I found the treefrog egg clusters of last week with little difficulty. They had been jostled slightly out of position, probably during a wrestling match between males, or during the amorous embrace of a couple.
This embrace, known as amplexus, is visually suggestive of an activity we humans would blush at, but since frogs practice external fertilization, it simply serves to ensure that only one male's DNA is the sole contributor to the next generation's diversity. Males develop horny pads (no pun intended) on their thumbs, and this enable them to tightly grasp the female until she lays her eggs. If this were not the case, I suspect it would be a free-for-all around a releasing female.
And they have grown! Notice in the photo how they have become elongated little grubs. Their yolk is clearly visible, between an obvious head and tail. I expect them to be swimming free in another week.
Pacific treefrogs not only have multiple color morphs within a population, but Wente & Phillips (2003, 2005) found that certain individuals within a population of "fixed" morphs were capable of gradual color change within a season, while most of the others remained the same color pattern from the juvenile stage. The color of these distinct morphs changed in response to the background brightness, not the hue, which they say makes it a seasonal adaptation, rather than one that might have evolved to help them when simply crossing from one background to another.
Last week's post showed a picture of the common classic bright green tree frog. But today, my son Evan was mowing the lawn and just caught a gray morph before it hopped under the mower. I have handled hundreds of Pacific tree frogs in northwestern Oregon, yet this is the first time I have ever seen this morph.
A little while later he brought in a dark green, but more normal striped pattern.
While photographing it with the flash on, it began changing to a paler but still green form. It maintained this coloration even when released into the pond.
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| (dark throat typical of actively singing males) |
I wondered if this might be the variable morph mentioned by Wente and Phillips, although it seems to be a bit more rapid than what they described.
References:
- Wente & Phillips (2003). Fixed Green and Brown Color Morphs and a Novel Color-changing Morph of the of the Pacific Treefrog. The American Naturalist.
- Wente & Phillips (2005). Seasonal Color Change in a Population of Pacific Treefrogs. Journal of Herpetology, Vol 39, No. 1.
I enjoyed reading this entry. I have been almost positive that I have seen our little tree frogs (hyla pseudocris) change background color from brown <=> green and have been flummoxed when reading in books that they cannot do this. Your experience confirms my strong but unscientific impressions (founded in memory but having no written records).
ReplyDeleteWhat you say about the "enthusiasm" about male frogs more than ready for amplexus is certainly true of Eastern Wood Frogs. I found my first Salamander one Spring in Michigan when I came across a large "wad" of wood frogs in a small woodland pond. The wood frogs were mating up a storm all over the pond, but here was this one wad of them, slowly writhing and rolling. I picked it up carefully after suddenly seeing a marbled flattend section of ... something. I was seeing the tail of a tiger salamander. I was very excited because I had looked for salamanders for many many years with no success. The male Wood Frogs were gripping the soft tender salamander flesh with an almost death-like grip. She must have felt just like a female frog to them! As I began to remove the wood frogs, I was afraid I was going to injure them because I had to pull so hard to get them to let go. I slowly removed them one by one. The Tiger Salamanders also mate and lay their eggs in such woodland ponds, although probably a week or two earlier in the year; so I suspect this was a female who had been captured by the wood frogs after laying her eggs and remaining in the pond too long.
Addendum: I found Tiger Salamander eggs in that pond that same day (after the frog incident), and raised three to adulthood in a 15 gallon aquarium. These larvae were voracious feeders. By the time they were morphing into adults, I was feeding them nightcrawlers. The largest of my three "foster children" was seven inches from snout to tip of tail.
Interesting -- would have been really weird if they'd turned out to be wood frog-tiger salamander hybrids. :)
ReplyDeleteLee