Canadian Journal of Zoology 74,1098-1109 (1996)
Diving performance of male and female Japanese cormorants
Yutaka Watanuki, Akiko Kato, and Yasuhiko Naito
Sexual differences in the diving behavior of the sexually dimorphic Japanese Cormorant, Phalacrocorax capillatus (males are 26% heavier than females), were studied at Terui Island, Hokkaido, using time-depth recorders. A typical dive cycle involved a rapid descent phase, a bottom phase where they remained for a while, an ascent phase, and postdive surface phase. Depth and duration across individual birds were greater for males (15.1 ± 3.7(mean ± SD) m,37 ± 5 s, respectively) than those for females (7.2 ± 2.4 m , 24 ± 4 s, respectively). While submerged, females spent a similar propotion of time during the bottom phase to males, hence foraging efficiency (propotion of time at the bottom to total dive cycle time) did not differ berween the sexes. No sexual differences were found in descent and ascent rates, dive bout duration, or time spent underwater per day. No significant effects of dive duration on postdive surface time were observed for either sex, indicating that birds dived within an aerobic dive limit. However, mean dive durations and maximum dive durations for individual birds were a function of body mass to the power 1.49 and 1.87, respectively, suggesting that body mass partly constrains the diving behavior of this opportunistically feeding cormorant.
BD96-2
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