Abstract
AbstractAn animal's body condition will affect its survival and reproductive success, which influences population dynamics. Despite its importance, relatively little is known about the body condition of large whales and its relationship to reproduction. We assessed the body condition of humpback whales (Megaptera novaeangliae) at a breeding/resting ground from aerial photographs recorded using an unmanned aerial vehicle (UAV). Photogrammetry methods were used to measure the surface area of individual whales, which was used as an index for body condition. Repeated measurements of the same individuals were not possible; hence, this study represents a cross‐sectional sample of the population. Intraseasonal changes in the body condition of four reproductive classes (calves, immature, mature, and lactating) were investigated to infer the relative energetic cost that each class faces during the breeding season. To better understand the costs of reproduction, we investigated the relationship between female body condition (FBC) and the linear growth and body condition of their dependent calves (CBC). We documented a linear decline in the body condition of mature whales (0.027 m2/d; n = 20) and lactating females (0.032 m2/d; n = 31) throughout the breeding season, while there was no change in body condition of immature whales (n = 51) and calves (n = 32). The significant decline in mature and lactating female's body condition implies substantial energetic costs for these reproductive classes. In support of this, we found a positive linear relationship between FBC and CBC. This suggests that females in poorer body condition may not have sufficient energy stores to invest as much energy into their offspring as better conditioned females without jeopardizing their own body condition and survival probability. Measurement precision was investigated from repeated measurements of the same animals both from the same and different photographs, and by looking at residual errors in relation to the positioning of the whales in the photographs. The resulting errors were included in a sensitivity analysis to demonstrate that model parameters were robust to measurement errors. Our findings provide strong support for the use of UAVs as a noninvasive tool to measure the body condition of whales and other mammals.
Highlights
The body condition of animals in a population has important implications for individual survival and reproductive success, and population dynamics
We documented that the body condition of mature and lactating whales decreased significantly through the breeding season, while there was no change in the body condition of immature whales and calves
Similar differences in energy storage between reproductive classes have been documented in minke whales, fin whales, and right whales and are likely to reflect differences in energetic costs during the breeding season (Lockyer 1986, 1987b, Vikingsson 1990, 1995, Miller et al 2011, Christiansen et al 2013)
Summary
The body condition of animals in a population has important implications for individual survival and reproductive success, and population dynamics. For capital breeding animals, where the costs of reproduction are met by endogenous energy stores during a period of fasting (Stephens et al 2009), the link between body condition and reproduction is strong (Festa- Bianchet et al 1998, Bonnet et al 2002). Not documented for baleen whales, maternal body condition in other marine mammals, such as pinnipeds, influences offspring birth and weaning mass (Boltnev and York 2001, Bowen et al 2001), which, in turn, influences offspring survival (McMahon et al 2000)
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