Discovery of the male wood frog.

April 4. 

Male Wood Frog (Rana sylvatica) amplexing a female

P. M. —Go to the cold pond-hole south of J. P. Brown’s, to hear the croaking frogs. 

They are in full blast on the southwest side, where there have been some birches, etc., cut the past winter, and there is much brush fallen in the water, whose shelter they evidently like, and there they have dropped their spawn on the twigs. I stand for nearly an hour within ten feet on the bank overlooking them. 

You see them lying spread out, or swimming toward one another, sometimes getting on to the brush above the water, or hopping on to the shore a few feet. I see one or two pairs coupled, now sinking, now rising to the surface. 

The upper one, a male, quite dark brown and considerably smaller than the female, which is reddish--such part of her as I can see--and has quite distinct dark bars on its posterior extremities, while I cannot discern any on the male.

But the greatest commotion comes from a mass of them, five or six inches in diameter, where there are at least a dozen or fifteen clinging to one another and making a queer croaking. From time to time a newcomer adds himself to the mass, turning them over and over. [It was an incessantly struggling mass. You could have taken up a dozen or fifteen in your two hands.]The water is all alive with them for a couple of rods, and from time to time they croak much more generally than at others, evidently exciting one another to it, as do the R. halecina. 

Before I caught any of them I was only struck with the fact that the males were much smaller and very much darker, though I could see only one female partially. At length, when all the rest had been scared to the bottom by nearer approach, I got near to the struggling mass. They were continually dropping off from it, and when at length I reached out to seize it, there were left but two. 

Lifting the female, the male still clung to her with his arms about her body, and I caught them both, and they were perfectly passive while I carried them off in my hand. To my surprise the female was the ordinary light-reddish-brown wood frog (R. sylvatica), with legs distinctly barred with dark, while the male, whose note alone I have heard, methinks, was not only much smaller, but of a totally different color, a dark brown above with dark-slate colored sides, and the yet darker bars on its posterior extremities and the dark line from its snout only to be distinguished [on] a close inspection. Throat and beneath, a cream white, like but clearer than the female. 

In color, a small bullfrog which I had caught [probably R. fontinalis], and any other frog that I know, was more like the female than these males were. I have caught the female in previous years, as last spring in New Bedford, but could find no description of him and suspected it to be an undescribed frog. 

It seems they were all (of this mass) about one female, and I saw only one other in the pool, but apparently only one had possession of her. There was a good deal of spawn firmly attached to the brush close to the surface, and, as usual, in some lights you could not see the jelly, only the core. 

I brought these frogs home and put them in a pan of water.

Sophia has brought home the early large-catkinned willow, well out; probably some yesterday at least.

H. D. Thoreau, Journal, April 4, 1858

Go to the cold pond-hole south of J. P. Brown’s, to hear the croaking frogs.See April 13, 1855 ("The small croaking frogs are now generally heard in all those stagnant ponds or pools in woods floored with leaves, which are mainly dried up in the summer. . . . We, hear them at J. P. Brown’s Pond, which is edged with ice still on the north.")

To my surprise the female was the ordinary light-reddish-brown wood frog (R. sylvatica), with legs distinctly barred with dark., while the male, . . .was not only much smaller, but of a totally different color, a dark brown above with dark-slate colored sides.  See April 4, 1857 ("Caught a croaking frog . . . Above it was a uniform (perhaps olive?) brown, without green, and a yellowish line along the edge of the lower jaws."); See also  May 27, 1852 ("Catch a wood frog the color of a dead leaf. "); June 29, 1852 ("The mud turtle is the color of the mud, the wood frog and the hylodes of the dead leaves, the bullfrogs of the pads, the toad of the earth, the tree-toad of the bark."); May 30, 1854 ("Wood frogs skipping over the dead leaves, whose color they resemble."). September 12, 1857 ("I brought it close to my eye and examined it. It was very beautiful seen thus nearly, not the dull dead-leaf color which I had imagined, but its back was like burnished bronze . . . and reddish-orange soles to its delicate feet. There was a conspicuous dark-brown patch along the side of the head,"); October 16, 1857 ("I see a delicate pale brown-bronze wood frog. I think I can always take them up in my hand. They, too, vary in color,") See also A Book of the Seasons, by Henry Thoreau, the Wood Frog (Rana sylvatica)

~ According to Wikipedia female wood frogs are larger than males. Adult wood frogs are usually brown, tan, or rust-colored, and usually have a dark eye mask. The underparts of wood frogs are pale with a yellow or green cast. Individual frogs are capable of varying their color.

Females tended to be redder and bigger than the males, with the males being more brown and smaller. ~A Frog of a Different Color: Sexually Dimorphic Color in Wood Frogs by thelizardlog