Project Name Dolphin facts page 3

Male bottlenose dolphins reach sexual maturity at lengths of 245 to 260 cm and aged 10 to 13 years. Females become sexually mature at lengths ranging from 220 to 235 cm and 5 to 12 years of age.
In Florida (and possibly elsewhere), most calving and mating activity apparently occurs from February to May, with a secondary peak in late summer.
Gestation lasts about 12 months. Calves (studied off Florida) typically remained with their mothers for 3 to 6 years, and one calf remained with the mother for over 10 years, during which time the mother was not seen with another calf. This suggests that the interval between successive calves may vary, and is probably longer than 2 years.
In a captive born dolphin, suckling lasted for 18 to 20 months, but evidence from wild dolphins suggests that nursing could continue well beyond the age of three years old, possibly lasting up to seven years in some individuals.

The social organisation of the bottlenose dolphin is very complex. Bottlenose dolphins may be observed in groups ranging from 2 to 1,000 individuals, but are most frequently observed in groups of between 2 and 25 individuals. Large groups often seem to be composed of several smaller pods dispersed over several square kilometres; some groups may inhabit limited geographical areas. School size tends to increase with water depth, and may alter through the day, with activity and with age or sex class (females, especially with calves, for example, tend to occur in larger schools than males). Sexually segregated groups are sometimes reported.
Although the composition of schools may change and fluctuate over the course of a day, many associations are relatively stable and long-term. Membership of a female band is relatively stable, and sub-adult and adult males may form lasting associations among themselves, often with other males from their natal pod. The associations of the longest duration, however, are between cows and calves.
Bottlenose dolphins are highly social and tactile; During bouts of aerial activity, dolphins have been seen swimming side by side while touching, with at least one of the animals swimming upside down (belly up). Individuals were seen to nudge each other with their snouts, and rub against each other. From some studies, it seems that such behaviours occur more frequently, when two sub-groups which had moved separately for several hours, joined again. Noisy leaps (splashing into the water) seemed to be used to attract the attention of the rest of the group. Rubbing and touching may be associated with play and copulatory activity, although close interpersonal association need not always indicate copulatory behaviour.
Adult dolphins have been observed in rough and tumble with calves. Calves are sometimes tossed into the air by the adults. Some researchers have interpreted this kind of behaviour as disciplinary behaviour. Calves were also seen to swim for extended periods with adults other than their mothers. Typically, the 'baby-sitter' played with the calf while the mother fed, often at a distance of several hundred metres.

The bottlenose dolphin is without doubt the best-studied odontocete. Much of our present understanding of echolocation and communication in odontocetes comes from studies of captive specimens of this easily-trained, gregarious species. The use of echolocation by marine mammals was first demonstrated in bottlenose dolphins, and later work has revealed impressive echolocation capabilities, for example the detection of a 14 cm radius metal sphere at over 250m. It is for this species that the 'signature whistle' hypothesis was first proposed, in which each individual repeatedly uses a distinct whistle to identify itself in a group.
PULSIVE : Peak frequencies of echolocation clicks are in the ranges 40-80 kHz and 110-130 kHz, and the spectra of individual clicks may be bimodal. Duration of individual clicks is from 50-80 msecs. Maximum source level is 228 dB re 1mbar @ 1m. Bandwidths range from 20-60 kHz and reportedly increase as peak frequency increases. Pulse rates of up to 780 per sec are reported. Narrow-band low frequency pulses have been reported, with peak energy between 0.3 and 3 kHz; these are heard in series of between 3 and 30 pulses at around 6-12 per second, and are believed to be social sounds, not involved in echolocation.
TONAL : A study over several regions worldwide found overall fundamental frequency of whistles between 0.94 kHz-21.61 kHz (mean 11.25 kHz), lasting 0.05-3.2 secs, with up to 37 contour inflections. Maximum source level is 178 dB re 1 mPa @ 1m. In the North Atlantic, mean values of 16.2 kHz (max. freq), 7.3 kHz (min. freq.), 11.26 kHz (start freq.), 10.2 kHz (end freq.), and a mean duration of 1.3 secs have been found
BEHAVIOUR : During echolocation, the inter-click interval is usually greater than the two-way travel time to the target, indicating that these animals wait to receive an echo before broadcasting the next click (some odontocetes do not, for instance the beluga).
Certain vocalisations seem to have a specific social function, for example, the 'distress call' (a certain pair of whistles) reportedly produces a consistent reaction in nearby individuals, namely to push the calling animal towards the surface. Low-frequency broadband pulses are thought to be a threat vocalisation.
There is an apparent repeated use by bottlenose dolphins of an individually distinctive whistle contour, and most subsequent research has been on this species. The 'signature whistle hypothesis' is that every dolphin has an individualised whistle which identifies it in a group, and accounts for about 75%-95% of its repertoire. Although some whistle parameters vary (e.g., rate and intensity increases in stressful situations), the 'basic contour' remains the same. Signature whistles have also been shown to play an important part in the maintanence of the mother-calf bond.
However, the extent to which the signature whistle forms an individual's repertoire has been challenged. Researchers point firstly to evidence that free-ranging wild animals have much more diverse repertoires than those of captive, restrained and isolated animals (as used in most signature whistle studies); and secondly that the proportion of shared calls appears to be greater in wild groups. Bottlenose dolphins are adept vocal mimics, and field evidence suggests that at least part of an individual's repertoire derives from mimicry of con-specifics.