Mark H. Deakos

Acoustic properties of humpback whale songs

A vertical array of five hydrophones was used to measure the acoustic field in the vertical plane of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of at least 10 m. The spacing of the hydrophones was 7 m with the deepest hydrophone deployed at a depth of 35 m. An eight-channel TASCAM recorder with a bandwidth of 24 kHz was used to record the hydrophone signals. The location (distance and depth) of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 151 and 173 dB re 1 microPa. One of the purposes of this study was to estimate potential sound exposure of nearby conspecifics. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicated that the sounds are projected in the horizontal direction despite the singer being canted head downward anywhere from about 25 degrees to 90 degrees. High-frequency harmonics extended beyond 24 kHz, suggesting that humpback whales may have an upper frequency limit of hearing as high as 24 kHz.

Male humpback whales in the Hawaiian breeding grounds preferentially associate with larger females

The competitive group appears to be a major component of the mating system of the humpback whale, Megaptera novaeangliae, bringing together a single female (nuclear animal or NA) and multiple males (escorts) that compete for physical proximity to her. We examined the relation of body size of the NA to the number of attending escorts and, separately, we determined the relation of a female’s body size to the size of her calf. Using underwater videogrammetry in Maui waters during 1997–2002, we measured the body length of the NA in each of 42 competitive groups. We also measured the lengths of the mother and her calf in each of 92 mother–calf groups. The number of initial escorts in a competitive group was positively correlated with NA body length. Longer mothers were associated with longer calves, even after accounting for seasonal differences in calf length. We conclude that male humpback whales prefer to associate with larger females and that larger females produce larger calves. Theoretically, larger calves have a greater chance of survival than do smaller calves. The choice of a larger female may therefore increase the reproductive success of an escort that succeeds in mating.

Size-assortative pairing and discrimination of potential mates by humpback whales in the Hawaiian breeding grounds

Assortative pairing, and its relation to mate choice, has rarely been documented in mammals. Using data collected during 1998–2007, we investigated size-assortative pairing as it relates to discrimination amongst potential mates in humpback whale, Megaptera novaeangliae, dyads in the Hawaiian breeding grounds. Across 67 male–female dyads in which both individuals were measured using underwater videogrammetry, male length was positively correlated with female length. Detailed analyses on the assessment of maturity by comparisons with whaling data revealed that mature-sized females associated almost exclusively with mature-sized males and had a significant preference for large mature-sized males. In contrast, mature-sized males were less discriminating in their associations with females and showed no significant preference for mature-sized females. However, mature-sized males that associated with immature-sized females were significantly smaller than males that associated with mature-sized females. Finally, immature-sized males tended to associate with immature-sized females. The sex differences in size preference by mature whales probably reflect the relatively high costs of mature females mating with small or immature males compared to the lower costs of mature males mating with small or immature females. Body size appears to influence the adoption of alternative mating tactics by males such that smaller mature males avoid the costs of competing for the highest-quality females and instead focus their attentions on smaller females that may or may not be mature. Overall, our results provide the first quantitative evidence of size-assortative pairing and female discrimination amongst potential mates in humpback whales and indeed in any cetacean species.

The reproductive ecology of resident manta rays (Manta alfredi) off Maui, Hawaii, with an emphasis on body size

In resident manta rays (Manta alfredi) off Maui, sexual maturity appears delayed until growth exceeds 90% of maximum size, an indicator that large body size provides a reproductive advantage at the expense of a shorter reproductive time period. In this study, 286 surveys were conducted between 2005 and 2010 using photo-identification and photogrammetry to study the reproductive ecology of a resident population of manta rays off Maui, Hawaii, and investigate the reproductive benefits of large body size in each sex. Although reproductive activities occurred year-round, mating trains and late-term pregnant females were significantly more likely to be observed during the winter months. Some females were pursued by males during both winter and summer of the same year, suggesting multiple ovulations may be possible in a single year. Males likely detect a female’s reproductive state by positioning directly behind her, or passing through her bodily excretions. The mean pregnancy rate was estimated at 0.56 pregnancies/adult female/year with larger females pregnant more often, and more likely in consecutive years. The operational sex ratio was heavily skewed with 2.68 adult males per reproductively available female. Although males appear to compete with one another for females within a mating train, no direct physical competition was ever observed between males. Evidence of highly dynamic mating trains lasting more than one day suggests endurance rivalry may be the primary mating strategy among males, during which larger males may benefit from greater energy reserves. The study area appears to be an important staging area for mating individuals in this population.

The acoustic field of singing humpback whales in the vertical plane

A vertical array of five hydrophones was used to measure the acoustic field of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and to position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of 10 m. The spacing of the hydrophones was 7 m with the deepest hydrophone deployed at depth of 35 m. An 8‐channel TASCAM recorder having a bandwidth of 24 kHz was used to record the hydrophone signals. The location of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 180 and 190 dB. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicate that the sounds are projected in the horizontal direction with the singer’s head canted downward 45 to 60°. High‐frequency harmonics extend...

Habitat preferences by individual humpback whale mothers in the Hawaiian breeding grounds vary with the age and size of their calves

We investigated whether calf age and calf size influence habitat choice by humpback whale mother–calf pairs in their breeding grounds. During 1997–2008, we conducted focal follows of mother–calf pairs in Hawaiian waters. Tail-fluke identification photographs and calf lengths (measured through videogrammetry) were obtained. Water depth and sea-bed terrain type were derived from GPS data. Identification photographs were matched so that the habitat choices could be established within breeding seasons. Across 72 mother–calf pairs resighted over various intervals within a breeding season, magnitude of depth change between initial and final sightings increased significantly with resighting interval. There was a significant increase from initial depth to final depth for relatively long resighting intervals (27–51 days), but no significant difference for relatively short resighting intervals (2–26 days). Although there was no preference for sea-bed terrain type by mother–calf pairs at their initial sighting, there was a preference for rugged terrain at their final resighting. A resource selection model indicated that the relative probability of a location being used by a mother–calf pair increased (as a function of water depth and rugged sea-bed terrain type) from initial to final sighting; a finding supported by subsequent tests of habitat preference versus availability. For 96 measured calves, calf length and water depth were positively correlated, even when ordinal day of measurement was controlled for statistically; a finding confirmed by a general linear model that simultaneously investigated the relationship between water depth, sea-bed terrain type, number of escorts, ordinal day and calf size. Thus, both calf age and size influence habitat choice by mother–calf pairs in their breeding grounds. The movement of mothers and their maturing calves into deeper waters where they favour rugged sea-bed terrain appears to be part of a suite of behavioural changes during the pre-migratory phase of residency in the breeding grounds.