Breaking the fast: first report of dives and ingestion events in molting southern elephant seals

Abstract

Southern elephant seals (SES) experience a 'catastrophic molt', a costly event characterized by the renewal of both hair and epidermis that requires high peripheral vascular circulation. Molting animals are therefore constrained by high metabolic heat loss and are thought to fast and remain on land. To examine the ability of individuals to balance the energetic constraints of molting on land we investigate the stomach temperature and movement patterns of molting female SES. We find that 79% of females swam and 61% ingested water or prey items, despite the cost of cold-water exposure while molting. This behavior was related to periods of warm and low wind conditions, and females that dived and ingested more often, lost less body mass. We conclude that the paradigm of fasting during the molt in this species, and the fitness consequences of this behavior should be reconsidered, especially in the context of a changing climate.

Fig. 1. Relationships between meteorological conditions and behavior of female southern elephant seals using.

A GLM predicted daily proportion of swimming females with daily mean wind speed (m/s). B GLM predicted daily proportion of swimming females with daily mean temperature (°C). C GLM predicted daily proportion of females that ingest with total daily sunshine duration (min/day).

Fig. 2. Daily mass loss per unit of body mass (g/kg/day) of female southern elephant seals according to their behavior.

A Diving score (PC1 as individual values on the first component of the PCA) and B number of ingestions per individual. Individuals with high PC1 values perform more dives, last longer, and show more surface swimming.

Fig. 3. Diving parameters during different at-sea behaviors: diving or surface swimming.

Surface swimming occurs between 1 and 5 m, while a dive is characterized by a maximum depth of more than 5 m. A dive or a surface swimming cycle is characterized by at least two successive dives or surface swimming separated by less than 20 min at the surface. A represents a surface swimming cycle and different profiles of dive cycles (B–D) of a female southern elephant seal in 2017. Each dive is defined by the succession of a submerged phase (blue area) and a surface phase (green area) (B).

Fig. 4. Stomach temperature variations (°C) during ingestion events recorded for two female Southern elephant seals in 2014 and 2019.

A Standard temperature variation. B, C Double ingestions. D Extreme temperature variations. T₁ is the initial temperature, T₂ the minimum temperature, T₃ the recovery temperature, ΔT the difference of temperature between T₁ and T₂, T_0.5 the half-way recovery temperature between T₁ and T₂ (T_0.5 = T₂ + ΔT/2) and t_0.5 the time of half-way recovery (black arrow: t_0.5 = t_T0.5 − t_T1). The area above the curve (integral) was calculated from the asymptote.

Fig. 5. Principal Component Analysis (PCA) including number of dives per individual, number of surface swims per individual, and mean dive or surface swimming duration.

The first component (PC1) accounted for 59.0% of the variation and the second component (PC2) accounted for 23.8% of the variation. PC1 was characterized by number of dives, number of surface swims, and mean dive duration. PC2 was defined by surface swimming duration. Individuals are reported on the PCA and identified through their behavior (red circles, diving; orange star, on land; blue triangle, surface swimming only). Orange star represents eight individuals on the same point.