Michimae Hirofumi, Wakahara Masami
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Evolution. 2002 Oct;56(10):2029-38. doi: 10.1111/j.0014-3820.2002.tb00129.x.
Larvae of the salamander Hynobius retardatus have two distinct morphs: normal and broad-headed, cannibal morphs. We performed three experiments to differentiate among the following hypotheses: The broad-headed morph is induced to allow: (1) feeding on nutritious conspecifics; (2) exclusion of strong competitors for food or space; or (3) feeding on large, tough prey when smaller prey items are unavailable. When newly hatched larvae were reared with a heterospecific, Rana pirica (an anuran amphibian) tadpoles, the broad-headed morph was induced more frequently compared with those reared with conspecifics. The phenotype expressed depended on the size of the tadpoles: The broad-headed morph occurred more frequently with small and the normal morph with large tadpoles. Metamorphosis occurred sooner in larvae fed conspecifics compared with those fed heterospecific tadpoles, and the mean growth rate of larvae fed conspecifics was significantly faster than that of those fed tadpoles, suggesting that the heterospecific tadpoles were less nutritive than the conspecifics. These results do not support the hypotheses that the broad-headed morph evolved for consuming conspecifics because of their better balance of nutrients or for excluding strong competitors for food or space. We tentatively conclude that the morph evolved to eat large, tough prey, including both conspecifics and heterospecific tadpoles. Because H. retardatus usually spawns very early in the spring in small ponds partially covered with ice and snow, newly hatched larvae may starve from the lack of proper food owing to extremely low water temperatures. Thus, the broad-headed morph of H. retardatus may represent a cold-habitat adaptation to overcome the severe circumstance when the only food items available are relatively large conspecifics or heterospecific tadpoles.
正常形态和宽头、同类相食形态。我们进行了三项实验，以区分以下几种假设：宽头形态的产生是为了：（1）以营养丰富的同类为食；（2）排除食物或空间的强大竞争者；或（3）在没有较小猎物时以大型、坚韧的猎物为食。当新孵化的幼体与异种（日本林蛙，一种无尾两栖动物）蝌蚪一起饲养时，与与同种幼体一起饲养的相比，宽头形态出现得更频繁。所表现出的表型取决于蝌蚪的大小：宽头形态在小蝌蚪中出现得更频繁，而正常形态在大蝌蚪中出现得更频繁。与喂食异种蝌蚪的幼体相比，喂食同种幼体的幼体变态发生得更早，并且喂食同种幼体的幼体平均生长速度明显快于喂食蝌蚪的幼体，这表明异种蝌蚪的营养价值低于同种幼体。这些结果不支持以下假设：宽头形态的进化是为了食用同类，因为它们的营养平衡更好，或者是为了排除食物或空间的强大竞争者。我们初步得出结论，这种形态的进化是为了食用大型、坚韧的猎物，包括同种和异种蝌蚪。由于日本山溪鲵通常在春季非常早的时候在部分覆盖着冰雪的小池塘中产卵，新孵化的幼体可能会因水温极低而缺乏合适的食物而挨饿。因此，日本山溪鲵的宽头形态可能代表了一种适应寒冷栖息地的特征，以克服在唯一可用食物是相对较大的同种或异种蝌蚪的严峻环境。
