Sharks and Birds: Showing Off For the Ladies
Satellite tagging allows both terrestrial and marine animals to be followed remotely over long periods of time. While financial and technological drawbacks to this methodology exist, satellite tags are effectively used to track many different species. For marine animals, among the most difficult to follow, satellite tags can provide geographic location, water depth and temperature, and ambient light levels. Integrating these data points can determine where an animal is and when, but typically cannot identify what the animal is doing there.
A recent paper by Jorgensen et al reanalyzes a large set of white shark (Carcharodon carcharias) satellite tag data. In the Northeastern Pacific, white sharks spend August through January near the coastal California habitat of seals and sea lions. During the rest of the year the animals range off-shore as far as Hawaii. Male sharks in particular, spend much of their time near an area named the “White Shark Café”, a communal meeting ground midway between Baja and Hawaii. While female white sharks also visit the Café, they typically stop there only briefly. What are the sharks doing at the Café -- feeding, mating, something else not yet documented? One clue to the purpose of this aggregation site comes from a unique behavior sharks display while there, a repeated series of deep, fast dives called Rapid Oscillatory Diving (ROD).
A detailed analysis of existing data from pop-up archival (PAT) tags identified four different modes of diving behavior exhibited by white sharks. “Coastal” mode was characterized by swimming in the upper 30 meters of the water, and appears to indicate sharks cruising for seals and other prey. “Travel” mode occurs primarily between California and the Café, reflecting near-surface, long distance migrations. “DVM” mode, for diel vertical migration, is found throughout the sharks’ range and represents feeding behavior as the sharks follow prey found between 350-500 meters during the day, which then rise to 200 meters at night. Lastly, and most interestingly, “ROD” mode occurs primarily in the Café and includes repeated diving between 30 and 200 meters. Most of these behaviors are performed by both sexes, but ROD is practiced largely by males within the Café.
Analysis of ROD patterns showed males converging at the Café to practice repeated ROD movements, while female white sharks make fewer and briefer passes through the Café. In the absence of evidence for a unique resource exploited by males at the Café, the authors instead propose a fascinating hypothesis. Although still to be tested, they compare the resident male/transient female aggregation of the Café to a lek-type breeding ground. Best known in birds, leks are established mating locations where males gather, and often compete in some manner, to find a mate. In grouse of the North American prairies, the competition involves dancing, calling or “booming”, and sometimes altercations between individuals. Females visit for briefer periods of time, typically only long enough to select a mate or mates.
What role does ROD play in potential shark lekking behavior? The authors propose that the purpose of ROD may be to allow males to survey the water column for the scent of reproductive females. As lek competition often favors the most fit males, however, and repeated ROD is likely to be energy intensive, this activity may indirectly select for high condition males.
The article is: Jorgensen SJ, Arnoldi NS, Estess EE, Chapple TK, Ruckert M, Anderson SD and Block BA. (2012) Eating or Meeting? Cluster analysis reveals intricacies of white shark (Carcharodon carcharias) migration and offshore behavior. PLoS ONE 7(10): e47819.
It is available open access at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0047819