The cost of reproduction is a key parameter determining a species' life history strategy. Despite exhibiting some of the fastest offspring growth rates among mammals, the cost of reproduction in baleen whales is largely unknown since standard field metabolic techniques cannot be applied. We quantified the cost of reproduction for southern right whales Eubalaena australis over a 3 mo breeding season. We did this by determining the relationship between calf growth rate and maternal rate of loss in energy reserves, using repeated measurements of body volume obtained from un manned aerial vehicle photogrammetry. We recorded 1118 body volume estimates from 40 female and calf pairs over 40 to 89 d. Calves grew at a rate of 3.2 cm d −1 (SD = 0.45) in body length and 0.081 m 3 d −1 (SD = 0.011) in body volume, while females decreased in volume at a rate of 0.126 m 3 d −1 (SD = 0.036). The average volume conversion efficiency from female to calf was 68% (SD = 16.91). Calf growth rate was positively related to the rate of loss in maternal body volume, suggesting that maternal volume loss is proportional to the energy investment into her calf. Maternal in vestment was determined by her body size and condition, with longer and more rotund females investing more volume into their calves compared to shorter and leaner females. Lactating females lost on average 25% of their initial body volume over the 3 mo breeding season. This study demonstrates the considerable energetic cost that females face during the lactation period, and highlights the importance of sufficient maternal energy reserves for reproduction in this capital breeding species.

Seasonal trends in energy storage of the minke whale (Balaenoptera acutorostrata), a capital breeder, were investigated in Iceland, a North Atlantic feeding ground. The aim was to better understand the energy acquisition strategies of minke whales and the energetic costs that different reproductive classes face during the breeding season. We modelled total blubber volume, using blubber thickness and morphometric measurements of individual whales. Blubber volume was influenced by body length, and was higher for pregnant females than mature whales. Blubber volume increased linearly through the feeding season at the same rate for mature (mean ± s.e.m.=0.0028±0.00103m 3 day −1 ; N=61 male, 5 female) and pregnant whales (0.0024±0.00100m 3 day −1 ; N=49), suggesting that minke whales aim to maximise energy storage while on the feeding grounds. The total amount of blubber accumulated over the feeding season (0.51±0.119m 3 for mature and 0.43±0.112m 3 for pregnant whales), together with energy stored as muscle and intra-abdominal fats, constitutes the total amount of energy available for reproduction (fetus development and lactation) on the breeding grounds, as well as migration, daily field metabolic rates, growth and body maintenance. No seasonal variation was observed for immature whales (N=4 male, 12 female), suggesting that they are investing most of their excess energy into growth rather than reproduction, in order to reach the length of sexual maturity faster and start reproducing earlier. Our novel modelling approach provides insight into large whale bioenergetics and life history strategies, as well as the relationship between single-site measurement of blubber thickness and total blubber volume. Supplementary material available online at

Body mass is a key life‐history trait in animals. Despite being the largest animals on the planet, no method currently exists to estimate body mass of free‐living whales. We combined aerial photographs and historical catch records to estimate the body mass of free‐living right whales (Eubalaena sp.). First, aerial photogrammetry from unmanned aerial vehicles was used to measure the body length, width (lateral distance) and height (dorso‐ventral distance) of free‐living southern right whales (Eubalaena australis; 48 calves, seven juveniles and 31 lactating females). From these data, body volume was estimated by modelling the whales as a series of infinitely small ellipses. The body girth of the whales was next calculated at three measurement sites (across the pectoral fin, the umbilicus and the anus) and a linear model was developed to predict body volume from the body girth and length data. To obtain a volume‐to‐mass conversion factor, this model was then used to estimate the body volume of eight lethally caught North Pacific right whales (Eubalaena japonica), for which body mass was measured. This conversion factor was consequently used to predict the body mass of the free‐living whales. The cross‐sectional body shape (height–width ratio) of the whales was slightly flattened dorso‐ventrally at the anterior end of the body, almost circular in the mid region, and significantly flattened in the lateral plane across the posterior half of the body. Compared to a circular cross‐sectional model, our body mass model incorporating body length, width and height improved mass estimates by up to 23.6% (mean = 6.1%, SD = 5.27). Our model had a mean error of only 1.6% (SD = 0.012), compared to 9.5% (SD = 7.68) for a simpler body length‐to‐mass model. The volume‐to‐mass conversion factor was estimated at 754.63 kg/m3 (SD = 50.03). Predicted body mass estimates were within a close range of existing body mass measurements. We provide a non‐invasive method to accurately estimate body mass of free‐living whales while accounting for both their structural size (body length) and relative body condition (body width). Our approach can be directly applied to other marine mammals by adjusting the model parameters (body mass model script provided).

The North Atlantic right whale Eubalaena glacialis (NARW), currently numbering <410 individuals, is on a trajectory to extinction. Although direct mortality from ship strikes and fishing gear entanglements remain the major threats to the population, reproductive failure, resulting from poor body condition and sublethal chronic entanglement stress, is believed to play a crucial role in the population decline. Using photogrammetry from unmanned aerial vehicles, we conducted the largest population assessment of right whale body condition to date, to determine if the condition of NARWs was poorer than 3 seemingly healthy (i.e. growing) populations of southern right whales E. australis (SRWs) in Argentina, Australia and New Zealand. We found that NARW juveniles, adults and lactating females all had lower body condition scores compared to the SRW populations. While some of the difference could be the result of genetic isolation and adaptations to local environmental conditions, the magnitude suggests that NARWs are in poor condition, which could be suppressing their growth, survival, age of sexual maturation and calving rates. NARW calves were found to be in good condition. Their body length, however, was strongly determined by the body condition of their mothers, suggesting that the poor condition of lactating NARW females may cause a reduction in calf growth rates. This could potentially lead to a reduction in calf survival or an increase in female calving intervals. Hence, the poor body condition of individuals within the NARW population is of major concern for its future viability.

Abstract. Considerable uncertainties often surround the causes of long-term changes in population abundance. One striking example is the precipitous decline of southern sea lions (SSL; Otaria flavescens) at the Falkland Islands, from 80 555 pups in the mid 1930s to just 5506 pups in 1965. Despite an increase in SSL abundance over the past two decades, the population has not recovered, with the number of pups born in 2014 (minimum 4443 pups) less than 6% of the 1930s estimate. The order-of-magnitude decline is primarily attributed to commercial sealing in Argentina. Here, we test this established paradigm and alternative hypotheses by assessing (1) commercial sealing at the Falkland Islands, (2) winter migration of SSL from the Falkland Islands to Argentina, (3) whether the number of SSL in Argentina could have sustained the reported level of exploitation, and (4) environmental change. The most parsimonious hypothesis explaining the SSL population decline was environmental change. Specifically, analysis of 160 years of winter sea surface temperatures revealed marked changes, including a period of warming between 1930 and 1950 that was consistent with the period of SSL decline. Sea surface temperature changes likely influenced the distribution or availability of SSL prey and impacted its population dynamics. We suggest that historical harvesting may not always be the ''smoking gun'' as is often purported. Rather, our conclusions support the growing evidence for bottom-up forcing on the abundance of species at lower trophic levels (e.g., plankton and fish) and resulting impacts on higher trophic levels across a broad range of ecosystems.

The eastern North Pacific gray whale Eschrichtius robustus experienced an unusual mortality event (UME) in 2019-2020, with 384 whales found dead along the Pacific coasts of Mexico, USA and Canada. A similar UME in 1999-2000 was speculated to have been caused by starvation, but body condition data were not available to test this hypothesis. Between 2017 and 2019, we used unmanned aerial vehicles (drones) and photogrammetry methods to measure the body condition of gray whales in San Ignacio Lagoon, Baja California Sur, Mexico. Body condition was calculated from the residual of the relationship between body volume and length. The body condition of gray whales was significantly lower in 2018 (-11.1%, SE = 1.74, n = 531) and 2019 (-9.7%, SE = 1.76, n = 628) compared to 2017 (n = 59) for all reproductive classes (calves, juveniles, adults and lactating females). Overall, lactating females were in good body condition. The reduction in body condition of whales in 2018-2019 is unlikely to have affected their survival, but could have reduced their reproductive rate by prolonging the post-weaning recovery time. This could explain the low number of mother-calf pairs observed in the San Ignacio Lagoon in 2018 and 2019. For juveniles and adults that arrived in the lagoons with less energy reserves, their reduced body condition may have been close to their survival threshold. This could explain the high proportion of juveniles and adults among the stranded dead whales in 2019-2020. Although the underlying cause of the reduction in gray whale body condition is unknown, starvation likely contributed to the 2019-2020 UME.

An animal's body condition provides valuable information for ecophysiological studies, and is an important measure of fitness in population monitoring and conservation. While both the external body shape of an animal and its internal tissues (i.e. fat content) can be used as a measure of body condition, the relationship between the two is not always linear. We compared the morphological body condition (external metric obtained through aerial photogrammetry) of migrating humpback whales (Megaptera novaeangliae) with their outer blubber lipid concentration (internal metric obtained through blubber biopsy sampling) off the coast of southwest Australia early and late in the breeding season (spanning ∼4.5 months). The external body condition index of juvenile and adult humpback whales decreased by 26.9 (from 18.8% to −8.1%) and 12.0 percentage points (from 8.6% to −3.4%), respectively, between the early and late phase. In contrast, we found no intra-seasonal change in blubber lipid concentration, and no difference between reproductive classes (juveniles, adults and lactating females); however, the small sample size prevented us from effectively testing these effects. Importantly, however, in the 33 animals for which paired metrics were obtained, we found no correlation between the morphometric body condition index and the blubber lipid concentration of individual whales. The lack of a linear relationship suggests that changes in outer blubber lipid concentration do not reflect external changes in body shape, thus limiting the utility of outer blubber lipid reserves for individual body condition evaluation. The wider spectrum of change in body morphometry captured with aerial photogrammetry supports the use of body morphometry as a reliable and well-tested method.

Animal body size and growth patterns play important roles in shaping the life history of species. Baleen whales include the largest animals on the planet, with somatic growth costs expected to be substantial. We used unmanned aerial vehicle photogrammetry and long-term individual sighting histories from photo identification (1991-2019) to estimate the cost of somatic growth for southern right whales (SRWs) Eubalaena australis. A Richards length-at-age growth model was developed, based on 161 calves, 20 yearlings, 1 juvenile and 23 adults, ranging in age from newborn to 27 yr. Predicted lengths were 4.7 m at birth, 12.5 m at minimum age of first parturition (6 yr) and an asymptotic length of 14.3 m. A volume-at-age curve was estimated from the body volume versus length relationship, and converted to a mass-at-age curve, using data on body tissue composition of North Pacific right whales E. japonica (n = 13). The energetic cost of growth was estimated using published estimates of tissue lipid and protein concentrations. The cost of growth for SRWs (in MJ d-1) was 2112 at birth, 544 at 4 mo, 314 at 1 yr (~weaning age), 108 at 5 yr (minimum age of sexual maturity), 51.5 at 10 yr and 5.2 at 30 yr. The cumulative cost to age 30 was 764.3 GJ, but varied widely (458-995 GJ) depending on the tissue energy content. Our estimates represent a healthy SRW population, and provide a baseline to investigate individual and population level impacts of anthropogenic disturbance (including climate change).