THE LIFE HISTORY OF FREE‐RANGING ATLANTIC SPOTTED DOLPHINS (STENELLA FRONTALIS): AGE CLASSES, COLOR PHASES, AND FEMALE REPRODUCTION

Abstract

Atlantic spotted dolphins (Stenella frontalis) were observed underwater and from the surface from 1985 to 1996 and photographed through successive years. Individuals were categorized into age classes by their degree of spotting and color phases. Dolphins spent an average of 3 yr in the two‐tone color phase, 5 yr in the speckled phase, 7 yr in the mottled phase and up to 10 yr or more in the fused phase. Sex ratios were close to parity, with old adults skewed towards females and juveniles and young adults skewed towards males. The average calving interval for 24 females was 2.96 years with a range of 1–5 yr. Females whose calves survived the first year had a significantly longer calving interval (3.56 years). The ages of first parturition for five females were estimated to be 10–12 yr. The age at sexual maturation was estimated to range from 8 to 15 yr. Pregnancy rate fluctuated annually, with an average rate of 0.25 (range 0.07–0.57). Annual average birth rate was 0.08 (range 0.07–0.14), average calf production was 0.33 (range 0.06–0.52), average fecundity was 0.23 (range 0.13–0.30), and average recruitment was 0.06 (range 0.03–0.08). Most females who lost a calf conceived the same or following year. Lactation lasted up to 5 yr, and 45% of visibly pregnant females were also lactating. Age of first parturition was associated with the mottled color phase. Average first‐year mortality rate of calves was 0.24.

Maternal and Alloparental Discipline in Atlantic Spotted Dolphins (Stenalla frontalis) in the Bahamas

“…Juvenile dolphins, age 4 -8, were considered speckled, young adults aged 9 -15years were mottled, and adults aged 16 and above were classified as fused. Most females reach sexual maturity during the mottled years (Herzing, 1997). It is unknown when male Atlantic spotted dolphins reach sexual maturity (Green et al, 2011).…”

“…Sex was determined when possible based on observation of the genital slit. Herzing (1997) classified Atlantic spotted dolphins into four age classes based on spot development with maturity. Age of each individual is estimated based on spot development, or known from tracking the animal since birth.…”

“…There are several reasons why this likely occurred. Spotted dolphin calves are age three and under (Herzing, 1997), and are dependent on their mothers for survival and integration into their social group. This is the case for many social mammals including other dolphin species and primates (Maestripieri, 2011;Mann, Stanton, Patterson, Bienenstock, & Singh, 2012;Rendell & Whitehead, 2001).…”

“…However, the majority of dolphins sampled in this study were adults, as about half of the dolphins studied were mothers. Atlantic spotted dolphins do not reach sexual maturity until their young adult years (mottled age class), and would obviously not be able to give birth before becoming adults (Herzing, 1997). Juvenile disciplinarians were also included in the sample, but were not observed performing discipline as often as adults.…”

“…The operational definitions of all independent variables and behavioral categories are listed in Tables 1 and 2. The disciplinarian was either a mother or alloparent. The mother was defined as the biological parent of an offspring, and was determined by visible pregnancies in females followed by resightings of female with a nursing calf (Herzing, 1997). All mothers were mottled (young adults) or fused (adult), as reproductive maturity occurred during the mottled age class.…”