Specific foetal growth rates of cetaceans

Abstract

The numerous data in the International Whaling Statistics relating foetal length to date of catch have been studied. They can be used as alternatives to cube roots of foetal weights to give rates of foetal growth and for the computation of data of conception and foetal ages. There are major differences between the Mysticeti and Odontoceti. The main gestation period for the former is 10.8.±1‐2 months and for the latter 14 ± 2 months. In both groups the greatest birth‐weight is attained by an increased foetal growth rate, to such an extent in the rorquals that the gestation period in the largest is actually shorter than in the smaller species. There are dietetic and metabolic differences between the two sub‐cohorts. The outstanding one is that pregnant mysticetes appear to stop feeding and lose body fat to the foetus during late pregnancy, at the time when the foetal increase in size is maximal. The causes of the differences in growth and metabolism of the two groups are unknown, apart from those imposed by differences in their ecology.

Female body condition affects foetal growth in a capital breeding mysticete

“…Instead of finding a linear increase in foetus length through the feeding season, which would correspond to the general mammalian foetal growth pattern of the late prenatal growth phase (Huggett & Widdas ; Frazer & Huggett , ), our results instead showed a curvilinear increase in foetus length of minke whales. Similar curvilinear growth patterns have also been found in other studies of mysticetes, including minke whales (Laws ; Sergeant ; Ivashin & Mikhalev ; Rice ; Wang ).…”

“…As foetuses were not aged, it could be argued that the measured effect of FBC on foetus size is the effect of variation in conception dates between females. Minke whales exhibit considerable variation in conception dates (Frazer & Huggett ; Wang ; Horwood ; Kato & Miyashita ; Kato ), most likely caused by their 14‐month reproductive cycle (Best ), which means that females gradually get out of sync with their seasonal migration pattern (Best ; Rice ). Females that conceive earlier also arrive earlier in the feeding grounds to start building up their blubber reserves, than those conceiving later (Kato & Miyashita ; Kato , ).…”

“…However, minke whales are considered annual breeders, with calving rates above 90% (Jonsg ard 1951 ;Horwood 1990). In the North Atlantic, the peak of conception occurs around February (Sergeant 1963;Lockyer 1984), although there may be considerable variation in conception dates (Frazer & Huggett 1973;Wang 1985;Horwood 1990;Kato & Miyashita 1991;Kato 1992). The gestation period is about 10 months (Sergeant 1963;Ivashin & Mikhalev 1978;Masaki 1979;Best 1982;Wang 1985;Horwood 1990), after which the female gives birth to a single calf between 240 and 280 cm in length (Sergeant 1963;Ivashin & Mikhalev 1978;Wang 1985) or about 305 kg in mass (Lockyer 1981).…”

“…To produce large offspring with high survival (McMahon, Burton & Bester 2003) and to have their reproductive cycle synchronized with their annual migratory cycle, balaenopterids have evolved some of the highest foetal growth rates among mammals (Frazer & Huggett 1959, 1973. Such high growth rates suggest that the energetic investment during gestation must be very high (Lockyer 1986) and that adequate nutrition will be key to reproductive success and recruitment (Pitcher, Calkins & Pendleton 1998).…”

The Evolutionary History of Cetacean Brain and Body Size

“…Hence, shifts in mysticete development and life history could alter EQ by shaping body mass evolution independently of overall brain mass. Although data are limited, mysticetes do appear to have divergent developmental trajectories compared to odontocetes, with extremely high rates of both pre‐ and postnatal body growth (Frazer and Hugget ). To fully interpret changes in EQ caused by changes in either body mass or brain mass, a better understanding of the mechanisms and selection pressures causing brain:body allometry is required.…”