Insights from life history traits of Risso's dolphins (Grampus griseus) in Taiwanese waters: Shorter body length characterizes northwest Pacific population

Abstract

Risso's dolphins (Grampus griseus) are widely distributed throughout temperate to tropical pelagic waters of the world and are one of the most frequently encountered cetaceans in eastern Taiwanese coastal waters. Because their life history is poorly known, the goal of this study was to investigate the relationship between age, body length, and sexual maturity of Risso's dolphins in Taiwanese waters. Ninety‐two carcasses of dead‐stranded or fisheries bycaught dolphins (1994–2008) were measured and dissected (total body length, TBL 125–290 cm); sexual maturity was assessed in 33 dolphins; and age was estimated by counting dentinal growth layer groups in routine histologically prepared tooth sections of 28 dolphins. Sexual dimorphism in TBL was not detected. The onset of sexual maturity occurred at 240–255 cm in females and 253–265 cm in males, which was at about 10 yr of age for both sexes. Our stranding, bycatch, and previous boat survey records suggest that Risso's dolphins occur year‐round and likely have a summer‐fall calving season in Taiwanese waters. The similar life history parameters and calving season in dolphins from Taiwanese and Japanese waters suggest a common population in the northwest Pacific, which has a noticeably shorter body length than in other regions.

Annual variability in dentin δ15N and δ13C reveal sex differences in weaning age and feeding habits in Risso's dolphins (Grampus griseus)

“…Growth layer group counts obtained from 12 individuals in the present study were compared with those made by Chen et al . () on stained tooth sections from the same individuals to confirm our estimated age. Three of our age estimates were higher by 1–2 yr and three others were lower by 1–2 yr than those of Chen et al .…”

“…The suggested differences in weaning age between male and female calves in the present study is contrary to expectations for a species that does not exhibit sex‐specific differences in early growth rates and adult body size (Amano and Miyazaki , Chen et al . ). Efficient maternal provisioning allows neonates to reach a threshold weaning weight, which is about four times the birth weight, faster and therefore wean earlier (Clutton‐Brock et al .…”

“…Based on the reported age of sexual maturity in female Risso's dolphins (Amano and Miyazaki , Chen et al . ), we took the average δ 15 N cor value (12.58‰ ± 0.20‰; 95% CI: 12.49‰, 12.67‰) from the 8th to the last extractable GLG of all females in our samples as a proxy for baseline maternal values. Mean δ 15 N cor and δ 13 C cor ratios of the prenatal dentin and of the 1st GLG ( n = 65) were then compared with maternal values to test assumptions of isotopic enrichment.…”

“…Previous studies based on correlations between morpho‐anatomical data and age estimates suggested that sexual maturity is attained at 8–10 yr in females and 10–12 yr in males (Amano and Miyazaki , Chen et al . , Bloch et al . ).…”

Insight into niche separation of Risso's dolphin (Grampus griseus) in the southwestern South Atlantic via δ13C and δ15N values

“…Geographical variation in the body size of Grampus populations has been reported previously. Grampus individuals can live up to ∼35 yr (Baird 2009) but reach sexual maturity between 8 and 10 yr, which corresponds to between ∼240 and 300 cm in total body length (TBL) (Kruse et al 1999, Amano and Miyazaki 2004, Chen et al 2010). Currently, there are no published descriptions of Grampus individuals from South American waters.…”