Examining Bergmann's rule in a cosmopolitan marine mammal, the bottlenose dolphin (Tursiops spp.) Marine Mammal Science

Body size is a life-history trait with significant ecological and physiological implications. As a group, marine mammals are the largest mammals on earth. The smallest marine mammal (by mass) is the marine otter (Lontra felina), weighing 3–5 kg (Jefferson et al., 2015), which is over 10 times as much as the smallest terrestrial mammal, the Etruscan shrew (Suncus etruscus), weighing 1.8 g (Jürgens, 2002). Similarly, for the opposite end of the size spectrum, the blue whale (Balaenoptera musculus)—the largest animal to have ever lived—is approximately 150,000 kg (Sears & Calambokidis, 2002) compared to the 5,000 kg African elephant (Loxodonta africana; Laurson & Bekoff, 1978). The large body size attained by marine mammals represents an evolutionary trend to balance energy costs associated with thermoregulation and foraging (Gearty et al., 2018; Goldbogen, 2018; Williams, 1999) and selects for larger neonates (Christiansen et al., 2014, 2018). The lower limit of body size in marine mammals is thought to be constrained by the thermal demand of seawater, which conducts heat 25 times faster than air at the same temperature.