Genetic identifications challenge our assumptions of physical development and mother–calf associations and separation times: a case study of the North Atlantic right whale (Eubalaena glacialis)

Abstract

While photo-identification is an effective tool to monitor individuals in wild populations, it has limitations. Specifically, it cannot be applied to very young animals before their identifying features have stabilized or to dead, decomposed animals. These shortfalls leave gaps in our understanding of survival, parentage, age structure, physical development, and behavioral variability. Here we report on 13 case studies of North Atlantic right whale, Eubalaena glacialis, calves that required genetics to track their life history data. These case studies revealed unexpected variations in mother–calf associations and separation times, as well as calf physical development. Prior to this study, calves were assumed to have died if their mothers were always alone on the feeding ground in the calf’s birth year. Using genetics and photo-identification, four such calves were discovered to be alive; two of the four possibly weaned earlier than expected at 7.5–8.0 months. To put these early separations in context, photo-identification data were queried and revealed that mothers and calves are seen apart from each other on the feeding grounds in 10–40% of all spring/summer sightings; previously, there were no published data on how often pairs are seen apart in the calf’s birth year. Two dead whales initially logged as calves of the year were discovered to be juveniles, thus allowing skewed survival estimates for calves of the year to be corrected. Genetically sampling animals early in their lives before they disperse or separate from their mothers provides an important means of individual identification at a time when photo-identification is not reliable.

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

“…Satellite telemetry data from a single leopard seal (Hydrurga leptonyx), a species where lactation durations are unconfirmed but speculations range from 10 days to 8 weeks (Southwell et al, 2003), provides preliminary support for a 2-week lactation duration (Kienle et al, 2022). In North Atlantic right whales, the use of genetic identification revealed that at least two calves never observed on the foraging grounds with their mothers survived, indicating successful weaning as early as 7.5 to 8.0 months of age on the northbound migration (Hamilton et al, 2022) A particular challenge in estimating total lactation costs is that there is a high degree of intraspecific variation in lactation durations in some species. Bioenergetic models and associated outputs, such as dynamic models estimating reproductive parameters, can be sensitive to lactation durations (New et al, 2013).…”

“…In some cases, this can result in lactation durations that are not well resolved, particularly if the composition of fisheries catches is nonrandom (Kasuya, 1978). Even when direct observations are available, they are often based on associations between an adult female and her presumed calf, which could result in erroneous estimates if they remain associated for reasons other than nursing, are not always together (a common occurrence), or are in fact not a mother-calf pair (Hamilton et al, 2022).…”

“…Lactation duration is one example of a parameter that is not only influential on bioenergetic model output (New et al, 2013) but also one that can be estimated from free-ranging animals across all taxonomic groups. The recent work of Feyrer et al (2020) and Hamilton et al (2022) highlight how new data can alter our perceptions of lactation durations and relationships between lactation duration and offspring survival. Prey supplementation by odontocetes calves is another example of a parameter that deserves further attention, both in terms of understanding temporal dynamics in free-ranging animals and in how important that contribution is to a calf's energy budget.…”

A multistate capture-recapture model to estimate reproduction of North Atlantic right whales

“…The probability of calf loss ( κ ) was assigned a Beta(2, 8) prior to reflect knowledge that it should be low. The probability of a missed summer calf ( ρ calf ) was assigned a Beta(5, 445) prior consistent with the observation of 5 calves during summer surveys that were without mothers (individuals that could themselves be sighted without a calf, despite calf survival) out of 450 successful calving events during 1990-2019; this informed prior was a necessary approximation given the lack of an alternative approach to modeling summer calf detection (for additional insights, see Hamilton et al (2022)). To improve mixing we used a sum-to-zero approach for the random effects (Ogle and Barber, 2020).…”

Artificial intelligence for right whale photo identification: from data science competition to worldwide collaboration

“…Some photographic data remain too challenging for semiautomated photo identification of right whales at this time, such as dead or decomposed animals, those showing only a portion of the body while engaged in a surface active group, or newborns with features that have not yet stabilized (Hamilton et al 2022).…”