It is generally expected that animals, including marine organisms, travel at speeds achieving the minimum energetic cost of transport. However, several factors cause variation in speeds within the energetically acceptable range. Light intensity is known to affect movement speeds in some flying and walking insects, which reduce speeds at low light levels. This is explained as compensation for degraded temporal resolution of vision in dim light by maintaining the rate of information gained per unit of travelling distance. Such a relationship between ambient light intensity and movement speeds is expected for any visual system in principle, but has not been examined in any marine species. As a mesopelagic forager, king penguins Aptenodytes patagonicus regularly commute between their breeding colonies and productive foraging areas over several hundreds of kilometres. During these trips, they experience a wide variation in light intensity between day and night, as well as within daylight hours, as dives often reach deeper than 100 m. The present study investigated diel patterns in the swim speeds of king penguins in relation to light intensity experienced within dives. King penguins gradually decreased their cruising speeds around dusk and increased them again around dawn. This resulted in consistently slower speeds in nocturnal dives. Correspondingly, the underwater light levels estimated were always higher in diurnal dives, even at depths greater than 100 m. The slower swim speeds after dusk may facilitate travelling and occasional prey detection in the dark. These results suggest that a common behavioural response to ambient light levels has evolved in different taxa of animals.
- Diel cycle
- Diving behaviour