Keywords

11.1 Introduction

Studying the mating strategies and behaviors of free-swimming cetaceans is challenging due to the difficulty of observing and characterizing these behaviors in nature (Schaeff 2007; Lanyon and Burgess 2014). The mating tactics of cetaceans are diverse and vary between the sexes and in different ecological and social contexts (Dines et al. 2015; Orbach 2019). However, decades of research have provided a wealth of knowledge on sexual selection and mating strategies in cetaceans, including conceptive and non-conceptive sexual behaviors (Whitehead and Mann 2000; Furuichi et al. 2014; Orbach 2019; Ham et al. 2023, this book). Many studies rely on anatomical investigations or observations of captive animals (Glabicky et al. 2010; Hill et al. 2018, 2022; Manitzas Hil et al. 2023, this book; Orbach et al. 2023, this book).

Methods for studying cetacean mating behaviors often include boat-based underwater and surface observations (Herzing 1996; Bender et al. 2009; Orbach et al. 2015) and land-based platforms, such as bridges, for observing nearshore species (Keener et al. 2018; Webber et al. 2023, this book). Boat-based observations are the most prevalent method for studying mating and sociosexual behaviors of free-swimming cetaceans (Mann 1999). Long-term photo-identification and behavioral observations have been instrumental in uncovering the mating dynamics of cetaceans. Such studies have provided important insights into the complex social networks of species such as bottlenose dolphins (Tursiops spp.) and killer whales (Orcinus orca), revealing intricate associations influenced by relatedness and overlapping home ranges of kin (Connor et al. 1996; Randić et al. 2012; Connor et al. 2017; Miketa et al. 2018).

Recent advancements in technology have dramatically improved the ability to track the behaviors of marine mammals, both in shallow and pelagic habitats. Animal-borne tags and passive acoustic arrays enable the recording of fine- and broad-scale movements (Nowacek et al. 2016). In some cases, movements recorded with tags and acoustics can provide clues about mating behaviors, such as by measuring the distance between males and females at depth. However, these methods can be cost-prohibitive, and obtaining fine-scale observations of free-swimming cetacean behaviors remains challenging.

11.1.1 Aerial Perspectives

The use of unoccupied aerial vehicles (UAVs)/drones in marine mammal studies has significantly increased in recent years due to their versatility and multiple benefits. Drones consist of a multirotor or a fixed-wing aircraft equipped with cameras and other sensors, which are piloted remotely or autonomously. Drones have been used in a wide range of marine mammal research including photogrammetry to assess body size, health, and energetics (Christiansen et al. 2016; Torres et al. 2022), estimations of population or group size (Fettermann et al. 2022), tracking interactions with humans (Fiori et al. 2020; Pirotta et al. 2022), and observing the behaviors of cryptic species (Baird et al. 2022). Small (< 5 kg) multirotor drones flown over cetaceans may provide overhead views of behaviors inaccessible from boat-based observers or capture rare events, foraging patterns, socializations, and mother-calf interactions (Ramos et al. 2021). Drones can also facilitate the collection of blow samples for genetic, microbiome, and hormonal studies (Raudino et al. 2019; Centelleghe et al. 2020) and provide a means of photo-identifying taxa (Koski et al. 2015; Hartman et al. 2020; Landeo-Yauri et al. 2021). When combined with other methods of data collection, drones may provide valuable insights about the mating grounds of large whales (Cole et al. 2013) and the behavior of pelagic cetaceans (Smultea et al. 2018). Yet observations of mating are often brief and opportunistically gathered during surveys dedicated to estimating population distribution and abundance (Kingsley and Reeves 1998), often in remote and inaccessible regions (Angliss et al. 2018).

The widespread use of drone technology for behavioral studies in cetaceans requires careful consideration and adaptation to different contexts. A growing body of literature has been published on the disturbance impacts of drones on marine mammals, particularly on bottlenose dolphins and manatees (Ramos et al. 2018; Giles et al. 2021; Landeo-Yauri et al. 2021). The type of drone needed depends on the research question and target species. Numerous recent comprehensive reviews and synthesis of the benefits and pitfalls of drone use for marine megafauna studies provide insights of broad and specific applications (e.g., Nowacek et al. 2016; Raoult et al. 2020; King and Jensen 2022). We emphasize the value of drones compared to boat-based research when applied to studying sociosexual and mating behavior in cetaceans (Table 11.1).

Table 11.1 Comparison of different aspects of boat- and drone-based methods for studying mating and sexual behavior in cetaceans
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11.2 Drones Applied to Assess the Sociosexual and Mating Behavior of Cetaceans

An increasing number of studies have reported sociosexual and mating behaviors of cetaceans using drones (e.g., Ramos et al. 2021; Hill et al. 2022; Lonati et al. 2022). The use of aerial drones for enabling direct overhead views of cetaceans and for gathering high-resolution videos of their interactions provides numerous benefits to the study of cetacean mating behavior (Table 11.1). By enabling continuous observations of behavior at the surface and subsurface, it is possible to track individuals and groups and detect specific mating behaviors. High-resolution drone footage demonstrated that the sex-specific mating behaviors of dusky dolphins (Lagenorhynchus obscurus) varied with context (Orbach et al. 2020b); detailed analysis of footage enabled precise counts of mating behaviors, swimming speeds, bearing changes, and the percent of time dolphins spent at the surface (Orbach et al. 2020b). One male rough-toothed dolphin (Steno bredanensis) was video-recorded copulating with another dolphin on at least seven occasions within 7 minutes (Ramos et al. 2021). For some species like harbor porpoises (Phocoena phocoena), their tendency to avoid boats makes them particularly challenging to observe in nature; yet drone footage has captured mating attempts (Webber et al. 2023, this book).

Drones have captured non-conceptive mating, which occurs in several species of cetaceans (Ham et al. 2023, this book). Non-conceptive male-male sexual interactions (swimming belly-to-belly with erect penises) were video-recorded between an adult male killer whale and a calf (Sanvito and Galimberti 2022); aerial imagery enabled identification of the animals from an established photo-identification catalog. Non-conceptive copulatory behavior has also been video-recorded by drone for North Atlantic right whales (Eubalaena glacialis; Lonati et al. 2022); the penis of an adult male was recorded entering a calf’s genital slit (Lonati et al. 2022; Brown and Sironi 2023, this book). Distinguishing conceptive from non-conceptive mating is important to understand social bonding, dominance relationships, and social learning (Ham et al. 2023, this book).

The rapidly growing literature on applications of aerial drones to cetacean research supports their use for capturing unparalleled views of cetacean behavior in nature. However, the application of drones to explore sociosexual and mating behaviors is largely unexplored.

11.3 New Data Documented with Drones

To demonstrate the capabilities of drones to advance exploration of the mating behavior of cetaceans, we analyzed a selection of aerial footage from six different species of cetaceans (common bottlenose dolphins (Tursiops truncatus), rough-toothed dolphins, pygmy killer whales (Feresa attenuata), Risso’s dolphins (Grampus griseus), dusky dolphins (Lagenorhynchus obscurus), and gray whales (Eschrichtius robustus)). We employed various models of small (< 5 kg) multirotor DJI drones, ensuring compliance with all local laws and regulations for drone operations in Belize, Mexico, New Zealand, the USA, and Portugal (Table 11.2). All videos were filmed in 4 K resolution during manually operated flights at a maximum duration of ~20 mins (Table 11.2).

Table 11.2 Information on the six species of cetacean filmed using small aerial DJI drones during focal group follows
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Videos collected by drone were analyzed, and a subset that included repeated sociosexual or mating behaviors were reviewed in BORIS behavioral analysis software (Friard and Gamba 2016). To provide examples of different data acquired by drones, we reviewed video footage and conducted focal group follows using ad libitum sampling (Mann 1999). Sexual behaviors associated with different mating tactics of each sex were characterized according to behavioral ethograms (Table 11.3; Orbach 2016, 2019). Due to the limited duration of our follows and our overall dataset, we likely only captured a fraction of the sexual behaviors displayed by any of our study species.

Table 11.3 Mating tactics and associated precopulatory behaviors in male and female cetaceans identifiable by drone observations
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11.3.1 Sex Identification

Observations of the genitals of cetaceans, which are a necessity to verify the sex of an individual in the absence of genetic analyses, were often possible during post hoc video review of sociosexual interactions and mating behaviors (Fig. 11.1). All cetaceans analyzed rotated numerous times during sexual interactions, exposing their ventra at the surface (Fig. 11.1). Males were identified in videos of all six species based on observation of their everted penises (Fig. 11.1). It was comparatively easy to observe the large penises of large odontocetes and baleen whales (Fig. 11.1d).

Fig. 11.1
4 photographs identify the sex of different kinds of dolphins, a to c, and whales, d. A. It annotates the male and female dolphins. B. It displays 2 male and one female dolphin. C. It identifies one male dolphin. D. It indicates 2 male and one female whale.

Examples of sex identification of cetaceans from aerial drone observations. The extruded penis of males provides the most reliable indicator of sex in a wild cetacean from aerial drone observations. (a) Dusky dolphins (Lagenorhynchus obscurus), (b) common bottlenose dolphins (Tursiops truncatus), (c) rough-toothed dolphins (Steno bredanensis), (d) gray whales (Eschrichtius robustus). ♀ = female; ♂ = male

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Differences in mating behaviors within and across species resulted in varying degrees of visibility of the act of intromission (penile penetration). For instance, intromission was rarely visible in dusky dolphins as females swam ventrum-down and belly-to-belly with a male, thereby obscuring views of copulation from the overhead view of the drone (Fig. 11.1a). In contrast, intromission was sometimes visible for common bottlenose dolphins as males had a perpendicular orientation to a female while thrusting their pelvises toward her genitals (Fig. 11.1b). Males of all species we analyzed approached females with their penises extruded during copulation attempts, sometimes successfully achieving intromission (Fig. 11.2). Successful intromission was not observed in gray whales although it should be overt as mating occurs primarily at the surface and the penis is sizeable and highly visible (Fig. 11.1d). The gray whale penis may be sufficiently large to detect ejaculation from aerial drone footage. Caution may be warranted in determining sex based on behaviors alone during mating interactions since homosexuality is common among cetaceans (Ham et al. 2023, this book; Würsig et al. 2023, this book).

Fig. 11.2
2 sets of 4 photos, labeled a to h, display different species of cetaceans swimming in the water. A and C, have one male and a female dolphin. B and D, have 2 male and a female whale and dolphin, respectively. E, F, and H, have one male and female dolphin, whale, and dolphin, respectively. G has 2 male and a female dolphin.

Copulation attempts and copulations involving intromission in six species of cetaceans. Copulation attempts varied across species in orientation, intensity of approach, and receptivity of females. (a) Dusky dolphins (Lagenorhynchus obscurus), (b) gray whales (Eschrichtius robustus), (c) rough-toothed dolphins (Steno bredanensis), (d) common bottlenose dolphins (Tursiops truncatus), (e) Risso’s dolphins (Grampus griseus), (f) pygmy killer whales (Feresa attenuata), (g) common bottlenose dolphins, (h) rough-toothed dolphins. ♀ = female; ♂ = male

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11.3.2 Individual Identification

The ability to identify particular individuals during mating encounters can yield insights into sexual selection pressures. For example, certain males may have a particular attribute that is desirable to a female or increases copulation opportunities. Drones offer the potential for photo-identification or tracking of cetaceans that have scarring or unique body markings and features that are discernible from an aerial perspective (Hartman et al. 2020; Ramos et al. 2021). Rough-toothed dolphins, Risso’s dolphins, pygmy killer whales, and gray whales had individually identifiable features visible in drone footage that provided the ability to distinguish individuals in mating interactions. For example, we could distinguish which male gray whales pursued females and count the frequency at which a male interrupted a rival’s copulation attempt (Fig. 11.3). ImageJ (Abràmoff et al. 2004) was used to alter the color scheme of images to make scars appear prominent, and the measure particle tool was used to extract prominent features (Fig. 11.3c).

Fig. 11.3
6 photographs, and an image. A. It displays 3 male and a female whale. B. It displays the head part of the female whale. C. An image of the head part of the same female whale. D and E, have one male whale. F and G, have one male and female whale.

An example of photo-identification of individual female and male gray whales (Eschrichtius robustus) during mating behavior in a group of adults observed in San Ignacio Lagoon, Mexico. Whales are covered in markings that enable reidentification of individuals throughout drone-based focal follows and in future sightings. (a) A female gray whale as multiple males nuzzle her genitals with their rostrums. (b) The head and anterior body of the same female zoomed in (yellow box in A). (c) Features extracted from B using the analyze particle feature in ImageJ (Abràmoff et al. 2004) to illustrate some features for use in photo-identification. (d–g) Different male gray whales observed in a mating chase in pursuit of a single female. ♀ = female; ♂ = male

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We caution that the overhead angle of drones can reduce visualization of certain body features typically used for photo-identification (e.g., perpendicular photographs of dorsal fins, undersides of flukes). However, body scarring can often be used to match individuals to identification photos taken from a boat simultaneous to drone operations. When flown at a steep angle to one side of a group, it may be possible to photograph the dorsal fins of cetaceans with sufficient quality images to match boat-based photo-identification images (e.g., dwarf sperm whales, Kogia sima, Baird et al. 2022). While drones are unlikely to serve as an alternative to boat-based photography during behavioral follows, they serve as a complementary tool.

11.3.3 Female Cetacean Mating Tactics

Cetacean mating tactics are diverse and vary between the sexes, ecological conditions, and social contexts (Orbach 2019). The mating strategies and tactics used by female cetaceans to control paternity are not well understood nor known for most species (Boness et al. 2002). Given the energetic costs associated with producing large gametes and investing in parental care (Trivers 1972), females likely play an active role in the selection of mating partners to improve the fitness of their offspring (Orbach 2019). However, male intrasexual competition and sexual coercion can obscure female preferences, leading to the historic belief that females have more passive roles in paternity control than males (Clutton-Brock and McAuliffe 2009). Thus, research has largely focused on understanding the temporal and energetic investments females make in rearing viable offspring (Whitehead and Mann 2000). However, female cetaceans may use several mating tactics to control paternity.

Of the five female mating tactics characterized for cetaceans (i.e., signal discrimination, mate choice copying, evasive behaviors, polyestry/multiple matings, modified genitalia; Orbach 2019), we focus on examples of evasive behaviors, signal discrimination, and polyestry/multiple matings because of the possibilities to detect evidence of these tactics using aerial videos collected by small multirotor drones (Table 11.3).

11.3.3.1 Evasive Behaviors

To ensure the reproductive success of their progeny, females assess the quality of potential mates before selecting preferred mates and rejecting undesirable mates. Behavioral studies using drones could identify and quantify female responses to copulation attempts and levels of proceptivity, receptivity, or resistance to males. Thus, drone footage can provide a valuable tool to improve understanding of the complex and nuanced ways in which females express their mate choice. Our data demonstrate that female cetaceans display many short-duration, easily detectable evasive behaviors during mating chases that can be examined in aerial drone video across different species and environmental conditions.

Females may prevent copulations by changing their body position to make their genitalia inaccessible to suitors. Female dusky dolphins repeatedly make deep dives and swim inverted at the surface so that their genital opening is inaccessible to pursuing males (Orbach et al. 2015). Similarly, we observed from drone footage that female bottlenose dolphins frequently rolled while stationary to turn their ventra away from approaching males (Fig. 11.4c). We observed a single female repeatedly roll to her left and right to avoid copulation attempts from three pursuing males (Figs. 11.2d, 11.4c). In contrast, female rough-toothed dolphins sometimes evaded approaching males by rapidly swimming forward.

Fig. 11.4
2 sets of photographs, a to f and g to l, indicates the female and male mating behaviors, respectively. A to f display tail slap, passive, body roll, reorientation leap, accelerated swim, and direction change. G to L display mate guarding, grasping, chasing, interference, male-male aggression, and open mouth.

Examples of different mating behaviors of female and male cetaceans identified in aerial drone observations. (a) Common bottlenose dolphins (Tursiops truncatus), (b) dusky dolphins (Lagenorhynchus obscurus), (c) common bottlenose dolphins, (d–e) dusky dolphins, (f) Risso’s dolphins (Grampus griseus), (g) common bottlenose dolphins, (h) gray whales (Eschrichtius robustus), (i) common bottlenose dolphins, (j) gray whales, (k) common bottlenose dolphins, (l) rough-toothed dolphins (Steno bredanensis). ♀ = female; ♂ = male

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In most species for which we observed evasive female displays, females also exhibited receptive behaviors to mating attempts. In a group of 12 pygmy killer whales, a single female was not observed actively maneuvering or turning her ventrum to block male access to her genitals (Fig. 11.2f). Similarly, a female rough-toothed dolphin did not resist copulations by multiple males and maintained her swimming speed.

11.3.3.2 Signal Discrimination

Female selection of desired mates involves choices of heritable characteristics that can include access to ample resources, morphological traits, behavioral displays, and overall competitive abilities (Darwin 1871). Signal discrimination of secondary sexual characteristics is a common tactic used by females to choose high quality mates. For example, female dusky dolphins swim ventrum-down, which may regulate copulations by restricting the breathing rate of competing ventrum-up males attempting to copulate (Markowitz et al. 2010; Orbach et al. 2015). These behaviors seem to drive extended mating chases during which females may assess the fitness of potential mates based on their agility and behavior.

Our analysis revealed that several cetacean species display behaviors consistent with female signal discrimination in mating contexts. Dusky dolphins and gray whales engaged in long mating chases, with multiple males pursuing a female that was swimming ventrum-down (Fig. 11.2a, b). During copulation attempts, male dusky dolphins briefly approached females and maintained a ventrum-up posture to align themselves with a female and match her swim speed (Figs. 11.1a, 11.2a). Male dusky dolphins also leaped multiple times while in pursuit of females, which may function as a behavioral display; mating attempts may provide females with opportunities to assess potential suitors’ characteristics such as swim speed and body size (Markowitz et al. 2010). Male gray whales also actively competed to be proximate to the female and copulate, pushing away other males and grasping the female with their flippers (Figs. 11.1a, 11.2b); a female gray whale may evaluate a male’s ability to remain proximate to her. In rough-toothed dolphins, food sharing may have played a role in maintaining females nearby during copulation attempts (Ramos et al. 2021).

Alternatively, photogrammetric measurements of the size of individual males may help reveal mate choice driven by body characteristics of different possible mates (e.g., the choice of a large male). We did not consistently collect data on animal size, but future studies using drones equipped with a GPS/LiDAR sensor payload (e.g., Dawson et al. 2017) would enable precise measurements of male and female sizes to associate with behavioral data on individual success at copulation.

11.3.3.3 Polyestry and Multiple Matings

In some species and populations in which male cetaceans coerce females to mate, females may exert control over paternity through polyestrous cycling and multiple mating (Connor et al. 1996). Polyestry is hypothesized to obscure calf paternity, reduce sexual harassment, and prevent infanticide by conspecifics (Hrdy 1979; Connor et al. 1996; McEntee et al. 2023, this book). Repeated estrous cycles, coupled with limited periods of ovulation and mating with multiple males, may aid a female in siring the offspring of a desired mate while obscuring the paternity of her calf. Infanticide (intentional killing of non-descendent young) may induce lactating females to begin estrous cycling and thereby increase the potential for a male to sire an offspring (Hrdy 1979). Multiple species of dolphins commit infanticide (McEntee et al. 2023, this book).

Polyestry is a physiological mechanism and cannot be identified directly from drone observations. However, tracking individuals with drones paired with information on the estrous cycling of specific females could facilitate investigation of cetacean reproduction and heredity. Detailed behavioral observations of cetaceans during mating chases and infanticide attempts could be used to identify evidence of males driving specific non-receptive females into estrus (Table 11.3). Target populations require extensive photo-ID and life history information where observations of female choice of mates could be associated with behavioral data and genetic information on dolphins (Connor 2000).

Multiple matings were readily detectable from aerial drone observations with all species we observed, sometimes involving possible signaling of receptivity to copulation with one or multiple males at a time. In contrast to the ventrum-up orientation of female gray whales typically observed during evasive mating chases (Swartz 2018), the female we observed maintained a ventrum-down position during a mating chase, possibly indicating receptivity to the mating attempts by numerous competing males (Figs. 11.1d, 11.3). Male Risso’s and rough-toothed dolphins individually approached females resulting in multiple mating attempts (Fig. 11.5). In Risso’s dolphins and dusky dolphins, copulation attempts occur in quick succession with multiple males (Hartman et al. 2023, this book; Markowitz et al. 2023, this book). Data on the estrous cycling of these species could be associated with drone-based mating observations to disentangle the dynamics of mating.

Fig. 11.5
4 photographs, a to d, of the mating attempts of dolphins just beneath the surface water.

One example from mutual mating in Risso’s dolphins (Grampus griseus) captured by a drone in 2022 (Mavic Pro 2). (a) Copulation between Male 1 and female. (b) Male 1 leaves the female and her calf. (c) The distance increases between Male 1 and the female with her calf as Male 2 approaches. (d) Male 2 in mating position beneath the female with the calf nearby. ♀ = female; ♂ = male

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11.3.4 Male Cetacean Mating Tactics

Male cetaceans primarily use five competitive mating tactics: display, scramble, contest, endurance, and sperm competition. These tactics can be observed visually and are more amenable to drone-based studies compared to the covert tactics used by females (Table 11.3).

11.3.4.1 Display Competition

In display competition, males engage in courtship displays that use morphological or behavioral signals to attract the attention of females. These displays can reflect dominance, genetic quality, readiness to breed, and access to resources. For example, male humpback whales (Megaptera novaeangliae) have elaborate songs, and Amazon river dolphins (Inia geoffrensis) carry sticks (Martin et al. 2008; Allen et al. 2018). The improved vantage point provided by drones enables capturing rarely seen displays and allows for detailed tracking of male and female interactions, which is not possible from a boat perspective (Fig. 11.4).

Although we did not observe cetacean males displaying overt behaviors that were interpretable as evidence of display competition, males frequently pursued females with their penises extruded prior to attempting copulation. An everted penis may increase the chances of intromission when near a female and could also serve as a signal to females of readiness to mate (Keener et al. 2018). In gray whales, an extruded penis would likely be visible to females during mating chases and may partially explain why males maintained their erections while swimming in pursuit of females instead of only immediately prior to attempting intromission (Fig. 11.1d).

11.3.4.2 Contest Competition

Contest competition involves one or more males attempting to prevent other males from approaching reproductive females through aggression, sometimes escalating into violent intrasexual interactions (Tyack and Whitehead 1982; Orbach 2019). For some species, detailed and repeated observations of contest competition are available from boat-based studies (e.g., humpback whale competitive groups), as they typically occur near the surface and in clear water (Clapham et al. 1992). Common and Indo-Pacific bottlenose dolphins in many populations display aggression and violent behavior during male-male competition and sexual coercion, typically resulting in dolphins biting each other and leaving extensive tooth rake markings across bodies (Connor et al. 2006). Similar aggressive male-male interactions are reported in Risso’s dolphins, including headbutting (Hartman et al. 2023, this book).

Intense competition among males was seen in multiple species we examined based on drone footage. Several male common bottlenose dolphins and Risso’s dolphins engaged in aggressive exchanges with each other in the presence of a single female being pursued. Two male bottlenose dolphins in a group of eight engaged in repeated head-on charges, during which both males had their mouths open and attempted to hit each other with their flukes while passing each other (Fig. 11.4k). Multiple competing male gray whales repeatedly interfered with the copulation attempts of other males by using their rostrums to wedge between rivals and the female and pushing the competing male out of the way while occupying its previous position (Fig. 11.4j).

11.3.4.3 Endurance Competition

In endurance competition, multiple males attempt to outlast their competitors for durations long enough to cause major energetic and temporal costs. Male Indo-Pacific bottlenose dolphins in Shark Bay, Australia, work with alliance members to sequester and isolate the female for up to several months and aggressively copulate with her (Connor et al. 1992). Allied males coordinate to “herd” a female and restrain her movements by producing loud and threatening “pop” sounds (Smolker and Connor 1996) and aggressively charging, biting, and colliding with her. Alliance formation among bottlenose dolphins is reviewed in this book (Brightwell and Gibson 2023, this book) as is endurance competition in Risso’s dolphins (Hartman et al. 2023, this book).

We observed common bottlenose dolphins engaged in sociosexual behaviors in small (group size = 3) to large groups (group size = 16); some included male cooperative mate guarding and herding of females and intense aggressive fights between multiple males competing for access to the female (Fig. 11.6). Mating behaviors frequently involved high-energy chases of females and frequent surface displays (Fig. 11.6).

Fig. 11.6
6 aerial photographs, a to f, display the sociosexual behavior of dolphins at 0, 5, 14, 21, 30, and 35 seconds, respectively. 2 spectrograms plot the sound record of the dolphin group.

Aerial drone observations of common bottlenose dolphins (Tursiops truncatus) engaged in sociosexual behavior during concurrent boat-based acoustic recordings of their sounds. (a–f) The interactions involved active chases, surface displays, numerous copulation attempts, and aggressive intrasexual interactions among 16 dolphins. The timestamps are in mm: ss. The waveform and spectrograms below the panels depict the sounds recorded during this 35 second clip, primarily consisting of the vocalizations of dolphins (bright orange). Sounds were graphed using Raven 1.6.3 (K. Lisa Yang Center for Conservation Bioacoustics 2023)

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11.3.4.4 Scramble Competition

Scramble competition manifests as males rapidly finding and mating with as many reproductively ready females as possible over a short time. Males fight for the closest position to the reproductively ready female to mate with her, providing her opportunities to exert selection over mates during extended chases (Clapham et al. 1992). During scramble competition, male cetaceans typically engage in energetic chases or surface displays in pursuit of females, changing their swimming speed and direction frequently and incurring energetic costs for both sexes (Orbach et al. 2014). For example, groups of four sexually mature male dusky dolphins typically chased a single female for 10 minutes (Orbach et al. 2015).

In our data, evidence of scramble competition was most salient in mating interactions of dusky dolphins. Dusky dolphin males engage in high-speed chases and rapid copulation attempts with a target female, often including surface-active behaviors and leaps. Male gray whales display numerous behaviors associated with scramble competition. Most baleen whales, including humpback whales and North Atlantic right whales, migrate to breeding grounds annually (Clapham et al. 1992; Kraus and Hatch 2001). Similarly, gray whales are commonly observed in surface-active groups engaging in vigorous sexual activity during their breeding season. Drone footage captured many surface-active groups including multiple gray males simultaneously pursuing a single female (Fig. 11.4h, j). It is unclear from our observations alone if multiple males pursue multiple females over short times in the other four species of cetacean we observed, as most observations involved a single female pursued by multiple males. In Risso’s dolphins, scramble competition filmed with a drone showed that not all males participating during a chase were able to mate or obtain access to a female.

11.4 New Mating Behaviors Documented via Drone

Drones hold immense potential to capture new mating behaviors among cetaceans. Continuous observations of cetacean mating interactions revealed a variety of behaviors that were previously undetected from a boat or any other platform of observation. For example, a rough-toothed dolphin that copulated repeatedly with a female displayed an open mouth behavior, moving its jaw up and down with its mouth agape while aligned perpendicularly with the female (Fig. 11.4l); the open mouth behavior occurred eight times within 12 minutes of video footage during a single focal follow, indicating it may be commonly associated with mating in this species or population. Open mouth displays may signal threats to the female, as open mouth behaviors have typically been reported during agonistic interactions between conspecifics and heterospecifics in multiple species of cetaceans (Herzing 1996). The use of open mouth behaviors during copulation attempts could signal to the female potential negative consequences of resisting copulation attempts or signal to other nearby males to stay away.

We documented a variety of mating behaviors involving close physical contact between animals at the surface and subsurface. For example, male gray whales regularly used their pectoral flippers to grasp the female across her peduncle, appearing to slow her movement and prevent evasion (Fig. 11.4h). Similar behaviors were observed in multiple bottlenose dolphin males sandwiching a female between them (Figs. 11.1b, 11.4g) and during individual male approaches in pygmy killer whales (Fig. 11.2f). Grasping behavior may function to constrain female evasion, prevent copulations from rival males by limiting access to the female, be part of a male display to the female or other male, or aid in intromission by enabling the male to direct its penis more effectively toward the female’s genital slit (Fig. 11.1d). Male cetaceans may orient their bodies in specific positions to align their genitalia with females as physical alignment of genitalia at specific angles is essential for high likelihood of fertilization success (Orbach et al. 2020a).

In gray whales, common bottlenose dolphins, and pygmy killer whales, we observed males repeatedly nuzzling their rostra against the genital region of females and engaging in close physical contact with females; these sociosexual behaviors may play an important role in mating and were detected because of the overhead view from the drone coupled with high-resolution video recording. Clitoral stimulation is likely pleasurable to common bottlenose dolphins (Brennan et al. 2022). Dolphins of other species and populations engage in genital stimulation, sometimes coupled with a buzzing with the melon against the genital area of a conspecific (Herzing 1996). Dolphin echolocation sounds have high energy, which could provide extensive vibration and stimulation of the genitals.

11.5 Understanding Sound Production and Mating Behavior

The sounds produced by cetaceans can play an important role in mating and reproduction, sometimes functioning in sexual displays and competition (Clapham et al. 1992). During cooperative mate herding, pairs of male Indo-Pacific bottlenose dolphins consort females through aggression and produce “pop” vocalizations that threaten her to stay close (Smolker and Connor 1996; Vollmer et al. 2015). Associating specific sounds with behavior and attributing calls to specific individuals are often limited to classifying behaviors of individuals or a group during brief surfacings and comparing these with recorded sounds (Tyack 2000). Drones provide a valuable observational tool to identify behaviors associated with acoustic production in cetaceans and enhance our ability to contextualize the use of different sounds. Drones used to observe dolphin behavior during acoustic playback experiments have enhanced understandings of social associations of male Indo-Pacific bottlenose dolphins (King and Jensen 2022). Concurrent flights of multiple drones, some equipped with passive acoustic recorders (Frouin-Mouy et al. 2020), are a promising area of future research to associate acoustic sounds with mating specific behaviors.

We demonstrate an example of acoustic recordings of the vocalizations of a group of 16 common bottlenose dolphins with concurrent aerial observations in the lagoons of Turneffe Atoll, Belize, on August 1, 2016. An SQ26-08 (Cetacean Research Technology) hydrophone was suspended 1 m under the water’s surface from the boat, recording to a Tascam DR-05 digital recorder with a 96 kHz sample rate in 16-bit to WAV format files. Spectrograms of acoustics recordings were reviewed by EAR in Raven 1.6.3 (K. Lisa Yang Center for Conservation Bioacoustics 2023) to identify and classify dolphin vocalizations as tonal whistles, burst-pulse calls, or echolocation clicks (Tyack 2000).

The bottlenose dolphins engaged in vigorous mating and sociosexual behavior throughout multiple flights (Fig. 11.6). Numerous dolphins pursued a single individual within the group (possibly a female), swimming fast to stay close to each other as the group collectively turned in the same direction (Fig. 11.6a). Half of the group appeared to pursue the single dolphin, while the other half oriented toward the possible mating chase (Fig. 11.6c). The fleeing dolphin leaped several times (Fig. 11.6d) and accelerated to swim ahead of its pursuers close behind (Fig. 11.6e).

We plotted the waveform and spectrogram of a 35 second clip of sounds recorded in the time series of aerial imagery of the dolphin group (Fig. 11.6). Multiple dolphins produced longs bouts of low- and high-frequency burst-pulse signals, repeated bouts of echolocation clicks, and numerous frequency-modulated narrowband whistles (Fig. 11.6). Several low-frequency sounds have been associated with allied male bottlenose dolphin aggression toward females during consortships, such as “pops” (Smolker and Connor 1996; King et al. 2019; Casoli et al. 2022), and with attempted infanticides perpetrated by males (Perrtree et al. 2016).

The above observations illustrate the power of aerial drone video to identify the surface and subsurface activity of cetacean mating groups associated with their sound production. For instance, most of the bottlenose dolphins recorded were completely underwater during our observations and largely out-of-sight to surface observers (Fig. 11.6). Paired subsurface observations and underwater recordings of animal sounds provide a previously unattainable capacity to identify the behavioral context of sound production.

11.6 Conclusions

The improvement of our understanding of the diversity of mating strategies, tactics, and behaviors that evolved across whales, dolphins, and porpoises encourages the development of novel methods to quantify animal behavior in nature. We demonstrate how drone-based imaging can be applied to the study of sociosexual and mating behavior of free-ranging cetaceans, providing increased observational power in capturing behavior and enabling detailed animal tracking. The inclusion of aerial drones as a data collection tool allows for unparalleled views of animals, their behavior, and collection of robust video and imagery for multiple uses. The use of drones for observing the behavior of cetaceans paired with traditional methods of field data collection and laboratory analyses has the potential to help contextualize the activities of rarely observed species and optimize limited surface times with difficult to study species. Advancements in drone and imaging technologies continue to rapidly improve the flight time and resolution of imagery while decreasing in costs; growing commercial popularity makes drone purchases and use more available to the global community of scientists.