Results and figures taken from: Mann, J., Connor, R. C., Barre, L.M. and Heithaus, M.R. 2000. Female reproductive success in wild bottlenose dolphins (Tursiops sp.): Life history, habitat, provisioning, and group size effects. Behavioral Ecology, 11:210-219.

long term study has revealed:

* The months of October - December are the peak birth months, occuring just before the peak in surface water temperature. (Figure 1)

* Females give birth to their first calf at ages 12-15. However, a few females, such as a dolphin named 'Peglet', have given birth to their first calves at the age of 11.

* Interbirth intervals for surviving calves range between 3- 6.2 years. Interbirth intervals of 4 years were most typical.(Figure 5).

* One third of all females studied have not calved successfully in a 10 year period. Reproductive success was defined as the number of calves surviving to 3 years old within a ten year period.(Figure 6).

Figure 1: This figure shows that the peak birthing period occurs just before the peak in temperature. Number of births and average temperature by month (SD ranges for temperature= 0.9-1.6). If a birth could not be assigned to one month, it was divided equally among the possible birth months. (N = 74 calves, 47 mothers).Return to Top of Female Reproduction

Figure 5: Interbirth intervals when the first calf survived to weaning (N=33). The graph demonstrates that average birth interval is 4.55 years (SD=1.00, median= 4.07). Shaded bar represents a possible gap if she gave birth >1 year after weaning for first calf, then she could have lost a fetus in the interum. Return to Top of Female Reproduction

Figure 6: Graph demonstrates 30% of the females had no surviving calves, 37.5% had one surviving calf, 25% had two surviving calves, and 7.5% had three (N=40). Return to Top of Female Reproduction

CALF DEVELOPMENT

* When calves are born, they will have lumps or folds in their skin, referred to as fetal folds. These folds last for approximately one week. Fetal lines, white lines resulting from the folds, develop and will remain until approximately the third month.

* The majority of calves studied are weaned before their fourth birthday.

* Fourty-four percent of calves do not survive to age three. Mortality rate within the first year is the highest and declining thereafter. Mortality rates for juveniles older than three are not yet known.

Click Here to learn how calf mortality of the provisioned dolphins has declined.

* Mortality of calves born to provisioned females is higher than mortality of calves born to non-provisioned females. The study revealed that first year mortality was 56% for calves born to provisioned females compared to 24% for calves born to non-provisioned females. However, calf mortality for the provisioned dolphins has declined since feeding practices at the Monkey Mia Beach have changed.

* By 6 months, calves catch their own fish (mothers do not share food with calves). Calves seem to learn their foraging techniques from their mothers.

* Mothers and calves use distinctive whistles to mediate reunions after separations of up to several hundred meters. Vincent Janik and Janet Mann are currently trying to determine how and why mothers and calves sometimes reunite without whistling. The context (activity) and distance between mother and calf are probably important factors.

CLICK HERE FOR A RESEARCHER'S STORY ABOUT BRAND NEW CALVES

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Since 1987 I have focused my research on male alliances. In Shark Bay, alliances of 2-3 males cooperate to herd individual females for periods of up to over a month. Some alliance partners are nearly always found together and their bonds may last for years.

I have been following one such pair of males, Real Notch and Hii, for fifteen years (Hii is pictured with close associate Bottomhook on right). We think that by cooperating in an alliance, the males can prevent their female consort from mating with other males and other males from mating with her.

Unfortunately for the males, females appear to have countered this male strategy with one of their own: they come into estrus several times the year they conceive making it very hard for any group of males to insure that one of their members fathers her offspring.

Incredibly, we found that teams of alliances cooperate to attack other alliances to take their females. Thus, there are two levels of male alliances in our dolphin society, the alliances that herd females and teams of alliances that steal them.

This was a very exciting discovery!, because such nested levels of male alliances are common in human society but very rare even in non-human primates. The key is that both levels of alliance occur in the dolphin society, not between two different societies.

Many birds and mammals form groups to defend their turf but these are all 'us against them' interactions--nothing complicated about that. But in social groups, dolphin alliances can take on the dimensions of a soap opera, with individuals using friendly behavior to compete for favored allies in a strategic fashion to enhance their social position. Such alliances within groups are common in primates but rare elsewhere--and dolphin males have two levels of such alliances!

But the story does not end there. In the 1990s we documented a 'super-alliance' of 14 males that, not surprisingly, handily defeats other alliances in their area. Members of the super-alliance still get together in groups of 3 for the purpose of herding a female, but to our surprise, we found that after a trio of males finished herding one female, the males often joined a different trio to herd another one (but only with other members of the super-alliance). This came as quite a surprise after years of watching the stable alliances that always stuck together.

We suspect that the males have to cooperate with a larger number of super-alliance members to maintain a degree of cohesion in the group. My future research will focus on finding other large alliances to see if alliance stability correlates with group size and on discovering the ecological and genetic bases for alliance formation.

* Our observations of fish caught are biased towards large fish because they carry these long enough for identification. It can take as long as one hour to break up a large fish. Examination of stomach contents from Indian Ocean bottlenose dolphins caught in shark nets off Natal, South Africa found about 48 species of fish and cephalopods.

CLICK HERE FOR THE AMAZING STORY OF WEDGES

* A number of foraging techniques have been described for bottlenose dolphins, both in Shark Bay and elsewhere. They use echolocation or sonar to track fish, but vision may be quite important too. Dolphins might also listen to sounds that fish produce, and use these to locate their prey.

* Many dolphins develop individual foraging strategies. Sponge-carrying is most famous. About two dozen animals wear sponges on their rostra (beak) and appear to use the sponge to ferret prey from the sea floor. This is generally a solitary activity, but sometimes more than one sponger will be tens of meters apart.

CLICK HERE TO LEARN ABOUT 3 GENERATIONS OF SPONGERS!


* Spongers generally stick to sponging and don't use other foraging techniques, but there are some exceptions. This appears to be a "tradition" of sorts. The daughters of sponge-carriers clearly grow up to be sponge-carriers, but we aren't sure about the sons. New research from the genetics team at Univ. of New South Wales will shed light on the relatedness of spongers.

* Shark Bay dolphins also chase fish "belly-up" near the surface, a behavior we call "snacking." Calves engage in this type of foraging most often. At Point Peron, northwest from our main study area, a small group of dolphins appear to strand-feed, trapping fish in very shallow water. This behavior can be viewed on the National Geographic film, Dolphins: The Wild Side.

* In the shallow seagrass beds out east, dolphins will arch their tails high before driving them forcefully into the water, creating a several meter splash and a resounding 'kerplunk' sound. We think dolphins learn the location of fish hiding in the seagrass when they are startled by 'Kerplunks.'

* One might also argue that the provisioned females in Monkey Mia, Shark Bay, have developed a unique foraging strategy of begging for fish from boats and tourists. Such "traditions" have continued across at least three generations. This may be true for other foraging tactics as well.

* At other sites, dolphins can be seen corkscrewing into the sand after fish (Bahamas), strand-feeding on mud-banks in Portugal, Georgia and South Carolina, or stunning or killing fish with a tail-hit. Found worldwide in warm coastal waters, bottlenose dolphins have also learned to take advantage of human activity. For example, bottlenose dolphins have learned to feed on fish drawn to garbage barges, follow shrimp trawlers as they stir up the bottom, or steal bait from lines or crab pots. In Laguna, Brazil, fishermen and dolphins appear to cooperatively net mullet, with the dolphins herding the fish into the nets and feeding easily off the remains. Historical accounts of Australian aboriginal cooperative fishing with dolphins have also been reported.

* The tiger shark has been the subject of intensive study by Mike Heithaus (Florida International University) and Larry Dill (Simon Fraser University). They have found, using the "crittercam" developed by Greg Marshall at National Geographic, that tiger sharks hunt in deep and shallow habitats, but most of their prey is in shallow water (4-5m).

* Studies of their stomach contents by Australian scientist Colin Simpfendorfer, indicate that seasnakes, turtles, and dugongs are the main prey of tiger sharks, but dolphins have been found in tiger shark stomachs at other sites. Many Shark Bay dolphins bear shark bite scars of varying size, but this does not mean that sharks are the main cause of dolphin mortality.


* Researchers commonly see sharks and dolphins in the same area, and the reactions of sharks to dolphins and dolphins to sharks seems to depend on the size and number of sharks, the size and number of dolphins, and probably some element of surprise. We know that sharks sometimes eat dolphins but dolphins occasionally turn the tables. Researchers have seen sharks chase dolphins and dolphins chase or even mob sharks. Dolphin mothers sometimes chase small (1m) sharks from their young calves.

Another myth is that when dangerous sharks are around, dolphins are always aware of their presence. We observed a juvenile great white shark, some 2.5 meters long, swim lazily right into a group of resting mothers and older calves, who quickly dove, came up leaping, and didn't cease their flight until they had covered 3 kilometers 10 minutes later. Clearly they had no idea who or what was approaching them. On other occasions dolphins and sharks appear to ignore each other. Clearly the relationship between sharks and dolphins is complex and deserving of further study.