. The critical ratios (pooled data) were 25 ± 8, 23 ± 10, 21 ± 15, and 23 ± 16 dB for the respective frequencies. The arithmetic mean of the critical ratios in both media was 23 dB. This suggests that the seal is equally sensitive to pure tone signals in the presence of broad band noise in both air and water. Direct measurements of the critical bandwidth underwater were determined at 4, 8, 16, and 32 kHz, using a pure tone masker ranging from 96 to 120 dB re 1 μPa. In-air direct measurements of the critical bandwidth were measured at 2, 4, and 8 kHz, using a pure tone masker set at 80 dB re 20 μPa. The bandwidths, estimated at 23 dB below the masking level, were all under 2.25 kHz and become proportionately narrow at higher frequencies. These results show a narrow critical bandwidth for the harbour seal, thus indicating high frequency resolution in both media. The directly measured critical bandwidths from the two-tone masking study were not 2.5 times the critical bandwidth estimated from the critical ratios, as previously reported in some other mammals.
Interspecific hybridization is recognized as an important process in the evolutionary dynamics of both speciation and the reversal of speciation. However, our understanding of the spatial and temporal patterns of hybridization that erode versus promote species boundaries is incomplete. The endangered, endemic koloa maoli (or Hawaiian duck, Anas wyvilliana) is thought to be threatened with genetic extinction through ongoing hybridization with an introduced congener, the feral mallard (A. platyrhynchos). We investigated spatial and temporal variation in hybrid prevalence in populations throughout the main Hawaiian Islands, using genomic data to characterize population structure of koloa, quantify the extent of hybridization, and compare hybrid proportions over time. To accomplish this, we genotyped 3,308 double‐digest restriction‐site‐associated DNA (ddRAD) loci in 425 putative koloa, mallards, and hybrids from populations across the main Hawaiian Islands. We found that despite a population decline in the last century, koloa genetic diversity is high. There were few hybrids on the island of Kauaʻi, home to the largest population of koloa. By contrast, we report that sampled populations outside of Kauaʻi can now be characterized as hybrid swarms, in that all individuals sampled were of mixed koloa × mallard ancestry. Further, there is some evidence that these swarms are stable over time. These findings demonstrate spatial variation in the extent and consequences of interspecific hybridization, and highlight how islands or island‐like systems with small population sizes may be especially prone to genetic extinction when met with a congener that is not reproductively isolated.
1993. Repetition enhances hearing detection thresholds in a harbour seal (Plzoc-u ~>irulinu). Can. J. Zool . 7 1 : 926 -932. Pure-tone hearing thresholds of a harbour seal (Phocbc1 virulincl) were measured in air and underwater using bchavioural psychophysical techniques. A 50-111s sinusoidal pulse was presented in both white-noise n~askcd and unmasked situations at pulse repetition rates of 1 , 2, 4, and 101s. Test frequencies were 0.5, 1.0. 2.0, 4.0, and 8.0 kHz in air and 2.0, 4.0, 8.0, and 16.0 kHz underwater. Relative to 1 pulsels, mean threshold shifts were -1 , -3, and -5 dB at 2, 4, and 10 pulsesls, respectively. The threshold shifts from 1 to 10 pulsesls were significant (F = 12.457. df = 2.36, p < 0.001) and there was no difference in the threshold shifts between the masked and unmasked situations (F = 2.585; df = 1.50; p > 0.10).Broadband masking caused by meteorological or industrial sources will closely resemble the white-noise situation. At high calling rates, the numerous overlapping calls of some species (e.g.. harp seal, Phocu groenlonclic~u) present virtually continous "background noise" which also resembles the broadband white-noise masking situation. An implication of lower detection thresholds is that if a seal regularly repeats short vocalizations. the coriimunication range of that call could be increased significantly (80% at 10 pulsesls). This could have important implications during the breeding season should storms or shipping noises occur or when some pinniped species become increasingly vocal and the background noise of conspecifics increases. TURNBULL, S. D., et TERHUNE, J. M . 1993. Repetition enhances hearing detection thresholds in a harbour seal (Phocu virulina). Can. J. Zool. 71 : 926-932. Le seuil d'audition des sons purs chez un Phoque commun (Phocu ~irulinn) a ete niesure dans I'air et dans I'eau au moyen de techniques comportementales psychophysiques. Une pulsation sinusoi'de de 50 ms a eti. cmise dans des contextes de bruit blanc masque ou non masque, a des taux de repetition de I , 2, 4 et 101s. La frequence etait de 0,5, 1.0, 2,0, 4,O ou 8,O kHz dans I'air et de 2,0, 4,0, 8,O et 16,O kHz sous I'eau. Le seuil moyen. relativement B 1 pulsationls, a ete cvalue B -I DB a 2 pulsationsls, a -3 DB a 4 pulsationsls et a -5 DB a 10 pulsationsls. Le deplacenient du seuil entre 1 et 10 pulsationsls est significatif (F = 12,457; d.1. = 2,36; p < 0,001) et il n'y a pas de difference dans les deplacements du seuil entre les contextes de bruit masque et non masque (F = 2,585; d.1. = 1,50; p > 0.10).Le masquage cause par des bandes de grande amplitude provenant de sources meteorologiques ou industrielles risque de ressembler au contexte de bruit blanc. A des frequences d'appel elevees, les nombreux appels de certaines especes (e.g. le Phoque du Groenland, Phocu grc~enlundicu) qui viennent s'ajouter creent un bruit de fond presque continu, ce qui ressemble aussi au contexte de bruit blanc de grandc amplitude. La consequence d'un seuil de detection bas est que la repetition regulikre d'appe...
Predators can influence prey communities through direct predation and also through the costs of antipredatory behavioral responses or risk effects (Heithaus 2001a, Heithaus and Dill 2006, Wirsing et al. 2008. The risk of predation by sharks may be a selective pressure for social evolution (Norris 1994) and perhaps seasonality in calving rates (Fearnback et al. 2012) in dolphins. Sharks are considered to prey on dolphins when they attack free-swimming individuals as opposed to scavenging on terminally ill, severely wounded, or stranded animals (Heithaus 2001a). Stomach content studies have shown delphinid species consumed by at least five different shark species (Cockcroft et al. 1989, Monteiro et al. 2006. First-hand observations of shark attacks on small cetaceans (Mann and Barnett 1999, Maldini 2003, Gibson 2006, Silva-Jr et al. 2007, Turnbull and Dion 2012, Dudzinski 2 ) are rare and therefore alternative methods for assessing shark predation must be developed. One method is by assessing failed attempts; that is, documenting fresh injuries and scars on reliably reidentified dolphins that can be attributed to sharks.Here, we report on shark attacks on Atlantic spotted dolphins (Stenella frontalis) near Bimini, The Bahamas, and examine occurrence of attacks as a function of body parts, sex, and age class. We also discuss how the rate of scarring attributed to nonlethal shark attacks at our study site compares to others, and the possible shark species responsible for these attacks.The waters north and west of North Bimini, The Bahamas, have been the site of longitudinal, long-term research by the Dolphin Communication Project (DCP)
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