The Vocal Behavior of Mammal-Eating Killer Whales: Communicating with Costly Calls. Cayenne, Angela, Yiru, and Kyra

Transcription

1 The Vocal Behavior of Mammal-Eating Killer Whales: Communicating with Costly Calls Cayenne, Angela, Yiru, and Kyra

2 Objective of study To quantify how often resident and transient killer whales produced pulsed calls in order to determine how differential costs resulting from eavesdropping prey have shaped their vocal communication Predicted that transients should produce pulsed calls less frequently overall, due to much higher costs of vocal behavior To determine, within the transient killer ecotype, whether levels of vocal activity were elevated during or after a kill compared to other behavioral contexts Predicted that vocal communication is rare when transients are searching for prey, but increases after successful attacks and during social interactions Video

3 Distinct Ecotypes of Killer Whales (Orcinus Orca) Resident killer whales Feed on fish (which have poor hearing abilities) Live in large, stable groups Transient killer whales Hunt marine mammals (which have sensitive underwater hearing)

4 Costs of Acoustic Communication of Killer Whales Direct costs: Energy required to produce sound, energy lost by not feeding during time spent in communication Indirect costs: Passing information on to unintended receivers (eavesdroppers) (e.g. competitors, predators, parasitoids, potential prey)

5 Previous studies vs this study Previous studies evaluated the coevolution of predator and prey with bats and insects capable of hearing Other studies have examined the costs of prey eavesdropping on echolocation of toothed whales This study aims to investigate how the superior learning ability of mammalian prey has shaped the vocal behavior of transient killer whales

6 Types of Killer Whale Vocalization 3 functionally and structurally distinct types of vocalization: Echolocation clicks Short, quick pulses of sound, used for orientation and prey detection Whistles Tonal signals used for short-range communication Pulsed calls Most common, used in group recognition and coordination of behavior; Grouped into 3 categories: Discrete Variable Aberrant Spectograms of vocalization types

7 Methods Finding the killer whales Study performed June-December of , northeastern Pacific ocean Johnstone and Queen Charlotte Straits, British Columbia Glacier Bay, Icy Strait, and Stephens Passage, southeastern Alaska Locating and monitoring killer whale groups opportunistic sighting/encounter Scanning from boat or elevated points on shore Opportunistic sightings were often relayed by a network of observers Identification photographs were taken to confirm identity and size of group Monitored vocal behavior within 500 m of boat, using Offshore Acoustics hydrophone

8 Methods Behavior Observations (transients only) Behavior was noted for each pass (within 500 m of boat), and boat position was determined by GPS every 30 min Predatory behavior - Confirmed and possible kills of mammals Categories of behavior for quantification Swim speed Synchronicity and directionality of group members Presence of aerial and percussive behaviors Transient behavior classifications Surface-active Milling (not after kill) Milling after kill Slow travel Travel

9 Methods Acoustic Observations For transients, studying differences in call usage during different behavior contexts Did not investigate echolocation clicks For comparing call frequency between residents and transients, quantified level of vocal activity using formula for rate of vocal behavior (r) r = c/ti where r = rate of vocal behavior per individual per minute, c = number of pulsed calls recorded, t = time in minutes i = number of individuals in the group Compared level of vocal activity while transients were milling after confirmed kills, with level of vocal activity of other behavior categories during same encounter

10 Comparison of Resident and Transient Killer Whales Calculated rates of vocal behavior for resident killer whales using time and number of calls recorded while at least one group member was within 500 m of boat Calculated overall rate of vocal activity across all behavior categories for each encounter with transients Compared these rates using Mann-Whitney U test (which determines whether two independent samples were selected from populations having same distribution)

11 Results Call Frequencies of Resident vs Transient Killer Whales Encountered 10 groups of residents, 25 groups of transients Residents produced pulsed calls more frequently than transients Residents median call rate: 0.34 calls/individual/min Transients median call rate: 0.05 calls/individual/min Difference in call rate was significant

12 Results Context of Vocal Behavior in Transient Killer Whales Encountered 25 total groups Behaviors observed in encounters: Travel: 17 encounters (total time within 500 m: 152 min) Slow travel: 11 encounters (157 min) Milling after kill: 7 encounters (83 min) Surface-active: 4 encounters (50 min) Milling: 4 encounters (12 min) Vocal rates in each behavior: Highest for surface-active (median call rate: 0.63 calls/individual/min) Second-highest for milling after kill (median: 0.27 calls/individual/min) Silence during all other behaviors (median: 0.00 calls/individual/min) Vocal behavior for milling after kill was significantly higher than during slow travel; all other comparisons were not significant

13 Results: Test for Food-related Calling in Transients Successful attacks by transients on marine mammals confirmed during seven of 25 encounters Possible marine mammal kill indicated in 2 encounters Apart from one kill of unidentified marine mammal, killer whales produced pulse calls when milling after a kill (median vocal rate: 0.27 calls/individual/min) No calls recorded during any other behavior categories in same encounters Difference in vocal rate for milling after kill compared with other behavior categories was significant

14 Implications Prey hearing and Killer Whale Vocal Behavior Eavesdropping by mammalian prey has shaped both the usage of vocalizations used in foraging and usage of communicative sounds of transients Results comparing resident and transient vocalizations are consistent with difference in ecological cost for vocal communication Eavesdropping does not impact resident killer whales, cost is limited to energetic cost of generating calls Eavesdropping is costly for transients, results suggest transients have responded by restricting communication to contexts when cost is low Killer whale pulsed calls are essential to maintain group cohesion, but not essential to foraging process

15 Food Calling in Transient Killer Whales? Vocal activity levels were significantly elevated after each of the 7 confirmed kills No observed link between vocal behavior amount and amount of food available Question remains as to whether results indicate food calling No evidence of food calling in study May be food-related Elevated levels of excitement after a kill, either for social interactions during food sharing, or because of smaller cost of vocalizations Noisy attacks may already warn other potential prey of killer whale presence Limiting vocal communication appears to be the only effective strategy to minimize detection by all potential prey

16 Transients hunting in groups Stealth and surprise are important elements of transient hunting strategy Requires behavior to be coordinated Other predators (e.g. residents) face benefit for coordinating behavior, but with no extra cost for lack of coordination For transients, stealth substantially increases probability of successful attack Lack of coordination carries significant added cost due to risk of eavesdropping Video

17 Conclusion As predicted, transient killer whales were much less vocal than residents Transients were more vocally active after a kill, but also during surface-active behavior No evidence of food calling in this study, as no other whales appeared during post-kill vocalizations

18 Fin