Historical data on age, growth and reproduction of pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales stranded along the Southern African coastline, with additional information from Australian specimens.

Abstract

South Africa has one of the highest stranding records in the world for both pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales and as such offers a unique opportunity to study these little-known species. Data and samples from animals stranded along the South African and Namibian coastline between 1960 and 1999 were analysed to determine basic life history parameters for the two species. Teeth from 80 K. breviceps and 45 K. sima from South Africa and an additional 27 K. breviceps and one K. sima from Australia were available for age determination. A good correlation between cemental and dentinal age estimates was found for both species, although cemental readings may not be as reliable in K. sima as they are for K. breviceps. Length at birth for K. breviceps was about 120 cm and the weight around 53 kg, while it was about 103 cm and 14 kg for K. sima. The asymptotic length for K. breviceps was calculated as 306.04 cm by 286.08 cm for females and males. Assuming one growth layer group (GLG) to be equal to one year, both sexes reached physical maturity at about the same age of 15 years. A life expectancy between 16 and 23 years was determined for the species. For K. sima, the asymptotic length was 249.14 cm in females and 263.75 cm in males. This corresponds to 13 and 16 years of age for females and males, respectively. A life expectancy of 17-22 years was determined for K. sima. Reversed sexual dimorphism is suggested for K. breviceps, while there appears to be little size difference between the sexes in K. sima. Reproductive organs from 19 male and 25 female K. breviceps and 19 male and 26 female K. sima were examined to determine reproductive status. The onset of sexual maturity was estimated to be at about 262 cm and around five years in female K. breviceps and at about 215 cm and around five GLGs in female K. sima. The ovulation rate of 0.9 per year for K. breviceps indicated that, on average, ovulations occurred about every 13.3 months. The gestation length is approximately 11 months and conceptions occur from April to September and births possibly occurring from March to August in K. breviceps. In K. sima, the ovulation rate of 0.7 per year indicates that ovulations occur about every 17.1 months (or roughly one and a half years) and gestation length is 11-12 months. Both conceptions and births occurred between December and March and 11.5 % of mature females were found to be simultaneously lactating and pregnant. These data indicated that K. sima may also show annual reproduction, if the conditions are right, although that may be facultative and some animals may only reproduce every two years. The reproductive strategy determined for both Kogia species indicated that a relatively high percentage of females was simultaneously lactating and pregnant, but the accumulation rate of corpora showed that although K. breviceps may have an annual reproduction, at least some K. sima females may only reproduce every two years. Both species exhibited seasonal reproduction, but while K. breviceps appeared to have a protracted mating and calving season of six months, K. sima exhibits a shorter mating and calving season over the period of four months with births occurring during the warmest part of the year. Reproductive organs from 19 K. breviceps and 19 K. sima males were examined to determine their reproductive status. In male K. breviceps, attainment of sexual maturity (ASM) occurred between 2.5 and 5 years, 241-242 cm and 210.0-233.6 kg, while it occurred between 2.55 and three years of age in male K. sima, a body length of 197 cm and body weights between 111.8 kg-124.0 kg. The maximum combined testis weight made up 1.04 % of the total body weight in K. breviceps and 2.00 % in K. sima. Based on data on testes size, sexual size dimorphism, signs of intraspecific fighting, and group size a polygynous mating system with a roving male strategy was proposed for both species.