Diving Into Diversity: Considering Intra-Specific Variation in Water Immersion Activity of Aquarium-Housed Magellanic Penguins.

Behavioral monitoring is essential for evaluating species-appropriate functioning, with growing recognition of the importance of understanding individual-level variation in modern zoos and aquariums. Yet animals housed in large groups, such as penguins, are seldom assessed at the individual level because of the practical difficulty of observing multiple individuals simultaneously. Using animal-borne immersion loggers, we quantified long-term (mean 233 days) and high-frequency (10-min intervals across 24 h) water-use patterns in 23 Magellanic penguins Spheniscus magellanicus housed at Sumida Aquarium, Tokyo, Japan from October 2022 to October 2023, comprising 13 males and 10 females, with 11 aquarium-born and 12 wild-born individuals. Penguins exhibited predominantly diurnal immersion, although some individuals regularly showed nocturnal immersion to a lesser extent, suggesting inter-individual variation in activity chronotypes. Monthly fluctuations were modest but partly aligned with phenological periods. Individual variation was substantial: although the group-level mean immersion was 35.2%, individual values ranged widely from 20.4% to 70.1%. Immersion behavior did not differ significantly by sex or birth origin; however, aquarium-born individuals showed lower immersion on weekends than on weekdays, indicating that individual characteristics can mediate responses to external stimuli. Furthermore, immersion activity was not associated with whether an individual ever developed bumblefoot, although wild-born individuals tended to show lower susceptibility to the condition. Meanwhile, higher baseline immersion was associated with fewer diagnostic episodes in penguins with bumblefoot, reinforcing that water use likely mitigates foot problems. Overall, this study underscores the importance of considering individual variation in welfare assessments and demonstrates the utility of animal-borne tags for continuous behavioral monitoring.