Wilson Rory P, Quintana Flavio
Institut für Meereskunde, Düsternbrooker Weg 20, D-24105 Kiel, Germany.
J Exp Biol. 2004 May;207(Pt 11):1789-96. doi: 10.1242/jeb.00967.
Air-breathing animals diving to forage can optimize time underwater by diving with just enough oxygen for the projected performance underwater. By so doing they surface with minimal body oxygen levels, which leads to maximal rates of oxygen uptake. We examined whether imperial cormorants Phalacrocorax atriceps adhere to this by examining dive:pause ratios in birds diving for extended, continuous periods to constant depths, assuming that the oxygen used underwater was exactly replenished by the periods at the surface. Examination of the cumulative time spent in surface pauses relative to the cumulative time spent in diving showed that surface pauses increase according to a power curve function of time spent in the dive or water depth. In a simplistic model we considered the rate at which birds expended energy underwater to be constant and that the rate of oxygen replenishment during the surface pause was directly proportional to the oxygen deficit. We then worked out values for the rate constant for the surface pause before using this constant to examine bird body oxygen levels immediately pre- and post dive. The model predicted that imperial cormorants do not submerge with just enough oxygen to cover their projected dive performance but rather dive with substantial reserves, although these reserves decrease with increasing dive depth/duration. We speculate that these oxygen reserves may be used to enhance bird survival when rare events, such as the appearance of predators or discovery of large prey requiring extended handling time, occur. The form of the oxygen saturation curve over time at the surface means that the time costs for maintaining constant oxygen reserves become particularly onerous for long, deep dives, so the observed decrease in reserves with increasing dive duration is expected in animals benefiting by optimizing for time.
通过潜水时携带刚好足够的氧气用于预计的水下活动,进行觅食的空气呼吸动物可以优化水下时间。这样做它们浮出水面时身体的氧气水平最低,从而导致最大的氧气摄取率。我们通过研究帝鸬鹚(Phalacrocorax atriceps)在持续长时间潜入恒定深度时的潜水与暂停比例,来检验它们是否遵循这一规律,假设水下消耗的氧气在浮出水面的时段恰好得到补充。相对于潜水累积时间,对浮出水面暂停的累积时间进行考察发现,浮出水面暂停时间根据潜水时间或水深的幂曲线函数而增加。在一个简单模型中,我们认为鸟类在水下消耗能量的速率是恒定的,并且浮出水面暂停期间氧气补充的速率与氧气亏缺成正比。然后我们算出浮出水面暂停的速率常数的值,之后用这个常数来检验鸟类在潜水前后瞬间的身体氧气水平。该模型预测,帝鸬鹚潜入水中时携带的氧气并非刚好足以满足其预计的潜水活动需求,而是带着大量储备潜水,尽管这些储备会随着潜水深度/时长的增加而减少。我们推测,当出现罕见事件,如捕食者出现或发现需要较长处理时间的大型猎物时,这些氧气储备可用于提高鸟类的生存几率。浮出水面时氧气饱和度随时间变化的曲线形式意味着,对于长时间、深潜水来说,维持恒定氧气储备的时间成本变得格外高昂,所以在通过优化时间而受益的动物中,观察到储备随潜水时长增加而减少是意料之中的。