Presented by Deron Bauer, Matt Vanasse, and Scott Titus

The Bottlenose dolphin (Tursiops truncatus) is a marine mammal which can be found world-wide. They mainly inhabit temperate and tropical waters to approximately forty-five degrees latitude in both hemispheres (Martin 1977). These animals can range in size from eight to ten feet in length and weigh from 400 to 600 pounds.

Bottlenose dolphins will reach sexual maturity at about five to seven years for females and ten to twelve years for males (Sergent 1973). Females, however will not typically begin breeding until about ten to twelve years of age. Dolphins have a spontaneous ovulation cycle (two to seven per year) with cycle length being about thirty days. Estrogen levels will generally increase for five to seven days during ovulation. Female dolphins are seasonally polyestrus. Male dolphins on the other hand will have seasonal peaks in testosterone levels and sperm concentration (Schroeder 1989). Delphinids will produce a single offspring every four to five years with a twelve month gestation period. The offspring will continue to nurse to about four years of age (Martin 1977). these animals have a single or bimodal birth season with births occurring in the early summer and fall.

Bottlenose dolphins usually travel in small groups of about two to twenty individuals. These groups are known to join and split often. This is known as the fission-fusion pattern of association (Smalker 1992). Dolphins are considered to be in association with each other when two dolphins are within ten meters of each other(10 meter chain rule). The proportion of time any two individuals spend together is estimated using the "half-weight" coefficient of association (Smalker 1992). COA yields numbers from zero for individuals who never co-occur in the same group, to 100, for two individuals that are always found together. Males typically share high COA’s (seventy-five to 100) with one or two other males. Male pairs and trios have COA’s of about twenty-five to sixty-five with members of one or two other pairs or trios. Female-Female associations have COA’s of about thirty-five to seventy-five, and female -male associations have long term COA’s of less then twenty, but can vary with reproductive states (Smoller 1992). In this study, we will focus on the association between individual females and male associates.

This study had two general components. One was the systematic and ad libitum observations on three males and three females with offspring were provisioned in Shark Bay, Australia. Focal individual follows were also made on twenty-one primary study males who were encountered within the 130 km² study area. Data was also collected on the composition of groups encountered during these surveys. Observations of interesting behaviors were also recorded. Individuals were identified by dorsal fin scar patterns and dorsal fin shape. Males were sexed by observation of an erection, by the lack of mammary slits, and gap between the genital and anal slits (>2.5 cm). Females were sexed by observation of mammary slits or the presence of a dependent calf. finally the dolphins were divided into age classes using ventral speckling, body size and behavior. Immature individuals had not ventral speckling and were generally smaller then individuals with ventral speckling (mature).

Bottlenose dolphin consortships consist of male pairs or triplets with a single female. Consortships have been observed to last from a few minutes to several weeks. The data indicates that consortships were commonly maintained through herding events by the males. Consortships were recorded as herding events if any of the following were observed: 1) female capture by male alliance, 2) female attempts to bolt, 3) male produce vocal; threats, 4) physical threats directed towards female or 5) a theft of a female from another alliance.

During 1987-1989 observations of herding were made on ten alliances including the alliance of three provisioned males. 255 consortships the provisioned males were identified as herding events. Fifty-eight cases of herding by nine non-provisional alliances. Of the 208 cases of herding by provisioned males, 118 (57%) were documented by more then one criteria, compared to 16 (28%) of the fifty-eight cases of herding by non-provisioned males. The majority (94%) of consortships by the provisioned males were observed in Shark Bay where detailed observations were made.

Female Bottlenose dolphins have a five to seven day period of rising estrogen levels prior to ovulation. If female attractiveness is mediated exclusively by estrogen, we should expect the duration of courtship not to exceed the period of elevated estrogen levels. Attractive periods are not equivalent to consortships; if a female escaped but was consorted again the next day, both events would be considered part of the same attractive period but would be considered two separate consortships. Two conventions were used in order to define "attractive periods". Convention one was the number of consecutive days a female was consorted, while convention two attractive periods were the same as (1) plus the duration of any other attractive periods occurring within seven days. By convention one, consortship of known females by the provisioned males produced 170 attractive periods, ranging in length from one to seventeen days with seven exceeding seven days. Using convention two courtship’s by provisioned males produced 137 attractive periods, ranging in length from one to twenty-two days with seventeen exceeding seven days and six lasting fourteen days or more.

Attractive periods outside the breeding season between provisioned and non-provisioned males and females occured at various times throughout the year. Consortships were common during Austral winter months from May-August. These consortships were due to rises in estradiol, which is an estrogenic hormone that precede rising progesterone levels in the fall breeding season.

The resumption of attractive periods following the loss of an offspring happened during the span of three weeks in early 1980 in Shark Bay. Three infants disappeared are were presumed dead possibly due to septic tank pollution(EPA report, Western Australia, 1989). The intervals between infant death and female disappearance from the provisioned area were commonly 6-11 days. A secondary fall birth peak was reported and was the result of a spring-early summer infant loss. The secondary birth peak may also be produced as an outcome of reproductive cost/benefit decision on whether to conceive at a sub-optimal time of the year(fall) or to wait 6-8 months before conceiving.

A particular behavior the males directed towards the females in oestrus is termed "genital inspection". The male will typically approach and angle his rostrum toward the females genital area. The male begins to echolocate on the females genital area using sound produced in the monkey lips/dorsal bursa. The sound bounces off the male’s parabolic skull and is directed forward by the melon. The echolocation allows the males to detect tissue changes in the female associated with oestrus. Males may also be able to detect chemical signals associated with oestrus in females by tasting the females urine.

The attractiveness of post-partum female bottlenose dolphins in Shark Bay was mediated by low levels of progesterone and high levels of estrogen. Estrogen and progesterone are female sex hormones produced in the endocrine glands. During pregnancy estrogen and progesterone levels are elevated but after birth, levels fluctuate. These fluctuating hormonal levels may produce non-conceptive attractive periods between males and females in Shark Bay.

Consortships and female cycling were implications for mating strategy in Shark Bay. During herding, the males may force copulation either directly or by threatening aggression towards the females. Females in the Bay may prefer to mate with multiple males due to receiving a higher quality of sperm. Female cycling is a counter strategy to mating with multiple males. By the females going through their cycles more often they can ensure mating with the most fit provisioned or non-provisioned male. Multiple cycles may be a mechanism by the females to exert choice for preferred males and a higher quality of sperm.

Female attractiveness in free-ranging bottlenose dolphins was studied to help better further the knowledge on this very delicate subject. Based on the results of these observations and studies, it can easily be said that the behavior and social relationships of delphids are much better understood today. These studies did in fact show that there are certain behavioral indicators that will coincide with physiological oestrus in females. However the information is still limited. This limitation was due to certain barriers, such as not being able to observe these relationships below the surface of the ocean. In order to fully understand these processes as a whole these studies need to involve much more detail and precision.

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Martin, Richard M. Mammals of the Sea. 1977. B.T. Batsford Limited. London.

Schroeder, P.J. Seasonality of Serum Testosterone Levels and Sperm Density in Tursiops truncatus. Journal of Experimental Zoology. vol.249. p316

Sergent, D.E. Age, growth, and maturity of bottlenose dolphins from NorthEast, Florida. 1973. Journal of Fisheries and Resource Board of Canada. vol.30. p1009.

Smolker, R.A. Sex diferences in patterns of association among Indian Ocean Bottlenose dolphins. 1992. Behavior, vol. 123. p. 38.

Tolley, K.A. Sexual Dimorphism in wild bottlenose dolphins from Sarasota, Florida. Journal of mammalogy. Nov1995. vol.76. n4. p.1190.

Urian, K.W. Seasonality of Reproduction in Bottlenose Dolphins. Journal of Mammalogy. May 1996. vol 7. n2. p.394.

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