Temperature Sex Determination in Frogs?


Over the many years I’ve been working with frogs, I’ve noticed that certain types of frogs display a skew in the sex ratio.  Dendrobates tinctorius for instance are well known for their heavy female sex ratio, while most of the thumbnail dart frogs I have worked with are heavy on males. 

I’ve often wondered why these sex ratios occur, but assumed it must have something to do with their natural history or some strategy which helped each different locality or species group survive.  For the most part these sex ratio skews are very reliable and seem to consistently appear in the offspring of various different breeders all over the US and even in shipments of frogs bred in Europe, so this lends credence to the idea that this phenomena is inherent in the frogs natural history, not something related to diet, temperatures, water ph, or any of the various factors that might influence tadpole development.

However I also work with some “non dart frog” type frogs, various tree frogs and terrestrial frogs, and one species in particular came to my attention as having a heavy sex ratio skew shortly after I started working with it…the Vietnamese Mossy frog, or Theloderma corticale.  I started breeding these frogs back in the early 2000’s. My original group was around 20 frogs that were imported from Thailand, and this group was slightly male heavy but really nothing remarkable in this regard.  However as I produced babies, and sold groups to customers I found that a year or so later I was getting a few people coming back and asking how to take care of eggs their groups were producing, but these folks always had a 3.1 or even a 5.1 group.  Other customers got all males, and frogs I held back seemed to be running at around 7 to 10 males per female.  Then I started hearing about people who had bought groups of ten or twenty from other breeders, in the US and Europe, and had gotten ALL males!  In fact I very rarely heard of anyone producing females except myself. Conditions I was rearing tadpoles in were not special, they were the same as our dart frogs, with water temps running around 72-74 degrees. 

Some where in this time line, a few years after I had begun breeding these frogs, I wrote an article on keeping and breeding them for Reptiles Magazine, and in the course of writing this article, I did some research on their natural history.  This was pretty interesting, and I ran across an article originally written in Russian by a couple of Russian researchers/herpetologists who had observed these frogs in their native habitat….turns out these frogs experience some pretty cool temps there, in the highlands of Northern Vietnam.   In addition, the authors of this article suggested that these frogs might be temperature sex determined, (TSD) which was definitely a first for me, that is the first time I had seen any one suggest that an amphibian might be temperature sex determined. 

I also work with another frog, the Cinnamon frog, (Nyctixalus pictus), which is from the same family, and seemed to be having the same experience with these frogs. I kept back a couple of groups of frogs and got all males, and had a few complaints from customers that their groups were all males.  This was sort of hard to figure out, this frog comes from lowland rainforests, and while the range does extend up into the lower levels of some mountainous areas, it is of course unclear where imported specimens originate. 

So, since then I have done some experimentation with raising tadpoles at lower temps, and had some interesting results. In 2014 I raised eight mossy frog tadpoles in a basement closet, with the froglets emerging in the spring of 2015. Temperatures ran between mid and upper sixties.  One downside to this process is that the mossy frog tadpoles development slows to a crawl at these temps, with the complete process taking up to a year.   I have so far found that four of the eight frogs are males, but the remaining four are possibly females, so far these frogs are near adult size and no calling! 

The Cinnamon frogs are a much easier subject.  I moved them to the floor in my main frog breeding area, and got water temps down to between 70 and 72.  This was done around April of 2015, and last fall a couple of different groups of young Cinnamon frogs that came out of these tubs produced viable eggs, at the ripe old age of four to five months old!  I will continue to raise all of my Cinnamon frogs this way, and there should be plenty of females in the offspring offered for sale!

So, I’m raising a good percentage of my mossy frog tadpoles in the colder temps, and I’m making them available.  They will be a little more expensive due to the longer time required to raise them, and of course I  can’t guarantee the sex of the offspring, but I am very confident that there will be a much higher percentage of females in the juveniles we are offering now.