The likely explanation for this paucity is that during their early evolution in the mesozoic ( 245 million to 65 million years ago ) , mammals were small , secretive and nocturnal 哺乳动物锥色素减少的原因，可能是哺乳动物刚在2亿4500 ~ 6500万年前的中生代演化出来时，体型小、潜匿且夜行。
2.
" the calf was surfacing twice for every one of the mothers appearances at the surface . the reason for this is that the calf , having relatively small lungs , cannot remain underwater without breathing for any length of time 世界自然基金会海豚保护主任布莲诗博士解释：母豚每浮上水面一次，幼豚就得露面两次，这是因为幼豚的肺活量较少，无法长时间潜匿水中。
