<P> Adult males have been observed carrying objects in their mouths such as branches or other floating vegetation, or balls of hardened clay . The males appear to carry these objects as a socio - sexual display which is part of their mating system . The behavior is "triggered by an unusually large number of adult males and / or adult females in a group, or perhaps it attracts such into the group . A plausible explanation of the results is that object - carrying is aimed at females and is stimulated by the number of females in the group, while aggression is aimed at other adult males and is stimulated by object - carrying in the group ." Before determining that the species had an evident sexual dimorphism, it was postulated that the river dolphins were monogamous . Later, it was shown that males were larger than females and are documented wielding an aggressive sexual behavior in the wild and in captivity . Males often have a significant degree of damage in the dorsal, caudal, and pectoral fins, as well as the blowhole, due to bites and abrasions . They also commonly have numerous secondary teeth - raking scars . This suggests fierce competition for access to females, with a polygynous mating system, though polyandry and promiscuity cannot be excluded . </P> <P> In captivity, courtship and mating foreplay have been documented . The male takes the initiative by nibbling the fins of the female, but reacts aggressively if the female is not receptive . A high frequency of copulations in a couple was observed; they used three different positions: contacting the womb at right angles, lying head to head, or head to tail . </P> <P> Breeding is seasonal, and births occur between May and June . The period of birthing coincides with the flood season, and this may provide an advantage because the females and their offspring remain in flooded areas longer than males . As the water level begins to decrease, the density of food sources in flooded areas increases due to loss of space, providing enough energy for infants to meet the high demands required for growth . Gestation is estimated to be around eleven months and captive births take 4 to 5 hours . At birth, calves are 80 centimetres (31 in) long and in captivity have registered a growth of 0.21 metres (0.69 ft) per year . Lactation takes about a year . The interval between births is estimated between 15 and 36 months, and the young dolphins are thought to become independent within two to three years . </P> <P> The relatively long duration of breastfeeding and parenting suggests a strong mother - child bond . Most couples observed in their natural environment consist of a female and her calf . This suggests that long periods of parental care contribute to learning and development of the young . </P>

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