Abstract

Blubber thickness (BT) has a seasonal pattern in most small cetaceans in temperate and polar regions, which may be a crucial adaptive mechanism in response to environmental temperature changes. However, BT and environmental temperature correlations have never been tested experimentally in any cetacean species owing to logistical difficulties in the aquatic environment. The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis, YFP) is one of the smallest cetacean species worldwide, which exclusively inhabits the middle and lower regions of the Yangtze River in China. Here, we investigated BT variation patterns in YFPs and their relationship to environmental temperature changes using ultrasound imaging technology by longitudinally monitoring four YFPs in human care. We found that blubber was not evenly distributed in the skin of YFPs. BT increased along the craniocaudal axis from the head to the umbilical girth, and decreased towards the fluke, with relatively thicker blubber in the dorsal region than in the ventral and lateral regions. Significant negative correlations between BT and seasonal water temperature changes were observed in YFPs. However, different body regions display different sensitivities to seasonal temperature changes. The BT in the anal girth region exhibited noticeable seasonal changes. In contrast, the umbilical lateral and ventral regions showed relatively blunt seasonal changes, indicating different adaptive functions of the blubber in different regions. BT in the dorsal region decreased linearly with increasing water temperature. In the lateral and ventral regions, BT significantly changed with water temperature at a threshold of 18 ​°C. The YFPs had relatively thinner BT than similar-sized harbor porpoises that inhabit relatively high latitudes with much lower water temperatures. This further demonstrates the adaptive function of BT in response to environmental temperatures in small cetaceans. This study elucidates the seasonal pattern of BT variation in small cetaceans and provides insight into adaptation mechanisms of small cetaceans to temperature changes.