Female body condition affects foetal growth in a capital breeding mysticete

Abstract

Summary Understanding how female body condition (FBC) influences foetal development, and hence offspring production, is fundamental for our understanding of species reproductive physiology and life history. We investigated the effects of FBC on foetus growth in common minke whales. Pregnant minke whales were sampled around Iceland during the summer feeding seasons between 2003 and 2007 and the length and weight of their foetuses were measured. FBC was modelled as the relative difference between measured blubber volume and the average expected blubber volume of individual whales. Generalized linear models were used to test the effect of FBC on foetus length, while accounting for the daily growth in foetus size through gestation, as well as other covariates. Foetus length increased curvilinearly through the study period at an average rate of 0·964 cm day−1 (SE = 0·138). The effect of FBC on foetal length was nonlinear, showing an almost linear positive relationship for females in poorer body condition (FBC < 0), which levelled off at better body conditions (FBC > 0). The curvilinear relationship between FBC and foetus growth was confirmed by fitting a generalized additive model and by running separate analyses on two subsets of data separating females in poorer and better condition. Our findings suggest that females that are in poorer body condition reduce their energetic investment in their foetus proportionately to their condition, most likely to help maintain a high survival probability. That foetus length did not increase for females in better body condition suggests that females have an upper limit on the amount of energy they will or can invest in their foetus. Reducing the size at birth by reducing the gestation period is also unlikely, because the reproductive cycle of balaenopterids is strongly linked to their seasonal migration between feeding grounds and breeding grounds. This study is the first to demonstrate that FBC can affect foetus growth in a capital breeding mysticete.