High suckling rates and acoustic crypsis of humpback whale neonates maximise potential for mother–calf energy transfer

Abstract

Summary The migration of humpback whales to and from their breeding grounds results in a short, critical time period during which neonatal calves must acquire sufficient energy via suckling from their fasting mothers to survive the long return journey. Understanding neonate suckling behaviour is critical for understanding the energetics and evolution of humpback whale migratory behaviour and for informing conservation efforts, but despite its importance, very little is known about the details, rate and behavioural context of this critical energy transfer. To address this pertinent data gap on calf suckling behaviour, we deployed multi‐sensor Dtags on eight humpback whale calves and two mothers allowing us to analyse detailed suckling and acoustic behaviour for a total of 68·8 h. Suckling dives were performed 20·7 ± 7% of the total tagging time with the mothers either resting at the surface or at depth with the calves hanging motionless with roll and pitch angles close to zero. Vocalisations between mother and calf, which included very weak tonal and grunting sounds, were produced more frequently during active dives than suckling dives, suggesting that mechanical stimuli rather than acoustic cues are used to initiate nursing. Use of mechanical cues for initiating suckling and low level vocalisations with an active space of <100 m indicate a strong selection pressure for acoustic crypsis. Such inconspicuous behaviour likely reduces the risk of exposure to eavesdropping predators and male humpback whale escorts that may disrupt the high proportion of time spent nursing and resting, and hence ultimately compromise calf fitness. The small active space of the weak calls between mother and calf is very sensitive to increases in ambient noise from human encroachment thereby increasing the risk of mother–calf separation. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12871/suppinfo is available for this article.

From a calf’s perspective: humpback whale nursing behavior on two US feeding grounds

“…Both Zoidis & Lomac-MacNair (2017) and Videsen et al (2017) concluded that in the breeding grounds mother-calf pairs remained mostly stationary while nursing, and Videsen et al (2017) additionally concluded that nursing only occurred during the ''bottom phase'' of dives (similar to the term ''horizontal phase'' used in this study). Contrary to these findings, nursing on the feeding grounds during our study occurred while the pair was moving and during descending, ascending, and horizontal phases of dives.…”

“…Another study in the breeding grounds used archival motion-sensing tags to study suckling behavior; however, since actual nursing could not be visually verified, Videsen et al (2017) inferred suckling dives and active dives based on a 1.5 m/s 2 median trimmed and normalized minimum specific acceleration (MSA) threshold value determined by surface observations of peduncle dives (Gero & Whitehead, 2007). Therefore, we were restricted in our ability to compare the kinematic values of our ''nursing events'' to Videsen et al (2017) ''suckling dives'' because ''suckling dives'' by definition only included periods of low MSA values.…”

“…Another study in the breeding grounds used archival motion-sensing tags to study suckling behavior; however, since actual nursing could not be visually verified, Videsen et al (2017) inferred suckling dives and active dives based on a 1.5 m/s 2 median trimmed and normalized minimum specific acceleration (MSA) threshold value determined by surface observations of peduncle dives (Gero & Whitehead, 2007). Therefore, we were restricted in our ability to compare the kinematic values of our ''nursing events'' to Videsen et al (2017) ''suckling dives'' because ''suckling dives'' by definition only included periods of low MSA values. Acknowledging the differences in methodologies, Videsen et al (2017) estimated that ''tagged neonate humpback whales are in suckling position, and so potentially suckling, on average 20% of the time''; which is only slightly higher than our results for the combination of the percentage of time the calf spent in close proximity to its mother without nursing (14.16%) plus the time it spent nursing (0.54%).…”

“…Therefore, we were restricted in our ability to compare the kinematic values of our ''nursing events'' to Videsen et al (2017) ''suckling dives'' because ''suckling dives'' by definition only included periods of low MSA values. Acknowledging the differences in methodologies, Videsen et al (2017) estimated that ''tagged neonate humpback whales are in suckling position, and so potentially suckling, on average 20% of the time''; which is only slightly higher than our results for the combination of the percentage of time the calf spent in close proximity to its mother without nursing (14.16%) plus the time it spent nursing (0.54%). Videsen et al (2017) deepest ''suckling dive'' was much shallower (19.2 m) than our deepest nursing event (64.4 m); however, it occurred within the Exmouth Gulf in Western Australia where the water depth is less than 25 m. Our deepest recorded nursing dive occurred along the seafloor; therefore, the depth was likely influenced by the topography of the area and the mother's need to forage along the substrate, more so than the possible physical constraints of the pair's ability to nurse at even greater depths.…”

“…Historically, our understanding of cetacean lactation has been based on whaling data and surface observations, but technological advancements have enabled underwater documentation of nursing in humpback whales. Studies in the breeding grounds have used free divers and underwater video to document suckling behavior (Zoidis & Lomac-MacNair, 2017), as well as archival suction cup tags to record subsurface movement behavior of mother-calf pairs during periods inferred to be suckling based on surface observations (Videsen et al, 2017).…”