Little is currently known about mating systems in Irrawaddy dolphins. Since the animals live in small groups averaging between three to six individuals, it is presumed that breeding happens outside of those groups. The breeding season is between December and June; however, little is known about mating behavior. As in other dolphin species, it can be presumed that males mate with multiple females and compete over mates.
Mating System: polygynous
There is relatively little information on reproduction in Irrawaddy dolphins. The mating season in the northern hemisphere is reported to be December to June. The calving season seems to be from June to August, with noted exceptions. Data from Chilka Lake, India shows a low rate of breeding and production of only a single calf in three years. Additionally, gestation is approximately nine months, but data from two captive births estimate the gestation period at fourteen months. Captive born Irrawaddy dolphin calves measured 96 cm in length and weighed 12.3 kg. In the first seven months the calves increased in length by 59% and 266% in weight. Calves begin eating fish around six months and are fully weaned at about two years old. Adult length is achieved between three and five years old. Rate of sexual maturation is thought to be positively correlated with growth rate.
Breeding interval: Females may not reproduce yearly, in one population females give birth every 3 years.
Breeding season: Irrawaddy dolphins breed in December through June.
Average number of offspring: 1.
Range gestation period: 9 to 14 months.
Average birth mass: 12.3 kg.
Average weaning age: 24 months.
Range age at sexual or reproductive maturity (female): 3 to 6 years.
Range age at sexual or reproductive maturity (male): 3 to 6 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous
Average birth mass: 12300 g.
Average number of offspring: 1.
Irrawaddy dolphins are not completely weaned until 2 years. From birth to approximately seven months old, the calf survives solely on the nutrition from the mother. For the following seven months the calves stay within the pod and continue to receive nourishment from the mother while also eating fish. It is presumed the calves learn to prey on fish by copying behavior of the mother's and pod mates. There is little information on the rearing of calves. Currently it is not known if both males and females are involved in the upbringing of calves. Like most mammals, females invest heavily in their young.
Parental Investment: female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); post-independence association with parents; extended period of juvenile learning
- 1999. Mammalian Species Orcaella brevirostris. American Society of Mammologist, 616: 1-8. Accessed March 08, 2010 at http://www.science.smith.edu/msi/pdf/i0076-3519-616-01-0001.pdf.
- 2007. "Trey Psaut Cambodia’s Irrawaddy Dolphin" (On-line pdf). Accessed March 08, 2010 at http://faculty.washington.edu/banasn/animals/student_work/Julia%20Boubel,%20Trey%20Psaut.pdf.
- 2010. "Orcaella brevirostris (Gray, 1866)" (On-line pdf). Convention on Migratory Species. Accessed March 08, 2010 at http://www.cms.int/reports/small_cetaceans/data/O_brevirostris/O_brevirostris.PDF.
- Stacey, P., S. Leatherwood. 1997. Asian Marine Biology: 1997. Hong Kong: Hong Kong University Press.