Animal mating systems are diverse, complex, and variable. The key to understanding this variation in mating systems is the realization that individuals behave so as to maximize their lifetime reproductive success and that males and females maximize reproductive success in different ways. In general, males tend to maximize reproductive success by mating with as many females as possible to increase the number of offspring they sire. In contrast, females can produce only a limited number of young and tend to maximize their reproductive success by producing and successfully rearing high-quality offspring. Males and females are thus subject to different selective pressures and do not necessarily cooperate. They may, in fact, be in direct conflict over mating arrangements or try to deceive one another. Together, the mating strategies of males and females define the mating system of a species. This article presents an overview of male and female mating strategies and describes how the different groups of marine mammals solve the problem of finding mates.
Mating systems have traditionally been categorized on the basis of three criteria: the number of partners each sex copulates with during the breeding season, whether the male and female form pair bonds, and how long these bonds are maintained. We focus on a single criterion, the number of mating partners ior each sex during a breeding season, to facilitate comparisons among taxa. Thus, we define four mating systems: monogamy, in which each individual has a single partner; polygyny, in which some males have two or more partners; polyandry, in which some females have two or more partners; and polygynandry, in which some males and some females have multiple partners.
In marine mammals, as in most other mammals, maternal gestation and lactation provide the majority of nutrient requirements for developing young and males often contribute nothing but sperm to their offspring. Consequently, males are free to devote the bulk of their time and energy to competing for access to receptive females. Thus, theorv predicts that most marine mammals should be polygynous.
The potential for polygyny, and the extent to which that potential is realized, is determined to a great extent by the degree to which receptive females are aggregated in space and time. The distribution of females, in turn, is determined by phylogenetic constraints (such as the retention of terrestrial birthing in pinnipeds) and by ecological and social conditions. Among the most important of these conditions are the distribution of resources necessary for breeding, predation pressure, and the costs and benefits of group living. Several types of polygynous mating systems have been identified in marine mammals. Males may defend resources that are vital to females, such as parturition sites (resource defense polygyny), or they may defend females directlv either simultaneously (female defense polygyny) or sequentially (sequential female defense polygyny). They may aggregate on traditional display sites and advertise for females (lekking) or they may search widely and spend little time with females except to mate (roving).
It is becoming increasingly clear that females of many species mate with more than one male during a breeding season. Polygynandry is the most accurate term for these mating systems where at least some members of both sexes mate with more than one individual. However, the reproductive success of a female marine mammal does not vary with the number of times she mates per season, as she usually gives birth to only a single offspring and therefore can be inseminated by only one male. Thus, some marine mammals have polygynandrous mating systems but polygynous fertilization patterns.
I. Male Mating Strategies
Male competition for access to mates takes at least three general forms: aggressive interactions to limit the access of other males to females (contest competition), competition to disperse and find sexually receptive females (scramble competition), and competition in courtship to be chosen by the female (mate choice competition). Each of these behavioral strategies has, in turn, generated a number of structures and physiological adaptations. For example, marine mammal males are often distinguished by large body size, big canines, or tusks that can be used as weapons in combat with other males (Fig. 1). In most mammalian species, males disperse more widely than females, and there is increasing evidence that the pattern holds in some marine mammals [e.g., Dall's porpoise (Phocoenoides dalli), Escorza-Trevino and Dizon (2000); beluga (Delphinapterus leucas), O'Coriy-Crowe et al. (1997)]. Males are typically more active in courtship and are the more conspicuous and ornamented sex. They may attempt to entice and attract females through visual, acoustic, and pheromonal displays (Fig. 2).
More recently, several additional forms of male competition have been described. Males may attempt to outcompete other males by producing higher quality or greater quantities of sperm or by removing other male’s sperm (sperm competition). When a male cannot monopolize access to females on his own. males may cooperate and form alliances. These alliances effectively act as a single male in competing for access to females and have been described for Indian Ocean bottlenose dolphins from Shark Bay, Australia (Tursiops aduncus; Connor et al. 1996). and common bottlenose dolphin from Sarasota Bay. Florida (T. truncatus; Wells et al, 1987). The existence of male alliances is suspected in a growing number of other marine mammal species. Males may also form consortships with females in which the male attempts to associate and copulate with a female during the presumptive fertile period. In common bottlenose dolphins, such consortships often correlate with the later birth of offspring (Wells et al, 1999). In species in which males are larger than females, possess dangerous weapons, and aggressively pursue copulation, some males may forcibly coerce the female to mate [forced copulation: northern elephant seals (Mirounga an-gustirostris), Le Boeuf and Mesnick (1990)]. Moreover, some males “cheat.” They may sneak copulations when alpha bulls are distracted [northern elephant seals; Le Boeuf (1974)] or abduct females from the territories of dominant males [kleptogyny; northern fur seals (Callorhinus ursinus), Gentry (1998)].
Figure 1 Adult male northern elephant seals (Mirounga an-gustirostris) fight for positions in a dominance hierarchy that confers access to receptive females.
Figure 2 A singing adult male humpback whale (Megaptera novaeangliae). Singing by humpback males presumably acts to attract females, although whether songs contain cues to mate quality remains in dispute. Singing may also function to space males in a breeding area or to aid in the establishment of dominance hierarchies.
These forms of competition need not be mutually exclusive. It is likely, based on better-studied taxa, that individual males within a marine mammal population will utilize different strategies depending on their age, size, dominance rank, and the number and quality of available females. Although competition between males is obvious in many species of marine mammals, the possibility that males of some species may also prefer to mate with particular females is an area that deserves further attention.
II. Female Mating Strategies
Females of most marine mammals produce only a single offspring at a time. The interbirth interval ranges from 1 year in most pinnipeds and small cetaceans to 5, 6, or even 7 years in larger toothed whales such as sperm whales (Physeter macrocephalus), killer whales (Orcinus orca), and short-finned pilot whales (Globi-cephala iruicrorhynchus). Females can dius produce only a limited number of young and must maximize their reproductive potential by successfully rearing high-quality young. To give offspring a competitive edge, females can enhance their fitness by choosing males that offer resources or genetic benefits. This choice may occur either pre- or postcopulation (die latter may lead to sperm competition). At present, precopulation mate choice has been studied in only a few pinniped species and virtually nothing is known about postcopulatory choice by female marine mammals.
In other well-studied taxa, such as birds, females are highly discriminating in their choice of sexual partners. Moreover, most females choose in a similar way so that a few males achieve many copulations and many other males none. Females may choose among potential mates directly (based 011 resources, size, strength, dominance, or display) or indirectly (by mating with the winner of contests for access to females). Some marine mammal females actively seek out particular males and mate. For example, in California sea lions (Zalcrphus califomianus), some females change pupping locations from one year to the next to remain with a territorial male who changed territory location (Heath, 1989). Females may also incite male-male competition. By protesting male sexual advances loudly, female northern elephant seals instigate fights among males and subsequently mate with the winner of these battles.
It is difficult to establish the presence of female choice and even more difficult to determine why females choose particular mates or to quantify die benefit to females of exercising choice. Direct benefits to the female, in the form of nutritional resources to die female or parental care, are not known to exist in marine mammals. Females can, however, benefit by choosing males widi higher-quality territories, which provide better parturition or dier-moregulatory sites, or by choosing males that give protection from harassment by subordinate males, which provides uninterrupted time for lactation and reduces vulnerability to aggression from other conspecific males. Females can also benefit by discriminating among potential mates 011 the basis of indirect (genetic) benefits. These include choosing males of the correct species, males with immunologically compatible genes, males with “good genes” who can produce offspring of higher quality. Females can also choose males widi better fertilization ability or virility.
Females may make very different decisions regarding which males they associate with, which males they mate with, and which male sires their offspring. In land-breeding pinnipeds, for example, a female may reside widi one dominant or territorial male during lactation but later leave this male to copulate with another male elsewhere (extraterritorial copulation). I11 some marine mammal species, such as the bottlenose dolphin, sexual behavior is a frequent and important component of nonreproductive social life and has little to do with fertilization. As with males, female strategies need not be mutually exclusive and it is likely tiiat different females will utilize different strategies depending 011 their age, dominance rank, and the number and quality of available mates.
III. Mating Systems in Different Groups
For all pinnipeds studied to date, data support, or are highly suggestive of, a polygynous mating system. Pinnipeds are predisposed to polygyny because they give birth 011 land, which results in the spatial clustering of females, and have an annual birthing cycle, which results in reproductive synchrony among females. The degree of polygyny varies both within and among species with the extent of reproductive synchrony and spatial clustering. Most species have a peak availability of receptive females lasting about 1 month, but the availability of receptive females ranges from 10-15 days in harp (Pagophilus groen-landicus) and hooded (Cystophora cristata) seals to a period of several months for species that breed in tropical habitats such as monk seals (Monachus spp.) and Galapagos sea lions (Zalo-phiis ivollebaeki).
Variation in the degree of spatial clustering within and among species is due to a variety of factors, including the spatial distribution of suitable breeding sites, whether mating takes place on land or at sea, the intensity of male harassment, pre-dation pressure and/or thermoregulatory needs. Polygyny and sexual dimorphism are generally much more extreme in species that mate 011 land than in those that mate in the water.
1. Otariids Otariid females feed during lactation. Lactation is energetically costly so females must raise their young 011 sites near highly productive marine areas. Because these sites are limited, females typically occur in dense aggregations (numbering from a few individuals to more than a thousand) on beaches or rocky shelves on islands. Mating occurs on land, although evidence of at least some mating at sea exists for a few species [e.g., Juan Fernandez fur seal (Arctocephalus philippii) and the California sea lion]. The combination of dense female aggregations and terrestrial mating gives some males the opportunity to monopolize mating with many females. Sexual dimorphism among otariids is correspondingly extreme; males are 011 average three times, and sometimes up to six times, heavier than females and have other traits favored in physical combat over females; large canines, thick chests, and dense manes.
The northern fur seal is among the most polygynous of the otariids: a single male at the St. George Island rookery mated with 161 females and hundreds of males had no copulations at all (Gentry, 1998). The lowest levels of polygyny probably occur in the Juan Fernandez fur seal, the South American sea lion (Otaria flavescens), the Galapagos fur seal (Arctocephalus galapagoensis), and Hooker’s sea lion (Phocarctos hookeri), in which the ratio of sexually active adults ranges from two to six females per male (Boness et al, 1993).
Male otariids typically defend territories containing resources needed by females—parturition and thermoregulatory sites—rather than individual females (Fig. 3). However, female defense has been demonstrated in at least one otariid, the South American sea lion. The two types of polygyny are difficult to distinguish and are not necessarily mutually exclusive. There is some evidence suggestive of lekking in three species (the California sea lion, the South American fur seal (Arctocephalus australis), and Hooker’s sea lion), although this interpretation remains controversial.
A male’s ability to acquire and defend a territory depends 011 his size and age, his ability to compete with other males, and his ability to fast during his tenure (contest competition). Un- der most circumstances, the boundaries of territories are fixed and are delineated by breaks in the topography. Males use a species-specific threat display when defending the boundaries of their territory. A male that secures a territory will probably, but not necessarily, mate with many of the females that give birth on his territory. Climate and rookery topography also play important roles in determining a male’s mating success. Those males defending territories containing access to the water, tide pools, or shade acquire a disproportionately large number of females.
Figure 3 A Steller sea lion (Eumetopias jubatus) territory. Adult males defend resource-based territories that encompass female parturition and thermoregulatory sites. Females choose among males in a surprisingly consistent way. As a result, some males holding territories never or rarely mate, While a few males mate with many females.
Most otariid bulls fast while maintaining their territories, sometimes for the entire 2- to 3-month breeding season [e.g., Steller sea lions (Eumetopias jubatus) and northern fur seals]. Some males return to the same territory in subsequent years whereas others move to new territories or are not seen again. Territorial males may try to herd females to prevent them from leaving their territories, but in most species, females determined to leave generally can. The males of some species, however [e.g., northern fur seal and the South American and Australian (Neophoca cinerea) sea lion], are able to prevent females from leaving their territories by threats, herding, and sometimes physical aggression leading to injury.
The importance of male courtship displays in otariids is not well understood. For example, it is not known whether male displays, such as the incessant barking of male California sea lions, are used as threat displays for males, or as displays for females, or both. Alternative male mating strategies are widespread and generally thought to be practiced by subadult or subordinate males. These include gang raids by groups of non-territorial males (up to 40) to abduct or mate with females in the main breeding territories (South American and Australian sea lions), males stealing females from the territories of their neighbors (kleptogyny; northern fur seals), and males trying to sneak copulations (several species). How successful these strategies are in inseminating females is not known.
Female mating strategies are less well understood than male mating strategies, but several lines of evidence suggest that females exercise more choice among males than previously suspected. Female otariids choose which territory to haul out in and usually, but not always, move freely in and out of a male’s territory. Estrus occurs within 1 to 2 weeks postpartum in all but one species (California sea lion, about 21-27 days). When it is time to mate, females may leave the male’s territory in which they have given birth and mate with another male. This has been documented through behavioral observations in California sea lions and South American fur seals and by paternity studies in fur seals on Macquarie Island.
Climate, rookery topography, and the intensity of male harassment influence the ability of females to exercise mate choice. Females of species breeding in hot climates have more opportunity for mate choice due to their frequent thermoregulatory movements between their birthing site and the water. Intense male herding restricts female choice and may injure females. Female northern fur seals are thought to successfully reduce the risk of injury from males by forming dense aggregations and competing for central locations within these groups, which minimizes contact with males, and by acting submissive around males. In this species, females do not appear to choose males directly. Rather, by gathering on traditional mating grounds, the result is that males fight and females subsequently mate with the winners of these contests. Female otariids may also directly solicit and initiate copulation from males. In Steller sea lions, for example, females gain the sexual attention of males by lateral neck swings, dragging of the hindquarters, and sinuous movements of the female’s body against the male’s body. While females tend to direct most solicitation behavior toward the older “proven” territorial males, the extent of female choice remains unclear. Multiple mating is known in 30% of otariids studied (Boness et al., 1993) and suggests an important and variable role across species for sperm competition and mate choice.
2. Phocids Most phocid females fast during a short and concentrated lactation period, utilizing energy stored as fat before parturition. Because phocid females are not dependent on concentrated marine production during lactation like otariid females, they can mate in more dispersed locations. Moreover, in 16 of the 19 phocid species, the majority of mating takes place in the water near or after the end of lactation. Females of many of these species give birth on ice and do not aggregate as densely as those of terrestrially breeding species. Thus, during the breeding season, females are dispersed spatially (solitary or dispersed in small- to moderate-sized well-spaced colonies) and mobile during mating. Because males have less opportunity to defend and mate with multiple females, aquat-ically mating phocids are less polygynous and sexually dimorphic than otariids.
Moreover, the breeding season is short and, in species that breed on ice, mating takes place when temperatures are well below freezing. Reverse sexual dimorphism, with females larger than males, occurs in several species. Large female size may help a mother provide greater quantities of fat-rich milk to her pup and protect her from low polar temperatures. Small size in males is diought to facilitate agility underwater, where males may defend territories and mate with females. Nevertheless, aquatically mating species are considered to be slightly or moderately polygynous. Mating takes place within a few days of the weaning of the pup.
We have limited knowledge of male and female mating strategies in most aquatically mating phocids. In some cases, males defend the lactating female and her vicinity directly, a strategy akin to roving and sequential defense of a single female or a small group of females. A typical group consists of a female and her pup and an adult male who may have to wait before the female comes into estrus and is receptive to mating. Presumably, the male will mate with the female when she enters the water after weaning her pup. This system can be described as sequential polygyny, as males may leave after mating to search for another receptive female. It occurs in crabeater seals (Lobodon carcinophaga), spotted seals (Phoca largha), and hooded seals. Other males may surround these “triads” and they may compete for access to the female, typically with threats and sometimes bloody fights.
In some cases, males appear to defend aquatic territories (called “maritories”) off the beach or ice where females reside. Males spend considerable time in these territories giving vocal and visual display. This characterizes such phocids as bearded (.Erignathus barbatus), harp, and Weddell (Leptonychotes toed-dellii) seals and, in some cases, harbor seals (Phoca vitulina). A male may mate with any receptive female that enters his territory. Genetic studies of harbor seals on Sable Island show that male success is moderate to low, with most males fertilizing one or no females and the maximum number of females fertilized for any male being five (Coltman et al, 1998).
Males of many aquatically mating species are thought to use visual and acoustic displays to threaten other males and to attract females. “Eerie but melodious” songs have been described for male bearded seals and “trills,” “knocks,” “buzzes,” and “chirps” for male Weddell seals. Male hooded seals make numerous sounds underwater and also produce sounds in air as they inflate and deflate their hood and red nasal sac. In ringed seals (Pusa hispida), there is much social interaction and sniffing between males and females (males have a strong odor during the breeding season), which raises the question of whether there may be preferred mates. Virtually nothing is known of female mating strategies among aquatically mating phocids.
The northern and southern (Mirounga leonina) elephant seal and some populations of the gray seal (Halichoems gry-pus) are unusual among phocids in that mating takes place on land. These species exhibit a form of female defense polygyny. Males maintain a position near a receptive female or females and attempt to exclude other males from their vicinity. In elephant seals, males use visual and acoustic threats as well as physical fighting to compete for dominance in a social hierarchy that confers access to females. Polygyny in elephant seals is extreme; at the Afio Nuevo rookery in California, as few as five males may be responsible for 48 to 92% of the copulations observed during a breeding season (Le Boeuf, 1974). The lifetime reproductive success of most males is nil or low. Many die before reaching breeding age and higher-ranking males prevent some of those that survive from breeding.
Genetic analyses confirm that the proportion of pups sired by alpha males is consistent with that expected from obsen’ed mating success in southern elephant seals but show that behavioral observations overestimate the success of some northern elephant seal alpha males (Hoelzel et al., 1999). The relatively lower success of northern elephant seal males was probably due in part to the behavior of the Afio Nuevo females, which copulate more frequently, the greater success of non-alpha males, and/or reduced fertility of specific alpha males. Female elephant seals may exercise mate choice by competing for central positions in harems where dominant males reside and by inciting male-male competition and subsequently mating with the winner of these battles.
Mating behavior among the geographically widespread gray seal is difficult to categorize. Gray seal females do not cluster as tightly and are more mobile in the colony than elephant seal females. Dominant males maintain their proximity to females by using visual threat displays and occasional fights to deter other males. In the Scottish Islands, behavioral observations suggest a classical polygynous system. Genetic studies, however, reveal that many fathers spend little time at shore, that some pairs of seals show partner fidelity, and that dominant males to not father as many offspring as behavioral observations would suggest (Worthington Wilmer et al, 2000).
3. Walrus Walruses (Odobenus rosmarus) have the most elaborate courtship displays of all pinnipeds. Walruses show marked sexual size dimorphism and are thought to be strongly polygynous. Atlantic walruses in the Canadian High Arctic exhibit a mating system that resembles female defense polygyny. Pacific walruses in the Bering Sea may have a lek-like mating system. Groups of males cluster around females, which form dense aggregations on pack ice. Males are aggressive toward one another and produce intricate visual and vocal displays, consisting of barks, whistles, growls, and underwater bell-like sounds. The massive tusks of the male walrus also appear to play an important role as a symbol of rank (to threaten other males) and as a visual signal to females, who may choose among males partly by the size of their tusks.
1. Odontocetes In contrast to pinnipeds, which are relatively sedentary and clustered during the breeding season, female odontocetes are mobile and dispersed. This has two important consequences for male mating strategies: males have less opportunity to control access to aggregated females and less assurance of paternity. It is not surprising, therefore, that the basic mating strategy of male odontocetes appears to be one of searching for receptive females and spending little time with them other than to mate. It is likely that mate guarding, or monopolization of females long enough to ensure conception, also occurs, although the phenomenon has been well documented only in bottlenose dolphins.
Female mating strategies in odontocetes are little understood. Given their mobility and three-dimensional habitat, it is generally thought that females are able to exercise choice by outmaneuvering males or by rolling belly-up. Observational and hormonal evidence suggests that females of several species copulate frequently both during and outside the breeding season and may be polyestrous. Frequent copulation may function to induce sperm competition, aid in assessing future mates, or help to establish social bonds with potential future partners, In many odontocete species, sexual behavior is an important component of nonreproductive social interactions and often has little to do with fertilization, making it difficult to infer mating strategy from incomplete observations. Perhaps the best example is the intriguing “wuzzling” behavior of Hawaiian spinner dolphins (Stenella longirostris longirostris). Wuzzling refers to interweaving masses of caressing and copulating dolphins of both sexes and all ages, which are especially common in the summer months, when manv females come in estrus. Is the behavior social? Sexual? Both?
We know little about mating systems in the vast majority’ of odontocetes. However, there are substantial data on bottlenose dolphins, sperm whales, and killer whales. A mating system of female defense or sequential defense polygyny has been suggested for Indian Ocean bottlenose dolphins. Males form stable coalitions of a few males that may work alone or with other closely associated coalitions to form temporary consortships with individual females, often through aggressive herding (Shark Bay, Australia; Connor et al, 1996). Male common bottlenose dolphin individuals and members of long-term pair bonds form temporary consortships with females without obvious aggressive herding (Sarasota Bay, Florida; Wells et al., 1999). The extent to which this sequential female defense strategy is successful is uncertain, however, as individual females can cycle multiply and associate with several males during the season in which they conceive. These behaviors may facilitate female mate choice and promote sperm competition. In another location, male common bottlenose dolphin apparently do not form alliances or aggressively herd females, although single males may accompany groups of females throughout the breeding season (Moray Firth, Scotland; Wells et al, 1999). Among sites, the level of male bonding may be inversely related to male body size and the degree of sexual dimorphism (Tolley et al, 1995). At sites where animals are small, males may form alliances to gain and maintain access to females; where animals are large, males can do this on their own.
Most sperm whales in the Galapagos Islands appear to rove between groups of females searching for potential mates. One or more large mates may attend a group of females (sometimes simultaneously) for short periods of time ranging from a few minutes to several hours (Fig. 4). Rather than herding female groups, females have been observed to alter course and speed so that they could join a large male hundreds of meters away. Males did not interact aggressively with each other within female groups, despite several accounts in the literature of males fighting outside of groups. Given the apparent roving strategy of males, the role of the tremendously large nose of the male sperm whale and its possible use as a sound-generating organ remains unclear. The loud clicks ma)’ function in male-male competition or advertisement to attract females (Cranford, 1999).
Pods of resident killer whales in the Pacific Northwest are frequently observed associating with one another in the summer months when prey (and observer) abundance is high. In these multipod groups, there is much sexual activity among all pod members, young and old alike. Because no dispersal of either males or females occurs from resident pods, it is thought that mating takes place during these encounters. Considering that the entire pod engages in these encounters, it is likely that their function is both sexual and social. Similarly, genetic analyses of long-finned pilot whales captured in a Faroese fishery indicate that males remain in their natal groups but do not mate within them (Amos et al, 1993). Young were sired by males not captured with the group, implying that pilot whales must mate when two or more groups meet or when adult males pay brief visits to other groups.
Figure 4 Adult male sperm whales (Physeter macrocephalus) rove among female groups searching for receptive individuals and staying with each group for only a few hours at a time.
Very little is known about mating systems in the remaining species of toothed whales. However, we can infer something about the mating strategies of these species from the type and degree of sexual dimorphism and its association with other characteristics, such as bodily scarring and relative testis size. For example, testis size ranges dramatically among odontocete species, from less than 0.05% [several Mesoplodon species, the franciscana (Pontoporia blainvillei), the baiji (Lipotes vexil-lifer), and sperm whale] to 5% or greater [harbor porpoise (Phocoena phocoena), finless porpoise (Neophocacna pho-caenoides), and dusky' dolphin (Lagenorhynchus obscurus)]. These data suggest the importance of sperm competition in several odontocete species, especially among some of the delphinids and poipoises. The importance of mate choice competition, attempts to entice and attract females through elaborate displays, is suggested by differences between the sexes in song and exaggerated visual signals such as the postanal hump or enlarged dorsal fins. At present, sexually dimorphic acoustic signals are known only in sperm whales. However, because odontocetes produce a wide range of sounds, acoustic displays are likely to occur in several other species as well. The importance of contest competition for access to mates is suggested by sexual dimorphism in size, weaponry (teeth and tusks), and the presence of scarring of conspecific origin (tooth rakes). Sperm whales, the beaked whales, narwhal, and bottlenose whale exhibit these traits.
2. Mysticetes Among the mysticetes, substantial data on breeding behavior exist only for the humpback, (Megaptera novaeangliae), right (Euhalaena spp.), and gray (Eschriehtius ro-bustus) whales. Even in these species, virtually nothing is known about female behavior. The humpback whale has been studied most intensively. Male humpbacks adopt one or more of three primary strategies: display by singing long, complex songs; direct competition with other males for females in “competitive groups”; and escort of females, including those with newborn calves. Males escorting females are most likely waiting for mating opportunities or guarding females after copulation. Two secondary strategies, roving and sneaking, have also been suggested. The relative importance and success of each of these strategies are unknown.
Female humpbacks sometimes aggressively reject subadult males and they may incite competition among males. Although molecular analysis of paternity has shown that females are mated by different males between years (Clapham and Palsb0ll, 1997), it is unknown whether females mate multiply within a given breeding season. Singing by male humpbacks is an intriguing phenomenon, as songs change over time, yet all members of a population sing essentially the same song at any one time. Singing by humpback males presumably acts to attract females, although whether songs contain cues to mate quality remains in dispute. Singing may also function to space males in a breeding area or to aid in the establishment of dominance hierarchies. Whether the aggregation and displaying of humpback whales at specific sites constitutes lekking also remains controversial.
Little else is known about the mating systems or other bal-aenopterid whales. Blue (Balaenoptera musculus) and fin (B. physalus) whales seem to be widely dispersed during the winter breeding season. Male fin whales have a patterned call, which has been termed a breeding display. The question of whether male blue whales have specific calls that may function as mating displays is currently under investigation.
Right whales show sexual activity throughout the year, although calving is strongly seasonal. Because the gestation period is 1 year and there is no evidence of diapause, mating leading to conception presumably occurs primarily in the winter. The function of sexual activity during other seasons is unknown. Observations of multiple male right whales mating with single females, together with the huge (1 ton!) testes, strongly suggest that sperm competition is a principal mating strategy in these species, and also probably in bowhead (Balaena mysticetus) and gray whales (Brownell and Ralls, 1986). The level of aggression in male-male interactions in these species is low compared to that observed in humpback whales, data consistent with the predominance of sperm competition as a mating strategy.
C. Sirenians, Sea Otters, and Polar Bears
Male manatees (Trichechus spp.) and dugongs (Dugong dugon) tend to be solitary and search for potential mates by roaming over large areas that include the home ranges of several females. Groups of males sometimes follow and try to mate with a single female, forming a “mating herd.” In both manatees and dugongs, males in these herds threaten and fight with each other but it is still unknown whether this behavior is a form of scramble competition or is more akin to a type of lekking. In Shark Bay. Australia, dugongs associate in a more classical kind of lekking, with several males patrolling exclusive areas and engaging in activities usually indicative of both male competition and mate attraction, including acoustic signaling. In both manatees and dugongs, the mating season extends over several months and sexual dimorphism is slight. Interbirth intervals are at least 2 years and may be as much as 5 in some cases.
Female sea otters (Enhydra lutris) typically give birth annually. Births generally peak in the spring, although females in warmer areas may give birth in any month. Adult males are larger than females. Male sea otters establish territories, usually overlapping one or more female home ranges, that contain food resources and sheltered resting places. Males may defend territories seasonally or all year. Other males congregate in groups outside of the areas occupied by territorial males. Courtship and mating, as are typical for many mustelid species, are rough and females may be injured or killed by males. Copulation occurs with both the male and the female on their backs near the water’s surface. The male grasps the female’s head or jaws, including the nose, in his own jaws. Recently mated female typically have red, swollen noses. After mating, the pair may stay together for a few days in which they feed, groom, play, and rest in close company.
Polar bears (Urstis maritimus) are highly sexually dimorphic and polygynous; adult males may be over twice as heavy as adult females. Female polar bears have extensive home ranges, and males travel over large areas when searching for mates. Males apparently fight among themselves for access to females. Specific courtship behaviors are lacking or are yet undescribed. The largest and strongest males apparently do most of the mating, while other males sometimes wait in the distance. Polar bears are notable among marine mammals in that they are the only species in which females give birth to multiple young (one or two is the most common litter size and rarely three or four). The interbirth interval is about 3 years.
IV. Mating System Studies and the Future
This is an exciting time for the study of marine mammal mating systems as technical advances such as the use of molecular markers and underwater acoustic and visual recording devices are providing new insights and making it possible to investigate previously inaccessible species. The results of molecular studies have confirmed some hypotheses regarding marine mammal mating systems and refuted others. Paternity analyses in a number of species are revealing that dominant males are not as successful in siring offspring as expected from behavioral observations alone. These results suggest that female choice plays a more important role than was previously suspected. Fe male mating strategies, such as the incitation of male-male competition, extraterritory copulation, and the promotion of sperm competition by frequent mating, are gaining increasing attention. As our understanding of the physiology of female receptivity grows, we will be better able to interpret both female and male mating behavior. At the same time, our increasing ability to hear and see underwater will enable us to tap into the little known realm of underwater acoustic and visual displays.