Estimating reproductive costs in marine mammal bioenergetic models: a review of current knowledge and data availability

Abstract

Reproductive costs represent a significant proportion of a mammalian female's energy budget. Estimates of reproductive costs are needed for understanding how alterations to energy budgets, such as those from environmental variation or human activities, impact maternal body condition, vital rates and population dynamics. Such questions are increasingly important for marine mammals, as many populations are faced with rapidly changing and increasingly disturbed environments. Here we review the different energetic costs that marine mammals incur during gestation and lactation and how those costs are typically estimated in bioenergetic models. We compiled data availability on key model parameters for each species across all six marine mammal taxonomic groups (mysticetes, odontocetes, pinnipeds, sirenians, mustelids and ursids). Pinnipeds were the best-represented group regarding data availability, including estimates of milk intake, milk composition, lactation duration, birth mass, body composition at birth and growth. There were still considerable data gaps, particularly for polar species, and good data were only available across all parameters in 45% of pinniped species. Cetaceans and sirenians were comparatively data-poor, with some species having little or no data for any parameters, particularly beaked whales. Even for species with moderate data coverage, many parameter estimates were tentative or based on indirect approaches, necessitating reevaluation of these estimates. We discuss mechanisms and factors that affect maternal energy investment or prey requirements during reproduction, such as prey supplementation by offspring, metabolic compensation, environmental conditions and maternal characteristics. Filling the existing data gaps highlighted in this review, particularly for parameters that are influential on bioenergetic model outputs, will help refine reproductive costs estimated from bioenergetic models and better address how and when energy imbalances are likely to affect marine mammal populations.

Assessing the Population Consequences of Disturbance and Climate Change for the Pacific Walrus

“…As such, we have identified several data gaps in the Pacific walrus and marine mammal bioenergetic literature that had been filled with assumed values in the present model. Key research needs related to Pacific walrus physiology include activity-associated metabolic rates for growing calves, further research on walrus diet and prey density in the Bering and Chukchi seas, and a more robust characterization of assimilation efficiency, true metabolizable energy, and the heat increment of feeding (e.g., Booth et al, 2023). Despite the necessary assumptions we have made within this modeling framework regarding bioenergetics, its strength lies in its flexibility to compare and contrast different parameter values and projected environmental conditions.…”

“…During pregnancy, a female must cover the costs of fetal growth and maintenance (Equation 8) for the fetus to survive (reviewed in McHuron et al, 2023). Thus, the mass of the growing fetus is added to the structural mass of the pregnant female, and fetal maintenance is included in her overall metabolic cost ( FM t ), applying a resting metabolic rate to the fetus’s body mass.…”

Extraordinary diversity of the pinniped lactation triad: lactation and growth strategies of seals, sea lions, fur seals, and walruses

“…Otariids ( n = 4 species, Schulz and Bowen, 2004 ) provide significantly less daily energy relative to maternal mass 7‐23 (MJ/kg)/d; however, given this investment continues for months to years the overall energy investment is much higher compared with phocids. Despite numerous studies evaluating the cost of lactation in pinnipeds, daily energetic cost for the female remains a significant bioenergetics question ( McHuron et al, 2022 , 2023 ).…”

Using audio and UAS‐based video for estimating tidal lung volumes of resting and active adult gray whales (Eschrichtius robustus)

“…All monitored winter lagoon whales were observed in close association with calves of the year and were considered to be adult females, fasting and lactating (Rice & Wolman, 1971). These females were monitored from small boats as they “logged” at the surface or swam slowly against tidal currents, and were assumed to be in a resting state, as the metabolic costs associated with milk production, exclusive of the energy value of its lipid content, are assumed to be negligible (Costa & Gentry, 1986; McHuron et al, 2023).…”

“…"logged" at the surface or swam slowly against tidal currents, and were assumed to be in a resting state, as the metabolic costs associated with milk production, exclusive of the energy value of its lipid content, are assumed to be negligible (Costa & Gentry, 1986;McHuron et al, 2023).…”