1) Open the
.../your Dwarf Fortress folder/raw/objects/creature_standard.txt2) The very first entry is for the dwarves. Scroll down until you see [CASTE:FEMALE] and [CASTE:MALE]:
3) Copy the castes and paste below the vanilla ones:
[CASTE:FEMALE]
[FEMALE]
[MULTIPLE_LITTER_RARE]
[CASTE:MALE]
[MALE]
[SET_BP_GROUP:BY_TYPE:LOWERBODY][BP_ADD_TYPE:GELDABLE]
[BODY_DETAIL_PLAN:FACIAL_HAIR_TISSUE_LAYERS]
4) Change the names of the new castes to something different like [CASTE:MALE_DEATH].
5) Specify caste proportions. By default, each caste has an equal chance to appear, so your normal and "cold blooded" dwarves would be 50:50 in population. If you add [POP_RATIO:2] to vanilla castes and [POP_RATIO:1] to your new castes, then normal dwarves will be twice as frequent as the "cold blooded" ones. And so forth.
6) Add [PERSONALITY:STRESS_VULNERABILITY:0:0:0], [PERSONALITY:DEPRESSION_PROPENSITY:0:0:0], [PERSONALITY:ANGER_PROPENSITY:0:0:0], [PERSONALITY:ANXIETY_PROPENSITY:0:0:0] to your new castes (each one) to make them as resistant to stress as possible, while still capable of feeling emotions in general; you can also add [PERSONALITY:IMMODERATION:0:0:0] to lower the negative effects of your fort lacking the dwarf's favorite squirrel kidneys and such. Alternatively, just give them [NOEMOTION] to turn them into complete robots.