Weddell seals do not lengthen calls in response to conspecific masking

Some mammalian and avian species alter their vocal communication signals to reduce masking by background noises (including conspecific calls). A preliminary study suggested that Weddell seals (Leptonychotes weddellii) increase the durations of some underwater call types when overlapped by another calling seal. The present study examined the durations and overlapping sequences of Weddell seal calls recorded in Eastern Antarctica. The calling rate, call type (13 major categories), total duration, numbers of elements per call, and overlapping order of 100-200 consecutive calls per recording location were measured. In response to increased conspecific calling rates, the call durations and numbers of elements (within repeated-element call types) did not change or became shorter. Calls that were not overlapped were 3.8 plus or minus 6.1 s long, the first call in a series of overlapped calls was 14.4 plus or minus 15.7 s and subsequent calls in an overlapping series were 6.5 plus or minus 10.3 s. The mean durations of non-overlapped and overlapped calls matched random distributions. Weddell seals do not appear to be adjusting the durations or timing of their calls to purposefully avoid masking each others' calls. The longer a call is, the more likely it is to overlap another call by chance. An implication of this is that Weddell seals may not have the behavioural flexibility to reduce masking by altering the temporal aspects of their calls or calling behaviours as background noises (natural and from shipping) increase.

部分哺乳类与鸟类会调整其发声通讯信号，以降低背景噪音（包括同类鸣声）造成的掩蔽效应。已有预研究显示，威德尔海豹（Weddell seal，Leptonychotes weddellii）在受到其他海豹鸣声掩蔽时，会延长部分水下鸣声的时长。本研究针对南极东部海域录制的威德尔海豹鸣声，分析其鸣声时长与重叠序列。本研究量化测定了每个录音位点处100~200段连续鸣声的鸣叫速率、鸣声类型（共13大类）、总时长、单鸣声所含音节数以及重叠顺序。结果显示，当同类鸣叫速率提升时，威德尔海豹的鸣声时长以及重复音节型鸣声的单鸣声音节数并未发生变化，甚至有所缩短。未发生重叠的鸣声时长为3.8±6.1秒，重叠序列中的首段鸣声时长为14.4±15.7秒，而重叠序列中的后续鸣声时长则为6.5±10.3秒。未重叠与重叠鸣声的平均时长均符合随机分布规律。威德尔海豹似乎并未通过调整鸣声时长与发声时序，主动规避同类鸣声的掩蔽。鸣声越长，其偶然与其他鸣声发生重叠的概率越高。据此可推知，当背景噪音（包括自然噪音与航运噪音）加剧时，威德尔海豹或许并不具备通过调整鸣声时域特征或发声行为来降低掩蔽效应的行为灵活性。