Bottlenose Dolphin

Male-Female Interaction

Male dolphins are dominant over females and form alliances in some areas to increase their access to females. Alliances are particularly common in Shark Bay, Australia, less so in Sarasota Bay, Florida, and very rare in Moray Firth, Scotland. Some researchers have hypothesized that this difference may be due to body size of the males or the distribution of the individuals. In Shark Bay, males and females are much closer in size than they are in Sarasota Bay; therefore, the larger males in Florida do not need the alliance to mate with a female. An individual is large enough that he can successfully guard and mate with a female by without help from another male(Reynolds, et al. 2000).

Sexual behavior is not limited to interactions between mature males and females in estrus. It has been exhibited as mounting of males on other males, females on females, juveniles on adults and females on males. Furthermore, males have been seen mounting both pregnant females and those with young calves, while juveniles mount each other in play (Connor, et al. 2000). Researchers are unsure about the significance of the mounting for non-reproductive purposes. Displays of dominance have been suggested, but does not correlate with less dominant individuals mounting more dominant individuals (such as an infant mounting its mother or a female mounting an adult male) (Mann and Smuts 1999).

Dolphins give birth to a single infant after a 12-month gestation period (Whitehead and Martin 2000). Interbirth intervals average between 3 to 6 years (Connor, et al. 2000). One reason for this extended period of dependence appears to be the time necessary for a dolphin to learn to catch its own prey and be able to sustain itself. Female dolphins ovulate spontaneously, with anywhere from 2 to 7 cycles occurring in one year (Conner, et al. 2000). The mother typically weans the infant around 10 months into her next pregnancy, which usually occurs 2 to 6 years after the infant's birth. In Shark Bay, Australia, males are weaned earlier than females, perhaps because of the necessity of males to form strong alliances. Also, the female may receive the advantage of additional nutrients provided through milk, which can help increase the survival rate of the female's first calf (Whitehead and Martin 2000). The mother-infant bond is the strongest among dolphins and continues throughout the lives of both individuals. Mother's provide all of the necessary care for their offspring. As the infant ages, it slowly gains independence (Gubbins, et al. 1999). Infant mortality is around 45%, probably due in large part to the increased vulnerability of the calves and their mothers (Connor, et al. 2000).

During the first week of an infant's life, the mother is very protective of it. Infants will follow anything that moves and other female dolphins may attempt to "bolt" with the infant by swimming rapidly by the mother-infant pair. The infant typically follows the moving dolphin. The mother reacts quickly and aggressively, retrieving the infant and occasionally threatening the other adult. However, after the first week, a female dolphin does not respond. She allows the other dolphin to spend time with the infant. Researchers hypothesize that the reason for this difference arises because dolphins spend the first week imprinting to their mother. During that initial time, the infant must learn its mother's song to be able to later identify her (Mann and Smuts 1998).

There are two main positions that an infant will have in relation to its mother. The first is "echelon" position, or swimming at the mother's side. The second is "infant" position, or swimming directly below the mother. As the dolphin ages, it spends less time in echelon position and more time in infant position. Gubbins, et al. (1999) hypothesize that initially, the mother's movement creates water pressure that can help carry the infant in echelon position. As the infant ages, it becomes heavier and cannot be carried in the same manner. Infant position then provides other possible benefits for the older infant. It enables better access to nurse from the mother. It may provide protection from predators who view the mother-dolphin pair from below-the white bellies of the two individuals blend together and make the pair appear to be one individual. Also, the position could reflect the more dominant position of the mother or a remnant of the natural position of the infant in the dolphin's land-dwelling ancestors . This position also appears to be the where the infant recovers when stressed or frightened. This position is unique to mother and infant pairs.

As calves age, they become more independent. Calves play together, imitating various adult behaviors, including male alliance formation and herding of females. Switching sex roles can also occur. Calves that spend more time apart from their mothers have an increased rate of survival than those that spend much of their time with their mothers, likely because increase independence indicates physical strength(Connor, et al. 2000).

Mothers may help each other in raising their young. Occurrences of "baby-sitting" and creating "playpens" (where the adults surround the infants in a protective circle) have been observed. Again, the group nature of dolphin society is important. Mothers and infants in groups have a better chance of survival than solitary individuals (Reynolds, et al. 2000).

Females without young may interact with infants to learn how to care for their own future offspring. Mann and Smuts (1998) study of mother-infant spatial relations revealed that the majority of dolphins that attempted to bolt with infants were inexperienced females (those who had not given birth or successfully raised a calf beyond 6 months). Mothers do not appear to receive any benefit from this time without their infants, as their time foraging does not significantly change. Infants may receive the benefit of learning to socialize with other dolphins. Besides inexperienced females, other dolphins who spent time alone with the infant tended to be females related to the mother.

Other dolphins may help the mother in other, less understood, ways. Offspring have been observed returning to their mothers when she gives birth (Booth 2002). These individuals possible help protect the mother during the birth, during which she may attract predators who smell the blood in the water. The offspring put themselves at risk, demonstrating kin selection. Also, the offspring may actually participate in the birth process. Stillborn dolphins that wash up on beaches have been observed to have teeth marks along their sides. These marks may suggest that the infant is removed from the mother with the help of another dolphin (Connor and Norris 1982). Such an assisted delivery has been observed by Pacific striped dolphins in captivity (Brown, et al. 1966 as cited in Connor and Norris 1982).

Researchers have documented evidence that bottlenose dolphins have purposely attacked and killed harbor porpoises (Phocoena phocoena). Studies conducted on stranded porpoises in Moray Firth, Scotland, reveal that many of the individual's bodies have teeth marks matching those of the bottlenose dolphin. Many of the animals had extensive internal injuries, which resulted in their deaths. None of the stranded animals had been used for food. All of the individuals measured between 100 and 140 cm long, indicating that they were juveniles. Furthermore, several aggressive interactions between bottlenose dolphins and harbor porpoises have been observed. In one case, a porpoise was seen fleeing from a group of dolphins. In two other instances, the porpoise did not escape. A group of dolphins (two in on case and three in the second) chased the porpoise "was pursued and often butted clear of the surface on the head of a dolphin." The porpoise eventually becomes tired and disappeared below the surface in both instances. Ross and White note "in all observations the porpoises appeared to be the victims and seemed to be making every effort to escape from the dolphins." (Ross and Wilson 1996).

Recently, evidence demonstrates that this behavior may be related to infanticidal behavior in dolphins. In the same area, researchers have found five stranded bottlenose dolphin calves that died from similar injuries. All were under a year old and no older stranded dolphins had the same types of injuries. Also, researchers observed a possible infanticidal event in the waters nearby. Two adult dolphins were interacting aggressively with a newly dead calf. One adult "repeatedly jumped and lunged at the surface [of the water] and was seen gripping the calf in its jaws, pushing it around underwater, and butting it clear of the surface with its head." These actions are very similar to the above description of the dolphin's interaction with the harbor porpoises. The other adult, identified as a female, stayed near to the calf, but did not touch it (Patterson, et al. 1998).

Although there is much to be learned about infanticidal behavior in dolphins, there are several possible explanations for this behavior. If females are practicing it, it could be to preserve resources for themselves and their offspring. Males could practice infanticide to mate with a female. Females do become "attracted" to males soon after losing a calf. Furthermore, this theory could help explain why females mate with multiple males during estrus. By mating with different males, the female confuses the paternity of her offspring. Males will injury themselves by killing an infant that they might have fathered (Patterson, et al. 1998).

BOTTLENOSE DOLPHIN (Tursiops truncatus) Mating System: Hierarchical Promiscuity
Bottlenose Dolphin Home	Dolphins have a hierarchical promiscuous mating system, meaning that there are no strong mating bonds between individuals and that mating is determined by dominance. In ensuring reproductive success, the limiting factor for males is access to females and they compete with each other to get this access. The limiting factor for females is access to resources, as they invest heavily in each offspring. The female is the sole parental care provider (Connor, et al. 2000).	Photo by Caterina Fortuna, courtesy of the Adriatic Dolphin Project.






	Male-Female Interaction Male dolphins are dominant over females and form alliances in some areas to increase their access to females. Alliances are particularly common in Shark Bay, Australia, less so in Sarasota Bay, Florida, and very rare in Moray Firth, Scotland. Some researchers have hypothesized that this difference may be due to body size of the males or the distribution of the individuals. In Shark Bay, males and females are much closer in size than they are in Sarasota Bay; therefore, the larger males in Florida do not need the alliance to mate with a female. An individual is large enough that he can successfully guard and mate with a female by without help from another male(Reynolds, et al. 2000). Sexual behavior is not limited to interactions between mature males and females in estrus. It has been exhibited as mounting of males on other males, females on females, juveniles on adults and females on males. Furthermore, males have been seen mounting both pregnant females and those with young calves, while juveniles mount each other in play (Connor, et al. 2000). Researchers are unsure about the significance of the mounting for non-reproductive purposes. Displays of dominance have been suggested, but does not correlate with less dominant individuals mounting more dominant individuals (such as an infant mounting its mother or a female mounting an adult male) (Mann and Smuts 1999). Mother-Infant Interactions Dolphins give birth to a single infant after a 12-month gestation period (Whitehead and Martin 2000). Interbirth intervals average between 3 to 6 years (Connor, et al. 2000). One reason for this extended period of dependence appears to be the time necessary for a dolphin to learn to catch its own prey and be able to sustain itself. Female dolphins ovulate spontaneously, with anywhere from 2 to 7 cycles occurring in one year (Conner, et al. 2000). The mother typically weans the infant around 10 months into her next pregnancy, which usually occurs 2 to 6 years after the infant's birth. In Shark Bay, Australia, males are weaned earlier than females, perhaps because of the necessity of males to form strong alliances. Also, the female may receive the advantage of additional nutrients provided through milk, which can help increase the survival rate of the female's first calf (Whitehead and Martin 2000). The mother-infant bond is the strongest among dolphins and continues throughout the lives of both individuals. Mother's provide all of the necessary care for their offspring. As the infant ages, it slowly gains independence (Gubbins, et al. 1999). Infant mortality is around 45%, probably due in large part to the increased vulnerability of the calves and their mothers (Connor, et al. 2000). During the first week of an infant's life, the mother is very protective of it. Infants will follow anything that moves and other female dolphins may attempt to "bolt" with the infant by swimming rapidly by the mother-infant pair. The infant typically follows the moving dolphin. The mother reacts quickly and aggressively, retrieving the infant and occasionally threatening the other adult. However, after the first week, a female dolphin does not respond. She allows the other dolphin to spend time with the infant. Researchers hypothesize that the reason for this difference arises because dolphins spend the first week imprinting to their mother. During that initial time, the infant must learn its mother's song to be able to later identify her (Mann and Smuts 1998). There are two main positions that an infant will have in relation to its mother. The first is "echelon" position, or swimming at the mother's side. The second is "infant" position, or swimming directly below the mother. As the dolphin ages, it spends less time in echelon position and more time in infant position. Gubbins, et al. (1999) hypothesize that initially, the mother's movement creates water pressure that can help carry the infant in echelon position. As the infant ages, it becomes heavier and cannot be carried in the same manner. Infant position then provides other possible benefits for the older infant. It enables better access to nurse from the mother. It may provide protection from predators who view the mother-dolphin pair from below-the white bellies of the two individuals blend together and make the pair appear to be one individual. Also, the position could reflect the more dominant position of the mother or a remnant of the natural position of the infant in the dolphin's land-dwelling ancestors . This position also appears to be the where the infant recovers when stressed or frightened. This position is unique to mother and infant pairs. As calves age, they become more independent. Calves play together, imitating various adult behaviors, including male alliance formation and herding of females. Switching sex roles can also occur. Calves that spend more time apart from their mothers have an increased rate of survival than those that spend much of their time with their mothers, likely because increase independence indicates physical strength(Connor, et al. 2000). Mothers may help each other in raising their young. Occurrences of "baby-sitting" and creating "playpens" (where the adults surround the infants in a protective circle) have been observed. Again, the group nature of dolphin society is important. Mothers and infants in groups have a better chance of survival than solitary individuals (Reynolds, et al. 2000). Females without young may interact with infants to learn how to care for their own future offspring. Mann and Smuts (1998) study of mother-infant spatial relations revealed that the majority of dolphins that attempted to bolt with infants were inexperienced females (those who had not given birth or successfully raised a calf beyond 6 months). Mothers do not appear to receive any benefit from this time without their infants, as their time foraging does not significantly change. Infants may receive the benefit of learning to socialize with other dolphins. Besides inexperienced females, other dolphins who spent time alone with the infant tended to be females related to the mother. Other dolphins may help the mother in other, less understood, ways. Offspring have been observed returning to their mothers when she gives birth (Booth 2002). These individuals possible help protect the mother during the birth, during which she may attract predators who smell the blood in the water. The offspring put themselves at risk, demonstrating kin selection. Also, the offspring may actually participate in the birth process. Stillborn dolphins that wash up on beaches have been observed to have teeth marks along their sides. These marks may suggest that the infant is removed from the mother with the help of another dolphin (Connor and Norris 1982). Such an assisted delivery has been observed by Pacific striped dolphins in captivity (Brown, et al. 1966 as cited in Connor and Norris 1982). Infanticide Researchers have documented evidence that bottlenose dolphins have purposely attacked and killed harbor porpoises (Phocoena phocoena). Studies conducted on stranded porpoises in Moray Firth, Scotland, reveal that many of the individual's bodies have teeth marks matching those of the bottlenose dolphin. Many of the animals had extensive internal injuries, which resulted in their deaths. None of the stranded animals had been used for food. All of the individuals measured between 100 and 140 cm long, indicating that they were juveniles. Furthermore, several aggressive interactions between bottlenose dolphins and harbor porpoises have been observed. In one case, a porpoise was seen fleeing from a group of dolphins. In two other instances, the porpoise did not escape. A group of dolphins (two in on case and three in the second) chased the porpoise "was pursued and often butted clear of the surface on the head of a dolphin." The porpoise eventually becomes tired and disappeared below the surface in both instances. Ross and White note "in all observations the porpoises appeared to be the victims and seemed to be making every effort to escape from the dolphins." (Ross and Wilson 1996). Recently, evidence demonstrates that this behavior may be related to infanticidal behavior in dolphins. In the same area, researchers have found five stranded bottlenose dolphin calves that died from similar injuries. All were under a year old and no older stranded dolphins had the same types of injuries. Also, researchers observed a possible infanticidal event in the waters nearby. Two adult dolphins were interacting aggressively with a newly dead calf. One adult "repeatedly jumped and lunged at the surface [of the water] and was seen gripping the calf in its jaws, pushing it around underwater, and butting it clear of the surface with its head." These actions are very similar to the above description of the dolphin's interaction with the harbor porpoises. The other adult, identified as a female, stayed near to the calf, but did not touch it (Patterson, et al. 1998). Although there is much to be learned about infanticidal behavior in dolphins, there are several possible explanations for this behavior. If females are practicing it, it could be to preserve resources for themselves and their offspring. Males could practice infanticide to mate with a female. Females do become "attracted" to males soon after losing a calf. Furthermore, this theory could help explain why females mate with multiple males during estrus. By mating with different males, the female confuses the paternity of her offspring. Males will injury themselves by killing an infant that they might have fathered (Patterson, et al. 1998).





This webpage was completed in partial fulfillment of the requirements for Biology 323, Animal Behavior, at Davidson College in the Spring Semester of 2002.