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

A collaborative and near-comprehensive North Pacific humpback whale photo-ID dataset

We present an ocean-basin-scale dataset that includes tail fluke photographic identification (photo-ID) and encounter data for most living individual humpback whales (Megaptera novaeangliae) in the North Pacific Ocean. The dataset was built through a broad collaboration combining 39 separate curated photo-ID catalogs, supplemented with community science data. Data from throughout the North Pacific were aggregated into 13 regions, including six breeding regions, six feeding regions, and one migratory corridor. All images were compared with minimal pre-processing using a recently developed image recognition algorithm based on machine learning through artificial intelligence; this system is capable of rapidly detecting matches between individuals with an estimated 97-99% accuracy. For the 2001-2021 study period, a total of 27,956 unique individuals were documented in 157,350 encounters. Each individual was encountered, on average, in 5.6 sampling periods (i.e., breeding and feeding seasons), with an annual average of 87% of whales encountered in more than one season. The combined dataset and image recognition tool represents a living and accessible resource for collaborative, basin-wide studies of a keystone marine mammal in a time of rapid ecological change.

Pregnancy rate and reproductive hormones in humpback whale blubber: Dominant form of progesterone differs during pregnancy

To better understand reproductive physiology of humpback whales Megaptera novaeangliae that reside in Hawai'i and Alaska, enzyme immunoassays were validated for both progesterone and testosterone in free-ranging and stranded animals (n = 185 biopsies). Concentrations were analyzed between different depths of large segments of blubber taken from skin to muscle layers of stranded female (n = 2, 1 pregnant, 1 non-pregnant) and male (n = 1) whales. Additionally, progesterone metabolites were identified between pregnant (n = 1) and non-pregnant (n = 3) females using high pressure liquid chromatography (HPLC). Progesterone concentrations were compared between juvenile (i.e., sexually immature), lactating, and pregnant females, and male whales, and pregnancy rates of sexually mature females were calculated. Based on replicate samples from ship struck animals collected at 7 depth locations, blubber containing the highest concentration of progesterone was located 1 cm below the skin for females, and the highest concentration of testosterone was in the skin layer of one male whale. HPLC of blubber samples of pregnant and non-pregnant females contain different immunoreactive progesterone metabolites, with the non-pregnant female eluate comprised of a more polar, and possibly conjugated, form of progesterone than the pregnant female. In females, concentrations of progesterone were highest in the blubber of pregnant (n = 28, 28.6 ± 6.9 ng/g), followed by lactating (n = 16, 0.9 ± 0.1 ng/g), and female juvenile (n = 5, 1.0 ± 0.2 ng/g) whales. Progesterone concentrations in male (n = 24, 0.6 ng/g ± 0.1 ng/g) tissues were the lowest all groups, and not different from lactating or juvenile females. Estimated summer season pregnancy rate among sexually mature females from the Hawai'i stock of humpback whales was 0.562 (95 % confidence interval 0.528-0.605). For lactating females, the year-round pregnancy rate was 0.243 (0.09-0.59), and varies depending on the threshold of progesterone assumed for pregnancy in the range between 3.1 and 28.5 ng/g. Our results demonstrate the synergistic value added when combining immunoreactive assays, HPLC, and long-term sighting histories to further knowledge of humpback whale reproductive physiology.

Intra-individual variation in the songs of humpback whales suggests they are sonically searching for conspecifics

Observations of animals' vocal actions can provide important clues about how they communicate and about how they perceive and react to changing situations. Here, analyses of consecutive songs produced by singing humpback whales recorded off the coast of Hawaii revealed that singers constantly vary the acoustic qualities of their songs within prolonged song sessions. Unlike the progressive changes in song structure that singing humpback whales make across months and years, intra-individual acoustic variations within song sessions appear to be largely stochastic. Additionally, four sequentially produced song components (or "themes") were each found to vary in unique ways. The most extensively used theme was highly variable in overall duration within and across song sessions, but varied relatively little in frequency content. In contrast, the remaining themes varied greatly in frequency content, but showed less variation in duration. Analyses of variations in the amount of time singers spent producing the four themes suggest that the mechanisms that determine when singers transition between themes may be comparable to those that control when terrestrial animals move their eyes to fixate on different positions as they examine visual scenes. The dynamic changes that individual whales make to songs within song sessions are counterproductive if songs serve mainly to provide conspecifics with indications of a singer's fitness. Instead, within-session changes to the acoustic features of songs may serve to enhance a singer's capacity to echoically detect, localize, and track conspecifics from long distances.

Size-assortative mating in explosive breeders: a case study of adaptive male mate choice in anurans

Exploration of size-assortative mating (SAM) in animals has led to a near consensus that it arises through constraints in choice, such as preference for large females combined with a large male advantage during intrasexual competition. Although such ‘apparent’ SAM is well explored, whether SAM arises because of specific preferences for size-matched mates has been less thoroughly considered. We tested for ‘preference-based’ SAM in an explosively breeding frog (Rana sylvatica), quantifying how male and female sizes affected fertilization and if males preferred size-matched females. We found that size mismatch severely reduced fertilization. Furthermore, males preferred size-matched, not larger, females in mate choice trials. Because males that mated with much larger females fertilized fewer eggs overall than they would have with size-matched females, male preference for size-matched females may be adaptive. Our results expand understanding of the mechanisms underlying SAM, suggesting that multiple mechanisms may simultaneously cause size-assortative mating patterns to emerge.

Assortative mating in space and time: patterns and biases

Despite the important roles of assortative mating for understanding evolutionary processes, our knowledge on the variation in assortative mating across populations and breeding periods has been overshadowed by the greater attention given to general patterns. Obtaining data on mating pairs are difficult for most species; therefore, researchers often group data from different populations or breeding periods, which can increase positive biases in detecting and estimating assortative mating. We used a meta-analytic approach to investigate the biases caused by spatially or temporally pooling data and the assortative mating consistency across populations and breeding periods. We describe assortative mating patterns across and within animal taxa. We performed a systematic review to search studies reporting measures of size-assortative mating (SAM). Grouping data from multiple populations and seasons incurred positive biases. Overall, assortative mating moderately exhibited low repeatability in space and time, but it was inconsistent for most taxa. After excluding pooled measures, the average estimate for assortative mating was moderate and positive. Thus, our findings demonstrate that pooling data can produce misleading results. We also highlight the importance of further investigation of hypotheses that explain spatial and temporal variation in assortative mating, after its detection. Our study reinforces the significance of investigating mating patterns at various spatial and temporal scales before drawing broad conclusions.

Assortative pairing for boldness and consequences for reproductive success in Montagu’s harrier

Behavioural combination within pairs depending on personality and plasticity might influence reproductive success. However, studies testing this hypothesis are rare, especially in the case of monogamous species with bi-parental care in which the sexes exhibit different behavioural roles. In this study, we investigated the pairing patterns for both boldness and boldness plasticity in Montagu’s harriers (Circus pygargus), a species with sex-specific care, and the consequences for their reproductive success. We measured individual boldness and plasticity for both sexes, and we assessed the pairing pattern in the Montagu’s harrier population for these two traits. We calculated four indices to characterize the behavioural association within pairs: pair boldness, boldness similarity within pairs, pair plasticity and plasticity similarity within pairs. The relationship between the behaviour of the parents and the reproductive success was then tested through these four indices. We found a pattern of assortative pairing based on both boldness and plasticity in the Montagu’s harrier population. Within-pair similarity of plasticity had a significant effect on the reproductive success, which was higher for less similar pairs than for more similar pairs. Our results question the origin of this pairing pattern and suggest that ecological constraint and not sexual selection could be the major driver.

Corticosterone in central North Pacific male humpback whales (Megaptera novaeangliae): Pairing sighting histories with endocrine markers to assess stress

Developing a better understanding of the stress response is critical to ensuring the health and sustainability of marine mammal populations. However, accurately measuring and interpreting a stress response in free-ranging, large cetaceans is a nascent field. Here, an enzyme immunoassay for corticosterone was validated for use in biopsy samples from male humpback whales (Megaptera novaeangliae). Analyses were conducted on 247 male North Pacific humpback whale blubber samples, including 238 non-calves and 9 calves that were collected on the Hawaiian breeding and Southeast Alaskan feeding grounds from 2004 to 2006. Significant relationships were found when corticosterone concentrations were examined by year, age class and distribution between locations. When examined by year, corticosterone concentrations for male humpback whales were higher in Hawaii in 2004 than in 2005 and 2006 (p < 0.05). Corticosterone concentration also varied by age class with initially high concentrations at birth which subsequently tapered off and remained relatively low until sexual maturity was reached around age 8-10 years. Corticosterone concentrations appeared to peak in male humpback whales around 15-25 years of age. Blubber biopsies from Alaska and Hawaii had similar mean corticosterone concentrations, yet the variability in these samples was much greater for whales located in Hawaii. It is clear that much work remains to be done in order to accurately define or monitor a stress response in male humpback whales and that specific attention is required when looking at age, sex, and yearly trends. Our results suggest that a stress response may be most impacted by age and yearly oceanographic conditions and needs to be initially examined at the individual level.

The influence of physiological status on the reproductive behaviour of humpback whales (Megaptera novaeangliae)

For most cetacean species, there is little known about how an individual's physiology influences its behaviour. Humpback whales (Megaptera novaeangliae) are a good candidate to examine such links as they have a well-described distribution and behaviour, can be consistently sampled using remote biopsy systems, and have been the subject of several previous endocrine studies. The objective here was to examine whether a female humpback whale's social state (i.e. escorted by a male or not) is related to her endocrine condition, and whether male dominance ranking is related to testosterone levels. Skin and blubber biopsies were collected from the east and west Australian humpback whale populations in 2010-2016 (n = 252) at multiple times throughout the winter-spring breeding season. Steroid hormones were extracted from blubber and concentrations of progesterone (a marker for pregnancy), testosterone (a marker of male testicular activity) and oestradiol (a potential marker of ovarian activity) measured using enzyme-immunoassays. Principal escorts-the dominant males in mixed sex groups-had significantly higher blubber testosterone levels (mean ± SE; 1.43 ± 0.20 ng/g wet weight) than subordinate, secondary escorts (0.69 ± 0.06 ng/g wet weight). Females that were escorted by males typically possessed elevated blubber oestradiol levels (1.96 ± 0.25 ng/g wet weight; p = 0.014); few were considered to be pregnant (p = 0.083). 'Unescorted' females displayed characteristically lower blubber oestradiol levels (0.56 ± 0.06 ng/g wet weight). Together, these results are consistent with 'challenge hypothesis' theory and suggest the existence of associated reproductive patterns in humpback whales.

Someone like me: Size-assortative pairing and mating in an Amazonian fish, sailfin tetra Crenuchus spilurus

In the absence of constraints, preference for larger mates is expected to evolve, as larger individuals are typical of higher potential fitness. Large females are often more fecund and carry larger eggs (which result in higher number and better quality of offspring), whereas large males usually have more conspicuous ornaments and are better at defending resources. However, intrasexual competition can constrain the access to larger partners, especially when opportunities for mate takeover abound. Here we investigate the relationship between individual’s size and mate choice in relation to one’s own size and their respective mate’s size using the sailfin tetra, a sexually dimorphic Amazonian fish species. We show that ornaments of larger males are exponentially more conspicuous, and larger females are more fecund and carry larger eggs. Contrary to expectation, neither males nor females associated for longer with the larger of two offered potential mates. Instead, individuals of both genders chose opposite-sex individuals of similar sizes to themselves. Additionally, similar-sized pairs were more likely to spawn than couples with higher size asymmetries. Grounded on field observations, we propose that prudent choice should be particularly important in this system, since courtship is long (often taking several days), which offers opportunities for mate takeover. Intrasexual competition, however, cannot readily explain female choice for similar-sized males. We thus suggest that such preference might be best explained by avoidance of filial cannibalism.

Testosterone trends within and across seasons in male humpback whales (Megaptera novaeangliae) from Hawaii and Alaska

Understanding reproductive profiles and timing of reproductive events is essential in the management and conservation of humpback whales (Megaptera novaeangliae). Yet compared to other parameters and life history traits, such as abundance, migratory trends, reproductive rates, behavior and communication, relatively little is known about variations in reproductive physiology, especially in males. Here, an enzyme immunoassay (EIA) for testosterone was validated for use in biopsy samples from male humpback whales. Analyses were conducted on 277 North Pacific male humpback whale blubber samples, including 268 non-calves and 9 calves that were collected in the Hawaiian breeding grounds and the Southeast Alaskan feeding grounds from 2004 to 2006. Testosterone concentrations (ng/g) were significantly different between non-calves sampled in Hawaii (n = 182) and Alaska (n = 86, p < 0.05) with peak testosterone concentrations occurring in the winter (January-March) and the lowest concentrations occurring in the summer (June-September). Fall and spring showed increasing and decreasing trends in testosterone concentrations, respectively. Blubber testosterone concentrations in non-calves and calves sampled in Alaska were not significantly different. Blubber and skin from the same individual biopsies (n = 37) were also compared, with blubber having significantly higher testosterone concentrations (p < 0.05) than skin samples. We found variability in testosterone concentration with age, suggesting that male humpbacks reach peak lifetime testosterone concentrations in the breeding grounds around age 8-25 years. The testosterone profile of male humpback whales follows a predictable pattern for capital breeders, where testosterone begins to increase prior to the breeding season, stimulating the onset of spermatogenesis. Incorporation of reproductive hormonal profiles into our overall understanding of humpback whale physiology will shed additional light on the timing of reproduction and overall health of the recently delisted Hawaii distinct population segment (DPS).

The multiple functions of male song within the humpback whale (Megaptera novaeangliae) mating system: review, evaluation, and synthesis

Humpback whales (Megaptera novaeangliae) are seasonal breeders, annually migrating from high-latitude summer feeding grounds to low-latitude winter breeding grounds. The social matrix on the winter grounds is a loose network of interacting individuals and groups and notably includes lone males that produce long bouts of complex song that collectively yield an asynchronous chorus. Occasionally, a male will sing while accompanying other whales. Despite a wealth of knowledge about the social matrix, the full characterization of the mating system remains unresolved, without any firm consensus, as does the function of song within that system. Here, I consider and critically analyse three proposed functions of song that have received the most attention in the literature: female attraction to individual singers, determining or facilitating male-male interactions, and attracting females to a male aggregation within the context of a lekking system. Female attraction suggests that humpback song is an advertisement and invitation to females, but field observations and song playback studies reveal that female visits to individual singers are virtually absent. Other observations suggest instead that females might convey their presence to singers (or to other males) through the percussive sounds of flipper or tail slapping or possibly through vocalizations. There is some evidence for male-male interactions, both dominance and affiliative: visits to singers are almost always other lone males not singing at that time. The joiner may be seeking a coalition with the singer to engage cooperatively in attempts to obtain females, or may be seeking to disrupt the song or to affirm his dominance. Some observations support one or the other intent. However, other observations, in part based on the brevity of most pairings, suggest that the joiner is prospecting, seeking to determine whether the singer is accompanying a female, and if not soon departs. In the lekking hypothesis, the aggregation of vocalizing males on a winter ground and the visits there by non-maternal females apparently for mating meet the fundamental definition of a lekking system and its role though communal display in attracting females to the aggregation, although not to an individual singer. Communal singing is viewed as a form of by-product mutualism in which individuals benefit one another as incidental consequences of their own selfish actions. Possibly, communal singing may also act to stimulate female receptivity. Thus, there are both limitations and merit in all three proposals. Full consideration of song as serving multiple functions is therefore necessary to understand its role in the mating system and the forces acting on the evolution of song. I suggest that song may be the prime vector recruiting colonists to new winter grounds pioneered by vagrant males as population pressures increase or as former winter grounds become unavailable or undesirable, with such instances documented relatively recently. Speculatively, song may have evolved historically as an aggregating call during the dynamic ocean conditions and resulting habitat uncertainties in the late Miocene-early Pliocene epochs when Megaptera began to proliferate. Early song may have been comprised of simpler precursor sounds that through natural selection and ritualization evolved into complex song.

Mares prefer the voices of highly fertile stallions

We investigated the possibility that stallion whinnies, known to encode caller size, also encoded information about caller arousal and fertility, and the reactions of mares in relation to type of voice. Voice acoustic features are correlated with arousal and reproduction success, the lower-pitched the stallion's voice, the slower his heart beat and the higher his fertility. Females from three study groups preferred playbacks of low-pitched voices. Hence, females are attracted by frequencies encoding for large male size, calmness and high fertility. More work is needed to explore the relative importance of morpho-physiological features. Assortative mating may be involved as large females preferred voices of larger stallions. Our study contributes to basic and applied ongoing research on mammal reproduction, and questions the mechanisms used by females to detect males' fertility.

Habitat segregation by female humpback whales in Hawaiian waters: avoidance of males?

Humpback whales congregate annually in low-latitude winter breeding and calving grounds. While on these grounds, females with a dependent calf (‘maternal females’) are sometimes closely attended by one or more male escorts. Using data collected from a shore-based observation platform in the Hawaiian Islands, we tested the hypothesis that the spatial distribution of maternal females is driven primarily by avoidance of males. As predicted, we found that (1) pods containing a calf occurred in significantly shallower water than pods that did not contain a calf, (2) unescorted maternal females occurred in significantly shallower water than escorted maternal females, (3) the number of males escorting a female decreased significantly with decreasing water depth, and (4) the swimming speed of maternal females increased as a function of male presence, with escorted females travelling significantly more rapidly than unescorted females and a significant positive correlation between swimming speed and number of escorts. We suggest that maternal females incur increased energetic costs when escorted by males and consequently position themselves in shallow waters to reduce the likelihood of unwanted male attention.