你是否注意过大象厚重的掌上长着几个脚趾？许多博物馆里的标本都是五个，但近日英国一项最新研究说，对一些大象脚掌的仔细分析显示，它们都长有“六趾”，与中国的国宝大熊猫一样长有“六趾”。
英国皇家兽医学院等机构研究人员在新一期美国《科学》杂志上报告说，分析了许多大象脚掌的标本，确认了“第六趾”的存在，不过这不是一个像其他五趾那样真正的趾头，而是由籽骨变异而成的具有脚趾功能的结构。
籽骨是许多动物体内都有的一种小骨头，可以起到支撑肌腱的作用。但本次研究发现，许多大象脚后跟处的籽骨变大变长，已经形成了类似其他五个脚趾骨的结构，它伸出的方向与五个脚趾骨相反，在脚后跟的位置帮助支撑大象身体重量。
领导这一研究的约翰·哈钦森教授说，其实在很久以前就有研究人员在解剖大象时发现了籽骨结构，但都把这当作一块奇怪的软骨，并没有多加注意，很多博物馆在设置大象标本时也没有加上这个“第六趾”。
大象的“第六趾”被长期忽视还因为大象脚掌难以研究。活体大象不易麻醉，它厚厚的脚掌也不容易用X光等手段来透视。哈钦森从动物园获得了多头大象去世后的脚掌样本，经过比对分析才能够确认。
他还指出，这是进化中反复出现的一个现象，即可以将籽骨变异成一个具有手指或脚趾功能的结构，大熊猫的“六趾”现象也是如此。大熊猫的脚掌上有一个乍看像人手大拇指的结构，但仔细观察就会发现，它的五个脚趾都并排长在一起，这个“拇指”是由其脚掌边缘的籽骨变异形成的，用于抓握竹子。(生物谷Bioon.com）
doi:10.1126/science.1211437
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From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution of Elephant “Sixth Toes”
John R. Hutchinson, Cyrille Delmer, Charlotte E. Miller, Thomas Hildebrandt, Andrew A. Pitsillides1, Alan Boyde
The enlarged radial sesamoid bones of giant panda forefeet (1, 2) are classic examples of evolutionary exaptation (3, 4): co-option of old structures for new functions. It is less widely recognized that such “sixth toes” or “false thumbs” have evolved convergently in numerous tetrapods, such as moles and frogs (5, 6). They exist in numerous mammals in a less enlarged state, variably called the prepollex/prehallux (here called predigits), radial/tibial sesamoids, or other terms (such as falciform, accessory scaphoid, or navicular). Whether these sesamoids are ancestrally or convergently evolved in various tetrapod clades remains to be determined. The latter seems likely, given the absence of similar sesamoids in most fossil outgroups, yet a cartilaginous nodular precursor cannot be excluded. Regardless, enlarged sesamoids are quite prominent in both the manus (forefeet) and the pedes (hindfeet) of elephants, where they have been mistaken for sixth digits or otherwise presumed to play a role in foot support (7–9). Indeed, the recent discovery that moles have developmentally switched their radial sesamoid (prepollex) to a digit-like identity (10) intimates that elephants and other species may have done the same. Here, we report a multidisciplinary anatomical, histological, functional, and phylogenetic analysis (11) of the predigits in elephant feet. We hoped this would illuminate how elephants evolved their characteristic subunguligrade (nearly “tip-toed,” with only distal toes contacting the ground) foot posture and function, as compared with the plesiomorphic plantigrade (“flat-footed,” with wrists/ankles contacting the ground) foot posture in many other tetrapods.
