He lieth under the shady trees - Referring to his usually inactive and lazy life. He is disposed to lie down in the shade, and especially in the vegetable growth in marshy places on the banks of lakes and rivers, rather than to dwell in the open field or in the upland forest. This account agrees well with the habits of the hippopotamus. The word here and in Job 40:22 rendered "shady trees" (צאלים tse'eliym), is by Gesenius, Noyes, Prof. Lee, and Schultens, translated "lotus," and "wild lotus." The Vulgate, Syriac, Rosenmuller, Aben-Ezra, and others, render it "shady trees." It occurs nowhere else in the Scriptures, and it is difficult, therefore, to determine its meaning. According to Schultens and Gesenius, it is derived from the obsolete word צאל tsā'al, "to be thin, slender;" and hence, in Arabic it is applied to the "wild lotus" - a plant that grows abundantly on the banks of the Nile, and that often serves the wild beasts of the desert for a place of retreat. It is not very important whether it be rendered the "lotus," or "shades," though the probable derivation of the word seems to favor the former.
In the covert of the reed - It is well known that reeds abounded on the banks of the Nile. These would furnish a convenient and a natural retreat for the hippopotamus.
And fens - בצה bitstsâh - "marsh, marshy places." This passage proves that the elephant is not here referred to. He is never found in such places.

He lieth under the shady trees - This and the following verses refer to certain habits of the behemoth, with which we are and must be unacquainted,

He lieth under the shady trees, in the covert of the reed,
and fens. This may be thought to agree very well with the river horse, the inhabitant of the Nile, where reeds in great plenty grew, and adjoining to which were fenny and marshy places, and shady trees; and, as historians relate (e), this creature takes its lodging among high reeds, and in shady places; yea, the reeds and sugar canes, and the leaves of the papyrus, are part of the food on which it lives; and hence the hunters of them sometimes cover their bait with a reed to take them; though it must be allowed that the elephant delights to be about rivers, and in clayey and fenny places (f), and therefore Aelianus (g) says it may be called the fenny animal.
(e) Ammian. Marcellin. l. 22. Bellonius & Achilles Tatius apud Bochart ut supra. (Apud Hierozoic. par. 2. l. 5. c. 14. col. 760.) (f) Aristot. Hist. Animal. l. 9. c. 46. Plin. l. 8. c. 10. Aelian. de Animal. l. 9. c. 56. (g) lbid. l. 9. c. 24.

lieth--He leads an inactive life.
shady trees--rather, "lotus bushes"; as Job 40:22 requires.