Bottlenose
Dolphins
(Tursiops truncatus)
Breeding seasons
Bottlenose dolphins exhibit a promiscuous mating system, which is loosely hierarchical (Connor et al., 1996). This system most likely evolved because of the fluidity of dolphin schools, and the few interactions between males and females. Promiscuity confers advantages on both sexes, which will be discussed later. Female Tursiops in the wild reach sexual maturity between 10-12 years, males between 10-13. Females are seasonally polyoestrous, exhibiting oestrogen (followed by progesterone) peaks 2 to 7 times a year when a female is not pregnant or does not have a calf to rear (Moore and Ridgway, 1996). Oestrus cycles are 30 days. Birthing generally occurs in spring or early summer when waters are at their warmest. However, there is a birthing peak in the autumn as well, which may occur due to summer offspring losses. When females lose an infant they usually rejoin a breeding cohort within 2 weeks of the loss, and conceive again quickly.
Males form stable alliances of 2-3 individuals to aggressively herd females, especially when the latter are in oestrous. These alliances form temporary consortships with individual females from a breeding cohort. The consortships can last anywhere from under an hour to 4 weeks, with an average of 7 days (Connor et al., 1996). Consortships are maintained by the aggressive behaviour of the males in an alliance. Males will display threat behaviours towards a female, such as popping sounds, an open mouth, and jaw clapping. Other aggressive behaviours include herd jerking, charging, and tail-slapping a female. Several alliances may join together to fight off other male alliances or steal females from other males. Aggressive herding increases when more males are present in any one area, and when food is so abundant that males do not need to devote much energy to hunting. Connor et al. (1996) found that females in an Indian Ocean population mated with upto 13 males in one season, suggesting that there is much sperm competition. Seasonal peaks in male testosterone level, sperm concentration, and a high testis to body mass ratio also suggest sperm competition.
Photo credit: Andy
This is an example of the open-mouth aggressive display in Tursiops truncatus.
Hierarchical Promiscuity
This promiscuous system is loosely hierarchical because older, larger dolphins are dominant. These males have the highest success in keeping females from others and in mating with her, because their experience (age) and size give them increased resource holding power and increase their likelihood of winning in an agonistic encounter.
Non-reproductive mating
Mating often occurs in playful, non-reproductive contexts in bottlenose dolphins. For example, males may seek copulation with pregnant females, other males, non-reproductive juveniles, or mothers with young calves. Mother-calf sexual behaviour also occurs, as does mutual or self-stimulation of genitals by both sexes. Mating in the non-reproductive situations occurs in order to maintain strong social bonds within schools of dolphins. The importance of social bonds cannot be underestimated.
While coastal bottlenose dolphin schools can be relatively small (2 to 20 individuals, mean of 5), they are very fluid, flexible and temporary (Connor et al., 1996). Therefore, there is likely to be little genetic relatedness between all members of a pod at any one time. In order to facilitate cooperative hunting, and to protect offspring from infanticide (by making paternity uncertain), much sexual activity occurs outside of oestrous. Such behaviour establishes and reinforces strong social bonds in pods. Playful sexual behaviour may occur between juveniles and adult females to teach offspring the importance of social bonding at an early age. Such behaviour may also facilitate "babysitting" or allomaternal care of young by other females in a pod who may not be related. Finally, sex allows juveniles to form strong bonds at a young age, which may even last into adulthood when males form their cohorts.
Captivity
In captivity the entire mating system is shortened: dolphins begin cycling at a younger age (5-7), they nurse for less time (18-20 months), and interbirth intervals are shorter (Connor et al, 1996; Gubbins et al., 1999). In captivity, nutritional levels are high, and there is no competition for food, nor any need to expend energy hunting. Thus, more energy is devoted to reproduction and to rearing offspring.
Reasons for Promiscuity
Promiscuity in dolphins is advantageous for both sexes. Males are able to mate with as many females as possible, correlating with the low cost of producing sperm. By mating with many males, females make the paternity of their offspring unclear, thereby decreasing the chances of infanticide by males. The occurrence of infanticide is hard to detect and has never been proven, but it is a theory to explain why a promiscuous mating system evolved over others (24/81 webspirs) Promiscuity also increases a female’s opportunity to mate with the fittest male, so that she can produce the fittest offspring. Males are not needed in parenting, or gathering food since females help each other and food is abundant enough for a female and her offspring. Nor is the male needed to defend any kind of territory. Finally promiscuity reinforces the social bonds that are so important for cooperative behaviour in bottlenose dolphins.