Flexibility in animal foraging strategies can increase overall feeding efficiency. For example, group foraging can increase the efficiency of resource exploitation; conversely, solo foraging can reduce intraspecific competition, particularly at low resource densities. The cost-benefit trade-off of such flexibility is likely to differ within and among individuals. Reef manta rays (Mobula alfredi) are large filter-feeding elasmobranchs that often aggregate to feed on ephemeral upwellings of zooplankton. Over three years in the Maldives, we free-dived to film 3106 foraging events involving 343 individually identifiable M. alfredi. Individuals fed either solo or in groups with a clear leader plus between one and eight followers. M. alfredi were significantly more likely to forage in groups than solo at high zooplankton levels, and at certain locations. Both biotic and abiotic factors contributed to variation in group foraging. Within aggregations, individuals foraged in larger groups when more food was available, and when the overall aggregation was relatively small suggesting that foraging in large groups was more beneficial when food was abundant, and/or the costs of intraspecific competition were outweighed by the efficiency resulting from group foraging strategies. Females, the larger sex, were more likely to lead foraging groups than males. The high within-individual variance (over 55%), suggested individuals were unpredictable across all foraging behaviours, thus individual M. alfredi cannot be classified into foraging types or specialists. Instead, each individual was capable of considerable behavioural flexibility, as predicted for a species reliant on spatially and temporally ephemeral resources.

Collected in the field via video clips. Briefly:

We collected data on manta ray feeding behaviour at different times of day as part of the Manta Trust’s Maldives Manta Conservation Programme (https://www.mantatrust.org/maldives) during the Southwest Monsoon (May through November) when the highest numbers of mantas are present. Data collection lasted for 160 days in 2014, 156 days in 2015, and 155 days in 2016. We identified five sites on the eastern side of Baa Atoll that served as the main monitoring spots: Dhigu Thila, Hanifaru Bay, Hurai Faru, Reethi Falhu and Veyofushi Falhu. Due to the restriction on SCUBA diving at the main study site, Hanifaru Bay, all foraging data was collected by free diving. When we spotted foraging events, we free-dived beneath the animals to video their behaviour and record the unique identification spots on the ventral surface (Marshall & Pierce 2012), taking care not to impede or alter this behaviour. If, on entering the water, we observed more than one M. alfredi in the immediate vicinity, we dived to be in position before the first animal in the group swam overhead. If the animal was feeding, we filmed continuously until feeding stopped, until the last member of the group passed from sight, or we ran out of breath. We terminated filming once reaching the surface, and a clear view of the ventral markings was lost. Clips lasted between 30 and 180 seconds and were collected between depths of five and 16 metres. As feeding events could last up to a few hours, video clips of individual feeding groups were recorded on a consecutive basis as they arose until the feeding event finished or until the weather or daylight deteriorated. We filmed using a GoPro Hero 3, or a Canon PowerShot S110 compact camera. It was not possible to record data blind because our study involved focal animals in the field.