Agree with above answer, formalin for fixation is needed, take tissues first though for later DNA and freeze those is a good idea, you can then store in 70% ethanol. If you think about it, you cannot really expect anything that prevents deterioration to be healthy for living tissue, including us. Take precautions.

You must check these books

Wilson DE et al 1996. Measuring and monitoring biological diversity. Standard methods for mammals. Smithsonian Institution Press

the same title for Amphibians and Reptiles too

In both case there are recent editions

Thanks all for your suggestions, but which part of the body shall tissues be taken from, so that least damage is introduced to the specimen, and on which temperature should these be stored for further DNA extraction.  

For amphibians and reptiles collected for future preservation in the museums we use 72-75 ethanol. Volume of fixator and animal must be related as 4-5 and 1 as minimum. You also need to inject the absolute ethanol inside the animal belly. It would be good to keep the animals one-two days for completion of digestion before the fixation. Bad smell of fixed specimens can appear because of small amount of the fixator and/or inner decay processes. Other important rule is to change the fixator at least twice after primary fixation. As a rule, first time we change the fixator after day or even less and next time already before museum preservation. In the conditions of hot weather we use higher concentration of the ethanol and/or change the fixator additionally. To preserve the specimens for future DNA use you need to do the manipulations dscribed above but ONLY with 95 ethanol.

Tatjana, I have read (or someone has told me) that fixation in ethanol makes the specimen more stiff and fragile over the years (more than if fixed with formalin). Have you noticed something like that or any other kind of difference between fixing with ehatnol and formalin?

Haya, I suggest you to read Chapter Seven of this very useful book available at Google Books: https://books.google.com.br/books?id=I9R3R_DksFcC&

This PDF file may also be useful: https://www.for.gov.bc.ca/hts/risc/pubs/tebiodiv/voucher/assets/voucher.pdf

Anyway, I really would like to continue reading the personal experience from our colleagues here.

After formalin fixation and long time preservation there the specimens 1) become quite dark and often lost the peculiarities of their color diagnostic pattern; 2) become more stiff and fragile in case of formalin concentration more than 4-5% (for example, 10% neutral formalin is very good for histological study but not good for a work with external morphology).

In case of 95 ethanol fixation and long preservation we have more or less the same effect of stiff and fragile specimens as in case of high formalin concentration. However, fixation in 72-75 % ethanol is quite ":soft" and "smooth".

I worked with very old collections of herps in Russia, Italy, Germany and US doing histological work and studing lizard pholidosis. I can say that condition of that specimens was not so bad (even for histology!). 

For adult amphibian specimens (Larval specimens have a slightly different protocol) our standard protocol is:

This provides firm, yet flexible, specimens. The only drawback I have seen is that in the long run hydrosoluble pigments (mostly xanthines; "yellows") fade a lot.

In our museum we also use 90-96% ethanol. But take a small-sized samples of DNA. Put them into the Eppendorf and fill with ethanol in volume in 3-5 times bigger than the sample. After this sample in a tripod we place in the freezer, where it can persist for years.

Yes, Victor is also right describing another good method of fixation for collection. After taking a special sample for future DNA work and its fixation with 90-95% ethanol you may fixed the rest of specimen with filter paper saturated with 10% formalin in closed plastic box (we used may be half of day) and then placed it to 72-75% ethanol. This method was successfully used by our colleagues  from California Academy of Sciences.

Steam of formalin acting during a short time gives certain flexibility together with good fixation to the specimen and its tissue and didn't destroyed a general pattern of coloration formed with melanin pigment. However (and Victor here is again right) long preservation in ethonal will destroy the xantophore pigments. So, the colors of red, yellow and green composition will dissapear from the skin.

Dear Tatjana, victor and Ihor, thanks for the detailed protocols and useful notes, I think that these protocols even though for reptiles and amphibians they are still applicable for small mammals.

Dear Henrique, many thanks, the book you suggested is very useful as it gives detailed information on almost everything you would like to know while dealing with reptiles.