Female reproductive success and calf survival in a North Sea coastal bottlenose dolphin (Tursiops truncatus) population

Abstract

Between-female variation in reproductive output provides a strong measure of individual fitness and a quantifiable measure of the health of a population which may be highly informative to management. In the present study, we examined reproductive traits in female bottlenose dolphins from the east coast of Scotland using longitudinal sightings data collected over twenty years. From a total of 102 females identified between 1997 and 2016, 74 mothers produced a collective total of 193 calves. Females gave birth from 6 to 13 years of age with a mean age of 8. Calves were produced during all study months, May to October inclusive, but showed a seasonal birth pulse corresponding to the regional peak in summer water temperatures. Approximately 83% (n = 116) of the calves of established fate were successfully raised to year 2–3. Of the known mortalities, ~45% were first-born calves. Calf survival rates were also lower in multiparous females who had previously lost calves. A mean inter-birth interval (IBI) of 3.80 years (n = 110) and mean fecundity of 0.16 was estimated for the population. Calf loss resulted in shortened IBIs, whilst longer IBIs were observed in females assumed to be approaching reproductive senescence. Maternal age and size, breeding experience, dominance, individual associations, group size and other social factors, were all concluded to influence reproductive success (RS) in this population. Some females are likely more important than others for the future viability of the population. Consequently, a better knowledge of the demographic groups containing those females showing higher reproductive success would be highly desirable for conservation efforts aimed at their protection.

Female reproductive output in a Mediterranean bottlenose dolphin Tursiops truncatus population

“…The age of first pregnancy was recorded to be 7.5 yr, which is within the ranges reported for other bottlenose dolphin populations (Fruet et al 2015, Robinson et al 2017, Kemper et al 2019. In captivity, a bottlenose dolphin female typically begins to be reproductive at 7−10 yr, but may also begin earlier, between 5 and 7 yr (Schroeder 1990).…”

“…Groups of bottlenose dolphins with calves were previously found to be significantly larger than those without calves (Hubard et al 2004, Blasi & Boitani 2012, and in this study the number of calves produced was influenced by the mean maternal group size. The trend of finding calves in larger groups suggests that larger group size may benefit mother−calf pairs (Schaffar-Delaney 2004, Gibson & Mann 2008, Robinson et al 2017; however, in this study, the number of calves per female was not significantly related to the strength of the associations of the mother. In Sarasota Bay, Florida (USA), Wells (1991a) showed that survivorship of bottlenose dolphin calves was related to group size.…”

“…Moreover, the population could not be maintained by the 16 calves born in 7 years. The number of calves per female was highly variable, ranging from 1 to 4, and suggested that reproductive success might be related to several factors, as already found in other bottlenose dolphin populations (Kellar et al 2017, Robinson et al 2017, Baker et al 2018 2. Main principal components (Factors 1 and 2) calculated for the half-weight index (HWI), showing the association patterns among female bottlenose dolphins (in bold) and juveniles/calves (plain text) with the known mother.…”

“…Photo-identification (photo-ID) is a technique which uses scarring patterns and unique nicks on the dorsal fins of individuals to investigate the ecology and behaviour of dolphins (Würsig & Jefferson 1990). Several studies have applied photo-ID methods to investigate demographic and reproductive factors in wild bottlenose dolphin populations (Grellier et al 2003, Robinson et al 2017, Bezamat et al 2020. Hammond et al (1990) used the individual reproductive history of repeatedly sighted females over multiple years to assess various parameters, such as age at first reproduction, birth season, birth intervals, and productivity.…”

Stomach Content Analysis and Concentrations of Chemical Pollutants in the Clymene Dolphin (Stenella Clymene, Gray 1846) from the Coastal Waters of Ghana

“…Many dolphin species or populations are recognized internationally as vulnerable or threatened due to their low reproductive potential, which is, giving birth to one calf after a long gestation period of 11-12 months [15,16]. Oil spills and various fishing methods, most notably purse-seine fishing for tuna and the use of drift and other gill nets, results in large amounts of dolphins being killed accidentally [17].…”