Jack Terhune terhune@unb.ca
Dept. of Biology
University of New Brunswick
100 Tucker Park Road
Saint John, NB, Canada E2L 4L5
Popular version of paper 3aABa2
Presented Wednesday morning, May 25, 2011
161st ASA Meeting, Seattle, Wash.
Many Weddell seals (Leptonychotes weddellii) spend the winter under 2 metre thick sea ice close to the coast of Antarctica. A dominant male will permit females, submissive males (during the winter) and juveniles to share “his” breathing hole. All seals take turns in rasping the ice at the edge of the breathing hole to keep it from freezing shut. The seals stay in the water for most of the winter because it is too cold for them to haul out onto the ice. In the early spring, when it is warmer, adult females haul out, give birth and nurse a single pup on the ice. The dominant males begin to court the females near his breathing hole especially when the pups are 5 or 6 weeks old and being weaned. Throughout the dark winter and spring breeding season, both male and female seals regularly call to one another underwater. In the spring, males defend their territories first by calling and then by fighting with intruders if necessary. Sound is the only way they can communicate with each other once they are more than a few metres apart. Weddell seals have a very wide range of call types, (trills, whistles, chugs, grunts, tones, etc. and many of their underwater calls are loud enough that they can be clearly heard by a human standing on the ice.
Assuming that the hearing sensitivity of Weddell seals is about the same as for harbour seals (Phoca vitulina), seals calling in a deep water area without any islands or grounded icebergs nearby should be able to hear each other at distances of 30 km or more. Perhaps one reason the calls are so loud is so they can communicate over very long distances. Males, for example, would benefit if they could attract females from greater distances.
My graduate student, Phil Rouget, overwintered at the Australian base at Mawson in Eastern Antarctica. He recorded Weddell seal calls and background noise levels using a pair of underwater microphones (hydrophones). I used these recordings to measure the amplitudes of 280 seal calls and the background sounds during days and nights when the noise levels were very low, medium and very high. The highest noise levels occurred when the wind was high, changes in the tide height were cracking the thick sheets of ice and many seals were calling.
When sound spreads out underwater, it becomes quieter the farther away the listener is from the caller. Close to the caller the decrease in amplitude as the distance increases is very pronounced but as the distance becomes larger, the relative drop in amplitude per increase in distance, becomes much less. The drop in amplitude between 10 and 20 metres is the same as the drop in amplitude between 100 and 200 metres or 1 and 2 kilometres. A loud call that was 100 decibels (dB) above the hearing threshold at a distance of 1 metre, would be 60 dB above threshold at 100 metres and 40 dB above threshold at 1000 metres (if there was no background noise).
Background noise will mask or block a call if the call and the noise have the same pitch and the noise level is close to, or above, the sound level of the call. To be clearly heard, a seal call will have to be about 20 dB above the noise level. The average communication ranges for underwater Weddell seal calls using the source levels of the 280 calls and the individual noise levels within each of the three general noise bands (low, medium and high) were calculated. In this model, 10,000 pairings of call levels and noise levels from each of the three categories were made in a random manner.
Under very quiet conditions and where there are no islands or icebergs to block the sound transmission, Weddell seals can hear each other at an average distance of 2.8 kilometres. When the noise levels are medium the average communication range is 430 metres but when noise levels are high, the average range drops to 83 metres. For all three noise levels, the communication distances are shifted towards the shorter ranges and very few of the calls would be detected at very long ranges. This suggests that the high amplitudes of Weddell seal calls have evolved to ensure a practical communication range with seals in the immediate area (as in those seals sharing a breathing hole in the winter or a male defending his underwater territory against an intruder during the breeding season) under noisy conditions. Although some seal, and other marine mammal, calls can be detected at great distances, the conditions would have to be almost perfect for that to occur. In practice, many Weddell seals occupy shallow water fjords or coastal areas where islands or grounded icebergs would preclude long range communication.
This study does not address the possibility that Weddell seals can increase the amplitude of their calls (that is, shout) when it gets noisier or that males may be increasing the loudness of their calls to impress, and thus attract, nearby females during the mating season.