Temperature Sex Determination in Naja

This subject has been written about in scientific papers on turtles and lizards, and in one I read it mentioned that they didn't think incubation temps had any effect on snake eggs, but they hadn't done any experiments. Here is what I have experienced in my limited breeding experience.

Up to and during the 2004-2005 breeding season, I was keeping my hot rooms at 29.4 - 30 degrees C. at all times. These temps are what I was told to use since I started keeping Cobras and other PNG elapids in 1999. I cooled my snakes, then warmed them back up to 29.4 - 30 C. The only Cobra I bred that year were Sri Lankan Spectacled Cobras. I introduced the female to the male, and they copulated at 29.4 - 30 C. The snakes were kept at those temps for the duration of egg formation inside the female. When the eggs were laid, I incubated them on a shelf in my hot room at those same temps. Here is the result:

Sri Lankan Spectacled Cobra - 12.5

I kept my hot rooms at those temps, and during the 2005-2006 breeding season, I did everything the same way, at the same temps. Here are the results from the 3 clutches of Naja eggs I got in 2006.

N. siamensis (B&W Phase) - 12.3
N. siamensis (Black Phase) - 11.6
N. sumatrana (Yellow Phase) - 5.1

When I saw these results, I wasn't very happy to say the least. Then, I remembered reading about lower incubation temps in some reptiles resulting in more females. So I immediately lowered the temps in my hot rooms to 27.2 - 28.3 degrees C.

So, for the next several months until mid October 2006, my snakes were kept at 27.2 - 28.3 degrees C. In mid October, I cooled the snakes for 45 days, then warmed them back up to 27.2 - 28.3 degrees C. I introduced the females to the males, they copulated, and the eggs formed in the females at those temps. When the eggs were laid, I incubated them on the same shelf, as the two years before, but at these lower temps. I had sold the Black Phase B&W Spitters, but I bred the exact same pairs of the other two. Here are the results.

N. siamensis (B&W Phase) - 7.13 compared to 12.3
N. sumatrana (Yellow Phase) - 2.2 compared to 5.1

Someone on a Cornsnake forum told me my results can not even be considered because I didn't do the experiment properly. It wasn't an experiment to begin with, but they said to do it properly, I have to separate each clutch into two even groups when the eggs are laid, then incubate them at the same time at different temps, and then see what the results are.

My argument to that was, what if sex was determined by temps during the time the eggs were forming in the female? They had no answer.

My females were kept at high temps during egg formation the first 2 years, and the eggs were incubated at high temps, and I got mostly males. When the eggs formed in the females at lower temps, and were incubated at the same low temps I got even or more females, even with the exact same pairs of snakes. I don't think you can say I have absolutely proven temps can determine sex rations on snakes, but I think it does show it may be possible.

I also bred several pairs of Cornsnakes and 2 pairs of Apalachicola Kingsnakes in the 2006-2007 season. All but one clutch resulted in close to even ratios, and that one was 3.11. I had never bred Corn or King Snakes before, but a buddy that has bred them for many many years, has noticed more even ratios or higher female clutches, since he lowered his temps.

I'm very interested in hearing what results you have obtained, especially in Naja species.

Hello Randy!

VERY interesting story! I can not give any information about Naja spec., but over the last three years I incubated nearly 300 Echis ocellatus-eggs, maybe it´s comparable.In the first year I incubated the eggs at an average temperature of 29,5 °C. The ratio:2/3 males, 1/3 females.
The next year I incubated the eggs at 28,5 °C, the result was the same. Last year I incubated the eggs at 26,5 °C to get possibly more females, but the result was still the same.
There are two possible conclucions:

1. There are gonosomes which determine the sex of the hatchings.
or
2. Your assumption is right and the sexes of the hatchings had been formed during the gestation of the female (I kept my Echis all the same over the years).

In Echis ocellatus the ratio I observed make sense, if you take a look on the mating behaviour of the males. They try to copulate and combat with everything that moves for nearly 3-4 month without any break! I´m pretty sure that in the wilderness males are much more frequently found and killed by enemies then the shy females. In this case a gonosomal sex determination would make sense to make sure, that everytime there are enough males to keep the population alive.
Maybe in Naja spec. it´s not as drastic, because most Naja´s are big and should not have as much enemies as a small Echis. So they don´t need such a fixed sex ratio and TSD is maybe possible.

As far as I know all studies about this theme result in the same: TSD does not exist for snakes (GORMAN 1973, BULL 1980, Xiang et al. 2001). For Boa constrictor occidentalis gonosomes have been detected, for other snakes I don´t know.
BUT there have been investigations on a lizard (Gekko japonicus) and it has been shown, that they have TSD and gonosomes togehter (YOSHIDA & MSAHIRO 1974, TOKUNGA 1985), so maybe it´s possible in snakes as well. In the past I spoke to some snakekeepers about this theme and some of them swear that they observed TSD in the snakes they breed (especially kingsnakes).
And as far as I know noone made investigations about the sex ratio of hatchings depending on the temperature of the female during the gastation. Definetly a highly interesting thing, hopefully someone can tell some more about that, or knows someone who can.

Best regards

Frank

Hey Frank,

Thanks for the info. I'd like to do more experiments, but it's hard to go back to the warmer temps after getting more even ratios at the lower temps. Especially when you get so few eggs from some species.

I've only bred 1 pair of Naja and 1 pair of Cornsnakes that were the same exact breedings as last year, so I will see if I get the same or close ratios from those. I made quite a few new breedings with Corns, Kings, and some Naja species, and they were all kept at the lower temps during the year, during copulation and gestation, as well as incubation.

Hi Randy,

I understand your descition not to go back to the warmer incubation very well. Who would do that after such results. It would be easier to do it with not such financially high-class animals, for example cornsnakes. I´m thinking about to build a new incubator to do experiments with my Echis ocellatus-eggs again. I have two pregnant females now, and both of them should lay around 50 eggs (what a clutch size for such small snakes!). This time I would split the clutches and incubate one half at 26°C and the other at 31°C. Maybe I find the time to do so, my females should lay their eggs at the end of May. But it depends as well, on the eggs of my other snakes. I don´t want to make that experiment with the Inlandtaipans, I think everyone understands
And at the moment I don´t have the financial background and space to build two new incubators just for this experiment.
As I said, hopefully someone can tell us more about the scientific background of that phenomenon you observed.

Best wishes

Frank

Wow, that is a lot of eggs. I had one E. ocellatus for a few days, then it died. It was shipped to me in a cloth bag, which was sitting on top of a 7' male Papuan Taipan, who was in a clotch bag. Then both bags were placed into a mesh bag, then placed in the crate. A heat pack was sitting beside the E. Ocellatus, on the Taipan. They were shipped from south Florida to North Carolina, where I was living at the time. The wood crate was lined with styrofoam, and the inside measurements were 26cm long x 15.8cm wide x 10.8cm deep. The air holes were covered by the airlines stickers. The Taipan was dead on arrival, and the ocellatus died a few days later. I called the dealer immediately upon finding the dead Taipan and emailed them photos of it. I told them the crate was way too small, and the air holes were covered. Also, that the heat pack was sitting on the Taipan, instead of having something between the heat pack and snakes, like newspaper, of which there was none at all. The owner of the company said he would give me a discount on future purchases. I lost $475 on that deal. Needless to say, I have not purchased from them since. How could I trust them next time.

Anyway, got off the subject there. The only way I think you can do the TSD experiment to check gestation temps as well as incubation temps is if you breed several pairs of snakes, after keeping them, breeding them, gestation occuring at, and incubating all the eggs at higher temps the first year.

Once the eggs are laid, start keeping those snakes at lower temps, and do everything the same, especially pairing the same snakes the next year, and see what you get compared to the year before. Then do it again at higher temps, and then at lower temps.

If you separate eggs of one clutch, or breed females to different males each year, it may cause the results to be questionable. If the same pair of snakes are bred year after year and you alternate the temps each year, and you get more males with higher temps, and more females at lower temps, then I think TSD is happening in snakes, though it may be during gestation, rather than incubation.

This will be the 3rd year breeding my original pair of Yellow Phase N. sumatrana. I got 5.1 at higher temps the first year, 2.2 at lower temps last year, and I have 6 eggs incubating now at lower temps, same as last year. If I get 5.1 or 4.2, maybe temps have nothing to do with sex. We will see. They hatched in 74 days last year, so they should hatch around July 3rd.

Hello Randy,

i will get my data together and post it here as well.
i have not observed TSD in Viper species so far... but i can tell you, with our Cobras, i got the same feeling as you for that matter. lower temps was resulting in more females as well at HerpaWorld over the years...


cheers
Mario

Thanks Mario

I will check with some friends here that have bred different species. See where they stand on the issue. I'm not positive, but I believe Mitch (Diamond Reptiles) once told me he has experienced more females at lower temps. I'll talk to him more about this and see what species he has seen it in.

Hi Randy
Interesting! As I have a friend that bred Aspidelaps Lubricus Lubricus for many years and he always had a very high male ratio! He was also incubating the eggs at about 29 C and I told him to lower the temps or hatch them at room temperature. He did this and from then on his male/ female ratio was just about even for about 5 years after which he gave up the Aspidelaps!
Years ago a local snake park bred Crotalus Simus and Adamanteus for a couple of years in a row and the cages had underfloor heating and were kept very warm all year round! 30 C and above and I know they had hardly any females from either species on both occassions!
I keep my Adamanteus between 27 and 30 all year round and I have such a high female ratio I try and sell trios or lone females. I never have enough males and that is 3 years in a row now!
We still have lots to learn I reckon!
Cheers
Gavin

yep - we do have much to learn... but, we gathering informations, and lets see what will come up - the main purpose of this board... i like it...

thanks randy for bringing it up, good job!


cheers
Mario

Thanks Gavin, Mario

This topic is something that is always on my mind. Some of my first breedings were very male heavy, and for some reason, in America, everyone wants females of whatever species of snake. Whether they think having more females in their collection is better, or they have only been able to find males, so they need females, I don't know.

I wonder what kind of research has been done, where they checked egg temps of wild snakes, and were then able to check the sex of the hatchlings. It seems to me that mother nature would make sure there were enough males and females in the wild. It would be hard to do this with most species as they hide in burrows and such. I know some temps of King Cobras eggs/nests has been done. I believe I saw Rom Whitaker doing that on a tv show. Whether they were able to catch the hatchlings I don't know.

Here is what I have so far. I've included info from above, so it's easier to compare, but only the info from breedings of the same exact specimens bred to each other for 2 or more years. Including a single breeding, tells us almost nothing, since we have no other years of breeding to compare it to.

The temps you see, are the temps the snakes were kept at all the time, day and night, except during brumation. They are also the temps at which the eggs formed in the females, and they are the temps the eggs were incubated at.

2005/2006 - 85 - 86F 29.4 - 30 C
Naja siamensis - 56 days from lay to hatch - 12 . 3
Naja sumatrana - 64 days from lay to hatch - 5 . 1

2006/2007 - 81 83 F 27.2 . 28.3 C
Naja siamensis - 62 days from lay to hatch - 7 . 13
Naja sumatrana - 74 days from lay to hatch - 2 . 2

2007/2008 - 80 - 82 F 26.6 - 27.7 C
Naja sumatrana - 78 days from lay to hatch - 2 . 4

It seems to me that temperature DOES have some bearing on sex determination. It may be during formation of the eggs in the female, or during incubation, or both, but I have to say these numbers don't lie.

This year I also got 1.4 N. oxiana hatchlings. I have not bred them before, so I have nothing to compare the results to, but I did get more females than males at 2007/2008 temps. Next year I will be using the same temps as I did this year or last year. I may experiment with some cheap Corn Snakes and King Snakes next year. I will not go back to the higher temps with my venomous and high end non-venomous.

Anyone else with results please speak up.

Here are the 2009 results of my Naja atra breeding, compared with breeding results from seasons 2006 - 2008.

2006 - 2008 incubation temperature 29 - 30 C -> 30 % males

2009 incubation temperature 24 - 25 C (first ten days, then 26 - 27 c.
Result: 100 % males!

Peter

Hey Peter,

First, sorry to hear you got all males at any temp. But, can you tell me at what temps you kept the snakes at before breeding, during breeding, and while the eggs formed in the females?

If you kept the snakes at different temps during egg formation and incubation, there may be some differences.

My findings have shown the opposite of your results. You got more males at lower temps, and I get less males at lower temps.

My N. siamensis eggs are just now hatching, but the female from earlier trials was bred to a different male this year.

The N. sumatrana eggs will start hatching around July 4th.