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

Molting is a crucial physiological process in various species, involving the renewal of integuments and requiring significant energy expenditure. In many seals, molting necessitates hauling out on land to maintain skin temperature above 17°C, leading to elevated metabolic rates and the assumption of fasting to conserve energy. Southern elephant seals (SES) undergo a particularly demanding 'catastrophic molt,' renewing both hair and epidermis. This process, coupled with the elevated heat loss in water, has led to the widely accepted belief that SES fast during their molt. However, this study aimed to investigate this assumption by examining the diving and feeding behaviors of molting female SES. The primary research question was whether SES fast during molting, or if they engage in at-sea activities to mitigate energetic constraints. Understanding the energetic costs of molting and individual adaptation strategies is essential, especially in the context of a rapidly changing climate impacting the Southern Ocean ecosystem. The study hypothesizes that females will remain on land and fast during early molting stages to minimize heat loss; however, it also explores the potential influence of weather conditions and body condition on at-sea behavior.

The literature extensively documents the high energetic demands of molting in various species, particularly marine mammals. Studies have highlighted the increased metabolic rate and heat loss associated with molting, necessitating adaptations like thick blubber layers to minimize energy expenditure. The prevailing view for SES has been that they fast on land during molting to conserve energy reserves in blubber. Existing research primarily focuses on the physiological aspects of molting, including metabolic rate measurements and energy reserve depletion. However, the literature lacks detailed investigations into the at-sea behavior and feeding patterns of molting SES. While studies on other seal species mention the potential for at-sea feeding during molting to supplement energy reserves, this aspect remains largely unexplored for SES. This study intends to fill this gap and provide crucial insights into the behavior of molting SES.

Between 2014 and 2022, 39 female southern elephant seals were captured at the Kerguelen Archipelago during their austral summer molt. Each seal was equipped with a Time-Depth Recorder (TDR) and a Stomach Temperature Pill (STP) to monitor diving behavior and ingestion events, respectively. The TDRs provided data on dive depth, duration, and surface intervals, enabling the distinction between diving and surface swimming. The STPs recorded stomach temperature changes, which were used to identify ingestion events. The researchers developed a Python algorithm to analyze the TDR data, distinguishing between surface swimming and diving based on depth and light level. The STP data were analyzed using an algorithm to identify sharp temperature drops indicating ingestion. Two published methods were employed to differentiate between water and prey ingestion events. The study also investigated the influence of environmental factors (temperature and wind speed) on the at-sea behavior using generalized linear models (GLMs). Principal component analysis (PCA) was used to create a swimming score that integrated various diving parameters. Linear mixed models (LMMs) were employed to analyze the relationships between physiological parameters, environmental conditions, and the occurrence of at-sea movements and ingestion events.

The study revealed that 79% of the monitored female SES engaged in swimming activities, and 61% exhibited ingestion events during the molting period. A total of 660 dives were recorded in 77 dive cycles, with an average dive duration of 7.7 minutes. Surface swimming events totaled 375, averaging 1.8 hours each. The average time spent at sea was 49.2%. The study identified 87 ingestion events in 80% of individuals, with 70% occurring during daytime. Among the ingestion events, 12% occurred during dives and 49% during surface swimming. Analysis indicated that 25-53% of the ingestions were potential prey ingestions. Environmental conditions significantly influenced SES behavior, with warmer temperatures and lower wind speeds increasing the likelihood of surface swimming. Surprisingly, the number of ingestion events and the amount of time spent at sea were not significantly linked to initial body mass index (BMI) or body mass. However, females that spent more time at sea and had more ingestion events exhibited less body mass loss. Analysis of stomach temperature showed that at-sea temperatures were higher than on-land temperatures, suggesting potential thermoregulatory benefits of at-sea behavior. Hyperthermia events were observed in some individuals, but these were not directly linked to immediate at-sea movements.

The findings challenge the traditional understanding that SES fast during molting. The significant occurrence of diving and ingestion events, even during early stages of molting, indicates that at-sea activities are not solely driven by energy restoration at the end of the molt. Rather, the behavior seems to be influenced by environmental conditions like temperature and wind, suggesting that thermoregulation may be a primary driver. The lack of a strong relationship between body condition and at-sea behavior further supports this. Females in better condition did not exhibit significantly more at-sea activity, suggesting that heat dissipation might outweigh the cost of cold-water immersion during molting. The observed similarity between diving profiles during molting and foraging suggests the possibility of prey consumption during at-sea excursions. This study underscores the importance of considering the interplay between environmental conditions and physiological needs in shaping the behavior of molting SES. Future research should focus on the precise composition of ingested materials and investigate the longer-term fitness consequences of these behaviors.

This study provides the first evidence that female SES do not consistently fast during their molt. A significant proportion of individuals engage in at-sea activities, likely driven by thermoregulatory needs and potentially supplemented by prey consumption. The results necessitate a re-evaluation of the accepted paradigm of obligate fasting during the molt, particularly in light of the changing climate. Future research should focus on determining the precise nature of ingested items, understanding the trade-offs between thermoregulation and foraging, and assessing the long-term fitness consequences of these behaviors.

The study's reliance on STP data for detecting ingestion events presents a limitation. Distinguishing between water and prey ingestion based solely on temperature changes is challenging, and the precise nature of the ingested material remains unknown. The Argos location data had limitations, particularly in precisely locating ingestion events. Moreover, the study focused solely on female SES; further research involving males would improve generalizability. Lastly, the study did not directly assess molting stage, potentially influencing the interpretation of some findings.