Chapter 5 - The life history of franciscana dolphins

Abstract

The franciscana, Pontoporia blainvillei, is the only extant species of its family and the most threatened small cetacean in the Western South Atlantic. Understanding life traits that shape the population dynamics and variation in the demography of threatened species is critical for their conservation. In this chapter, we provide a comprehensive review of the life history of the franciscana, integrating information from 40 years of studies. We also discuss the species´ mating system and make recommendations regarding future research. Franciscana longevity is ca. 20 years, but with very few animals reaching more than 12/14 years. Growth curves are available for franciscanas in all distribution range. Females are larger than males and the mean reversed sexual size dimorphism for all populations is 17%. Sex ratio at birth is 1:1 and the size at birth is around 70 cm, with few exceptions. The franciscana is an annual breeding and monoestrous species. Ovulations and conceptions are concentrated in the austral summer, about two-thirds occurring from December to February. Franciscana presents a typical birth-pulse pattern in most of its range, with births starting in early spring and decreasing gradually until the end of austral summer. Mean gestation length estimate is 10.5 months and lactation lasts 7–9 months. The average age at sexual maturity (ASM) ranges from 2.7 to 3.9 years, depending on the population and estimator. Survival rates for juvenile franciscana were estimated at 0.91 and 0.90, while for adults were estimated at 0.85 and 0.86. Multimetodological approach suggests that adult franciscanas form monogamous reproductive pairs of unknown duration, where a male performs mate guarding. A matrilineal society where offspring remain for some time in the same group of their mother is likely. Collectively, the franciscana can be considered one of the cetaceans with fastest life-history traits, mainly due to its remarkably low ASM, short reproductive interval, low survival rates, and short life span.