The heterogeneous phenomenon considered as intraspecific aggressive or agonistic behavior represents a conglomerate of social responses, including male disputes over territorial boundaries, female fights to protect an offspring, female harassment and forced copulations, and infant abuse and killing. Agonistic encounters:
1. Mediate competition for limited resources economically defendable and valuable to the fitness of an individual (Bartholomew, 1970). Finite resources that can be monopolized would lead to social conflict between individuals of different sexes and generations and of the same sex and similar age class and status. Most often, agonistic confrontation (at least the most conspicuous interactions) involves sexually mature males.
2. Are more common in some social contexts, such as breeding on land in a polygynous mating systems, in which competition for resources is typically solved via aggressive disputes. Size and strength (but also agility) correlate positively with winning a contest through exerting dominance over individuals subdued by the costs of rebellion.
3. Have a broad range of costs for actors and recipients, from simple rejection after a ritualized threat display to injury or even death after an overt physical encounter.
The form and frequency of agonistic behavior partially reflect the sophistication of a social system. Aquatic mammals vary widely in the complexity of their societies, thus in the manifestation of agonistic behaviors. The most openly competitive societies characterize the otariids, the walrus (Odobenus rosmanis), and pho-cids that live in crowded conditions (e.g., elephant seals, Mirounga spp., and gray seals. Halichoerus grypiis), a fertile ground for aggressive social interactions (Riedman, 1990). Conversely, polar bears (Ursus maritimus), all the mysticetes and river dolphins, and some other phocids (e.g., Ross and leopard seals, Om-niatophoca rosii and Hydrurga leptonyx) generally occur in smaller social groups, except for periods during reproduction in which breeding males engage in scramble competition over receptive females (Berta and Sumich, 1999). The most complex social systems in the aquatic mammals would characterize some of the odontocete cetaceans, such as killer whales, Orcinus orca, pilot whales, Globicephala spp., bottlenose dolphins, Tursiops spp., or sperm whales, Physeter niacrocephalus (Connor et al, 1998). These species live in stable social units and show coordinated, cooperative behaviors. The long-term shared history among individuals of the group would have ritualized many of the overt aggressive responses typical of the polygynus pinnipeds.
I. Male-Male Competition for Mates
Competition over limited resources to maximize reproductive success would be the most common origin of agonistic encounters. It is likely that in all aquatic mammals, males compete for access to reproductive females, by either direct or indirect monopolization, through achieving the best place for reproduction or the highest status in a dominance rank. Defense systems can set the stage for the evolution of sexually selected traits, such as dimorphism in size and in special morphological structures (e.g., tusks, manes, elongated snouts).
The behavioral manifestations of conflict directed to the intimidation of rivals is often referred to as agonistic display or agonistic social signaling. Behavioral displays include vocal signals, facial expressions, and stereotyped postures and movements. such as static open-mouth threats, open-mouth sparring. foreflipper raise or waving, and oblique staring. Overt fighting is commonly a last-resort solution to conflict.
The form of male agonistic encounters and their outcome lias been described in detail for several pinnipeds. Within the highly polygynous otariids and phocids there are examples of resource-defense (territorial) and female-defense polygynous systems (Riedman, 1990). Both types of polygyny may occur in the same species, such as in the South American sea lion, Otaria flavescens, as a function of different ecological conditions (Cam-pagna and Le Boeuf, 1988).
The establishment and defense of a territorv involve vocal displays, stereotyped postures and movements, and fights. During territorial displays, male contenders may rush toward each other with the mouth open or vocalizing, weave the head from side to side, puff out the chest, or perform the “oblique stare” posture at one another, but physical contact is usually avoided. Much of the fighting between otariid males takes place early in the breeding season, when territorial boundaries are being established. When physical contact occurs, it typically lasts a lew seconds but may be violent, particularly in the largest sea lions. Fights involve chest-to-chest pushing, vigorous biting of the neck and face, lunging, and slashing at the opponent’s flippers, chest, and hindquarters.
In female-defense polygyny, females cannot be sequestered or attracted to a particular place. Males then compete to achieve a position among the females in the breeding colony and move with the shifting population of females [see Boness and James (1979) for gray seals and Cainpagna and Le Boeuf (1988) for South American sea lions]. Association to a particular group of females is loose and may change even during the same day due to female redistribution related to the physical environment (high temperatures, variable space due to tidal movements) or to social behaviors (e.g., group raids of ousted males into the colony; Campagna et al, 1988).
In phocids such as elephant seals, males aggressively establish a dominance hierarchy rather than a resource or female defense system. Only the highest ranking individuals have undisturbed reproductive access to females (Le Boeuf and Laws, 1994). During the establishment of hierarchies, males attempt to intimidate each other with vocal displays. If none of the contestants retreats, then a chest-to-chest fight takes place. Fights in elephant seals are violent confrontations and may last half an hour. Each bull throws his weight against the other and slashes at his opponent’s face, neck, and back with long canines. Most fights end with multiple lacerations and bloody wounds or even a broken canine tooth; even death may occur on rare occasions.
Vocal threats are a common component of agonistic encounters. Pinnipeds vocalize both in air and underwater (Riedman, 1990). Harp (Pagophilus groenlandieus), ringed (Ptisa hispida). Weddell (Leptonijchotes weddellii), and bearded (Eiig-nathus barbatus) seals and the walrus have a rich underwater vocal repertoire. Males maintain underwater territories and vocalizations seem to be part of territorial displays. Vocal displays can be of a repetitive nature and are then termed songs. Otariid males, particularly among the fur seals (Arctocephalus spp.), have a rich variety of airborne threat vocalizations associated with boundary display postures. The California sea lion, Zalo-phns californianus, vocalizes both in air and underwater (several phocids also produce airborne and underwater sounds).
The strong airborne calls or barks of Zalophus occur during breeding and nonbreeding seasons and may serve to advertise dominance. In elephant seals, airborne threat displays consist of loud and directional pulsed sounds that tend to precede fights.
There is comparatively little description of agonistic encounters in the rest of the aquatic mammals. Agonistic behaviors to establish dominance relationships were described among dolphins in captivity. Observations of free-ranging cetaceans described a range of behaviors interpreted as agonistic, such as lobtailing, tail and flipper slaps to the body of other individuals, open-mouth postures, jaw claps, forceful exhalations, chases, body charges and leaps and body slaps, and vocal threat displays (Wells et al, 1999). Escalated agonistic displays involve striking with flukes, biting, and jousting with tusks, the latter in narwhals (Monodon monoceros). Humpback whale (Megaptcra novneangliae) males fight vigorously in surface-active groups and receive not only scrapes and scratches but also deep gouges and bloodv wounds as a result (Tyack and Whitehead, 1983).
The scar pattern of some odontocetes has been interpreted as the consequence of tooth marks and violent interactions. Several odontocetes have conspicuous scars. In Risso’s dolphins, Grampus griseus, narwhals and several of the beaked whales, most of the body is covered with scars. At least for the narwhal, scars have been associated with intraspecific agonistic encounters (see later). Scrape marks are also common in baleen whales. It has been suggested for the southern right whales, Eubnlaena australis, that males may use the thorny callosities during scramble competition over females.
Agonistic contests in cetaceans also involve vocal displays. Males of the humpback whale escort receptive females and vigorously rebuff other males bv threatening displays such as thrashing of the flukes. The underwater songs of humpback whales in Hawaiian breeding grounds are performed by males and likely serve as communication signals in the context of male competition.
An example of male-male competition involves Australian bottlenose dolphins, Tursiops aduncus (Connor et al, 1998). Males of this population form stable alliances of two or more individuals that cooperate to obtain and control reproductive females. Two alliances occasionally combine efforts to sequester or defend females from another alliance. Alliances in dolphins and group raids in sea lions (see later) represent special cases in which competition involves the participation of several individuals simultaneously.
C. Other Aquatic Mammals
Sea otters (Enhtjdra lutris), polar bears, and sirenians tend to be more solitary or live in low-density societies with little interaction among individuals (Berta and Sumich, 1999). Male sea otters are polygynous, establish breeding territories, and mate in the water. Females live in low-density areas chosen in relation to the distribution and abundance of food.
During the mating season, polar bear males rove to locate receptive females that are dispersed and solitary. Males access one female at a time. Competition involving physical interactions has been observed rarely but is indicated by broken teeth and scarring on the head and neck.
Manatees (Trichechus spp.) form mating groups in which several males compete for access to a receptive female by pushing and shoving each other. Physical competition for females also occurs in dugongs (Dugong dugon) with some males obtaining scars probably made by the tusks of other males.
II. Tusks as Special Structures for Aggression?
Two species of marine mammals have extraordinarily developed tusks: the walrus and the narwhal. The two upper canines in both male and female walrus are extraordinarily elongated (Riedman, 1990). The massive tusks of a male can weigh up to 10 pounds and be almost 1 m long. Both sexes use tusks in squabbles. to threaten one another, and. perhaps, to establish dominance. Males may force their way to selected places in crowded colonies by pushing and jabbing other walruses with their tusks.
The tusks of narwhals are even more exceptional morphological traits. As a general rule, the left canine in males extends anteriorly into a spiraled tusk to a length that may exceed 2.5 m. Some males have two tusks and a few females also develop one or even two shorter and less robust tusks. It has been suggested that narwhal tusks may be used to disturb or pierce prey, to open breathing holes in the ice, as defense weapons against predators, or as organs of sexual display. Although tusks may be used in more than one context, evidence shows that they serve in aggressive encounters (Silverman and Dunbar, 1980; Gerson and Hickie, 1985). Evidence includes direct observations of males crossing tusks and striking them against one another, scar patterns (with adult males having more and larger scars on the head after attaining sexual maturity), significantly higher incidences of broken tusks in mature males compared to immature individuals or females, and imbedded splinters and tusk tips found in the head of males. Tusks are also used to spear individuals of other species or, apparently, at times even female narwhals.
III. Sexual Selection and Special Morphological Traits
Pronounced sexual dimorphism in the direction of males being heavier and larger than females is common in all otariids, the walrus, and some phocids (e.g., elephant and gray seals). This kind of dimorphism often indicates direct physical confrontation among reproductive males involving pushing or strength contests. Dimorphism is not apparent, is slight, or is even reversed in most other phocids. A lack of or even reversed dimorphism is often accompanied by the defense of aquatic territories, aquatic mating, and serial monogamy. Females in these species are usually dispersed and breeding occurs over a protracted period. Social and ecological conditions do not favor frequent direct physical confrontations, but competition does occur, and may for more agile rather than larger individuals.
Among other aquatic mammals, males are much larger than females in some odontocetes, such as killer and sperm whales, whereas dimorphism is reversed in all the mysticetes. Mysticetes may have promiscuous mating systems in which competition for insemination takes place at the level of males displacing each other from the vicinity of a female and of spenn cells displacing or diluting sperm of other males. Gray (Eschrichtius robustus), right (Eubalaena spp.), and bowhead (Balaena mysticetus) whales have larger testes than expected based on their body weight, suggesting selection for sperm competition.
In addition to dimorphism in body size, males of some species evolved special secondary sexual features that may function in the context of competition for mates. Examples include the enlarged snouts of male elephant seals and gray seals and the inflatable nasal cavity of hooded seals (Cystophora cristata). Hooded seal males can blow a red, balloon-like sac from one nostril that is similar in shape to the long proboscis of elephant seals. These organs have visual or acoustic effects and may allow other males and females to judge the quality of a contender or a sexual partner. The developed neck and mane of sea lions with long and thick guard hairs also has visual effects and serves as a shield that protects internal organs from bites.
IV. Avoiding Fights
Competition for resources by direct aggression is a costly experience in species capable of inflicting serious injuries that could lower future fitness of the contestants. Thus, contenders with low chances of success should avoid physical confrontations. Theory predicts that the assessment of the fighting ability of competitors and of resource value prior to an escalation of violence may allow differential adaptive responses on the basis of the perceived asymmetries (Maynard Smith and Parker, 1974). Once a territory or social hierarchy is established, disputes tend to be asymmetric contests in which territory owners or high-ranking males almost always win. Threat displays may then serve as indicators of a quality and motivational state of a contender. Individual variation in vocal displays may help territorial males to recognize one another and to forgo direct competition if each knows its respective status.
In female-defense systems, the proportion of sexually receptive females accessible to a male is variable in space and time. Thus, the level of asymmetry can vary within the same day of a breeding season. This social context would favor behaviors that are unusual in strict territorial or hierarchical systems, such as group raids in South American sea lions.
V. Group Raids and Other Forms of Male Harassment of Reproductive Females
In the South American sea lion, losers in male-male competitions at times raid breeding colonies in groups of dozens of individuals (Campagna et al, 1988). Raiders abduct females from the harems of established males and attempt to mate with them. A male seizes a female in his jaws and hurls her into the air to a spot where he can hold his ground against other males while aggressively keeping her in place. In the process, females are often wounded and can be killed. Perhaps group raids represent a primitive stage of a male alliance or coalition.
Violent behavior toward females is relatively common in pinnipeds. Harassed females are injured and sometimes killed by males during mating attempts. Le Boeuf and Mesnick (1990) suggested some social conditions that can increase mortality risks to a female during mating: (a) marked male sexual dimorphism, (b) males outnumbering females, (c) use of force or potentially dangerous weapons in mating, and (d) monopolization of mating by a few individuals through direct or indirect control of resources (space, females, food, etc.) with forcible exclusion of the majority of the competitors. All of these traits are common in the most polygynous mating systems.
The majority of female deaths during the breeding season of elephant seals, the most sexually dimorphic of all the pinnipeds, occurs by traumatic injuries inflicted by males during mating attempts as the females depart the harems for the sea at the end of lactation. Male South American sea lions and elephant seals are three to five times heavier than females, have large canines, and often bite the neck of the female when copulating. Breeding colonies early and late in the season have a high number of males that intercept departing females and attempt to mate with them. Mating injuries inflicted by males to females have also been reported for several other species [e.g., gray seals, Boness et al. (1995), Hawaiian monk seals (Monachus schauinslandi), Hiruki et al. (1993)]. Male aggression toward females may be a selective force in shaping female behavior, female choice, maternal performance, and reproductive synchrony (Boness et al, 1995).
VI. Female Agonistic Behavior
In polygynous pinnipeds, females are aggressive toward one another and rarely tolerate neighbors close by, which helps to regulate density of a site. A common context of female agonistic encounters is that of protection of a pup in a crowded breeding colony Alien pups are often bitten by females. Aggressive mothers react rapidly and intensively to the threat to their pup by a neighbor, which enhances chances of pup survival by decreasing the risks of mother-pup separation and pup injury (Christenson and Le Boeuf, 1978).
At times, females threaten transient males when the latter approach or protest vocally when males mount them. As a result, a harassing male will then be more likely challenged by another male who hears the female vocalizing. These challenges generally interrupt a male’s approach or mount, and hence a potential copulation. By resisting male copulatory attempts, females increase their likelihood of mating with a dominant individual, which may be viewed as an indirect form of mate choice.
VII. Abuse and Killing of Young
Infanticide is the killing by conspecifics of young still dependent on their mothers. Infant abuse implies injury of a young either via active violent behaviors or via passive neglect. Violent abuse of pups by males (most often young individuals but also adults) occurs in several pinniped species, particularly in sea lions and elephant seals. The killing of young is most often the by-product of abuse, although it may also occur as a directed behavior. In addition to pinnipeds, infanticide has been described in polar bears and is inferred in at least one odonto-cete, the common bottlenose dolphin.