Male humpback whales switch to singing in the presence of seismic air guns

Anthropogenic noise, particularly from seismic air guns used in oil and gas exploration, poses a significant threat to marine mammal populations. While mortality is a key concern, sublethal effects on hearing, physiology, feeding, and migration have also been documented. This study focuses on the less-understood impact of seismic air guns on baleen whale breeding behavior. Seismic air guns emit intense pressure waves that overlap with the communication frequencies of baleen whales, potentially causing behavioral changes. Previous research has shown mixed results, with some studies reporting decreased vocalizations while others report increases in response to seismic activity. The current study examines humpback whale mating tactics, which involve a conditional strategy balancing singing (presumed advertisement) and non-singing behaviors (seeking and joining females). The research hypothesizes that air guns will affect this strategy, potentially reducing singing and joining interactions.

Existing research on the effects of seismic surveys on marine mammal breeding behavior is limited. Some studies suggest reduced joining interactions in humpback whales during air gun activity, potentially impacting breeding success. Other studies have reported varied vocal responses in different baleen whale species, with some ceasing singing while others increase vocalizations. This inconsistency highlights the need for further investigation into the complex interplay between factors such as proximity, noise level, duration, and the competitive context of the signaling whales. The flexible mating strategies employed by some species, such as humpback whales, may play a role in their resilience to environmental changes, including anthropogenic noise. The ability to adapt mating tactics may contribute to population recovery, as seen in humpback whales following whaling moratoriums. Male humpback whales exhibit a conditional mating strategy involving a balance between singing and non-singing behaviors. Singing is thought to function in attracting females or as passive male-male competition, while non-singing involves direct competition for access to females through joining and escorting them.

Data were collected during the humpback whale's return migration from breeding to feeding grounds in eastern Australia (2011, 2014, and 2015). Observations were made from land and vessels, tracking whale groups visually and acoustically. Groups of whales were tracked using a theodolite and VDASAR software. Acoustic recordings were collected from a hydrophone array. Seismic air gun experiments were conducted using various vessel and air gun configurations. Data were collected in 3-hour periods, categorized as 'baseline' (no air guns) or 'air gun' (during air gun activity). Whale sex was determined based on visual characteristics, presence of calves, and previous biopsy data, with some probabilistic assignments for uncertain cases. The number of singing and non-singing males, and females, were counted for each 3-hour period. Generalized linear models (GLM) and generalized additive models (GAM) with negative binomial distributions were used to analyze the relationship between the number of singing males and the number of males and females in the area, both in baseline and air gun periods. The payoff for each male tactic (singing or seeking) was assessed based on the number of joining interactions and additional joiners to the group. Statistical analyses involved GLMs and GAMs to examine the relationship between payoff, air gun presence, and the number of males and females present.

During air gun periods, there was a significantly higher number of singing males compared to baseline conditions. However, unlike baseline conditions, the number of singers was not significantly related to the total number of males or females during air gun periods. The payoff for seeker males (non-singing) was significantly reduced in the presence of air guns. Conversely, the total payoff for singer males increased significantly with increasing male numbers during air gun activity, but not during baseline conditions. The individual singer payoff was higher in higher male densities during air gun activity but not during baseline periods. In baseline conditions, seeker males increased joining interactions with more singers present. Singer payoff did not change significantly with air gun presence, but increased with more males present during air gun activity, while not doing so in baseline conditions. The number of observed joining interactions for seeker males declined in the presence of air guns.

The findings indicate a shift in male humpback whale mating tactics in response to seismic air gun activity. While the increase in singing during air gun periods might seem positive, the overall impact on breeding success is uncertain. The observed decrease in non-singing joining interactions may reflect avoidance behavior, while the increase in singing could be a compensatory response or a triggered reaction to the air gun noise. The possibility that air gun noise mimics other acoustic signals involved in breeding behavior, such as breaching, should be considered. However, the short duration of air gun exposure in this study might not fully capture the long-term effects of seismic surveys. The study also highlights the complexity of baleen whale vocal responses to seismic air guns, suggesting further research is needed to understand the mechanisms and long-term consequences of such noise exposure. The impact on female choice and the overall consequences for breeding success remain unclear.

This study demonstrates that seismic air guns alter male humpback whale mating tactics, causing an increase in singing behavior and a decrease in non-singing joining interactions. While the short-term effects appear to be a shift in strategy, rather than a complete cessation of breeding activities, the long-term impacts on breeding success and population dynamics require further research. Future studies should examine longer exposure durations, investigate the role of female choice in these altered interactions, and consider the potential for both positive and negative effects of this behavioural shift.

The relatively short duration of air gun exposure (1 hour) may not fully represent the effects of longer seismic surveys. The study's reliance on visual and acoustic observations introduces potential biases. The probabilistic assignments of sex for some whales introduce some uncertainty into the analyses. Further research is necessary to fully elucidate the long-term effects of seismic air guns on breeding behavior and population dynamics.