The conspecific avoidance strategies of adult female-calf humpback whales

Abstract

During migration, humpback whale (Megaptera novaeangliae) adult females and their calves use acoustic calling to help maintain contact. The signals produced by these pairs, however, may unintentionally attract nearby breeding males, which can result in interactions that have negative physical and physiological effects on the calf. Therefore, maternal females must choose the vocal and/or behavioral strategy that most effectively balances intra-pair communication with male avoidance. Here, we analyzed differences in adult female-calf vocal activity and movement behavior according to the presence of, and distance to, singing whales and other groups likely to contain males. The results of this study found that these pairs make only minimal changes to their vocal behavior in response to nearby males, suggesting that they have instead evolved calls that are naturally difficult to detect (i.e., produced at significantly lower rates and acoustic levels than other whale groups, resulting in a restricted active space). In addition, they maintain spatial separation from nearby groups by moving to shallower, inshore waters, increasing their proportion of time spent near the surface, and favoring a direct migratory course. This combination of cryptic strategies balances avoidance of unwanted conspecific interaction with the necessity of continued contact between maternal female humpback whales and their calves.

Humpback whale adult females and calves balance acoustic contact with vocal crypsis during periods of increased separation

“…They exhibit considerable differences in their acoustic parameters between age classes (Indeck et al, 2021) and the variety of call types produced suggests that they may serve several potential functions, including as contact, distress, and/or nursing calls. Additionally, Videsen et al (2017) noted that adult female–calf pairs produced calls at very low levels (on average 136 and 141 dB re 1 µPa, depending on call type) compared to previously reported calls by other humpback whale groups (e.g., Dunlop et al, 2013) and suggested that these resulted in a restricted active space with a radius of only 30 m. Cryptic calling, resulting in calls that are naturally difficult to detect, has been shown to be a vocal strategy for avoiding unwanted conspecific interaction (Indeck et al, 2021). However, such low‐level calls would be particularly susceptible to acoustic masking from anthropogenic activities, increasing the risk that a female may lose acoustic contact with her calf.…”

“…Here, we found that this risk was greatest for adult female–calf humpback whale pairs during periods of “resting/milling” as compared to periods of “travelling.” As expected, these pairs called significantly more often when resting; however, they did not increase their call levels between behavioral states. Due to their consistently low levels, the majority of adult female–calf calls were estimated to reach an SNR of 0 before 2 km, limiting their detectability by nearby conspecifics (Indeck et al, 2021). Therefore, we show that these pairs balance their need to maintain contact when separated (i.e., increased calling) with their effort to simultaneously maintain acoustic crypsis (i.e., quiet calling).…”

Preferred shallow-water nursery sites provide acoustic crypsis to southern right whale mother–calf pairs

“…This includes humpback whales and grey whales, which both visit seasonal calving sites in shallow, near shore environments [ 59 ]. Acoustic crypsis in signal production amplitude, similar to that of right whales, has been observed in migrating humpback whale mother–calf pairs in Australia [ 4 , 26 ]. Additionally, humpback whale mother–calf pairs have been observed moving to shallower waters and spending more time near the surface when there are breeding males nearby [ 4 ], behaviours which could allow mother–calf pairs to avoid acoustic detection by conspecifics.…”

“…Acoustic crypsis in signal production amplitude, similar to that of right whales, has been observed in migrating humpback whale mother–calf pairs in Australia [ 4 , 26 ]. Additionally, humpback whale mother–calf pairs have been observed moving to shallower waters and spending more time near the surface when there are breeding males nearby [ 4 ], behaviours which could allow mother–calf pairs to avoid acoustic detection by conspecifics. Grey whales are known to nurse their calves in shallow lagoons and research has suggested that avoidance of killer whale predators may drive migration to these habitats [ 60 ].…”

“…These quiet calls may protect vulnerable calves by reducing their detectability by, for instance, adult male conspecifics and killer whales ( Orcinus orca) . Adult male baleen whales may harass mother–calf pairs, with the potential for physical injury or adverse physiological effects [ 4 , 28 ], and killer whales are the primary predator of baleen whale calves [ 29 , 30 ].…”