Small quadrapeds/birds shouldn't be able to slip on things, nor should snakes.

data.c

@@ -17879,6 +17879,7 @@ void initrace(void) {
 	addflag(lastrace->flags, F_NATURALFLIGHT, B_TRUE, NA, NA, "");
 	addflag(lastrace->flags, F_CANWILL, OT_A_FLY, NA, NA, NULL);
 	addflag(lastrace->flags, F_SPELLCASTTEXT, OT_A_FLY, NA, NA, NULL); 
+	addflag(lastrace->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 	addflag(lastrace->flags, F_STARTSKILL, SK_FLIGHT, PR_BEGINNER, NA, NULL);
 
 	addrace(R_SWARMRAT, "swarm of rats", 40, UNI_SWARM, C_BROWN, MT_FLESH, RC_ANIMAL, "A large swarm of vile rodents.");
@@ -17899,6 +17900,7 @@ void initrace(void) {
 	addflag(lastrace->flags, F_MAXATTACKS, 5, 5, NA, NULL);
 	addflag(lastrace->flags, F_NOCORPSE, B_TRUE, NA, NA, NULL);
 	addflag(lastrace->flags, F_SWARM, NA, NA, NA, NULL);
+	addflag(lastrace->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_STR, AT_VLOW, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_IQ, IQ_ANIMAL, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_AGI, AT_GTAVERAGE, NA, NULL);
@@ -17928,6 +17930,7 @@ void initrace(void) {
 	addflag(lastrace->flags, F_MAXATTACKS, 7, 7, NA, NULL);
 	addflag(lastrace->flags, F_NOCORPSE, B_TRUE, NA, NA, NULL);
 	addflag(lastrace->flags, F_SWARM, NA, NA, NA, NULL);
+	addflag(lastrace->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_STR, AT_VLOW, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_IQ, IQ_ANIMAL, NA, NULL);
 	addflag(lastrace->flags, F_STARTATT, A_AGI, AT_HIGH, NA, NULL);

lf.c

@@ -10837,13 +10837,15 @@ char *real_getlfnamea(lifeform_t *lf, char *buf, lifeform_t * usevis, int showal
 }
 
 enum LFSIZE getlfsize(lifeform_t *lf) {
-	flag_t *f;
+	flag_t *f = NULL;
+	enum LFSIZE size = SZ_HUMAN;
 	f = hasflag(lf->flags, F_SIZE);
-	if (!f) f = hasflag(lf->race->flags, F_SIZE);
+	if (f) {
-	if (!f) {
+		size = f->val[0];
-		return SZ_HUMAN;
+	} else {
+		size = getracesize(lf->race->id);
 	}
-	return f->val[0];
+	return size;
 }
 
 float getlfweight(lifeform_t *lf, int withobs) {
@@ -11117,6 +11119,21 @@ int getracerarity(enum HABITAT hab, enum RACE rid, enum RARITY *rr) {
 	return rarity;
 }
 
+enum LFSIZE getracesize(enum RACE rid) {
+	race_t *r = NULL;
+	flag_t *f = NULL;
+	enum LFSIZE size = SZ_HUMAN;
+	r = findrace(rid);
+	if (r) {
+		f = hasflag(r->flags, F_SIZE);
+		if (f) {
+			size = f->val[0];
+		}
+	}
+	return size;
+}
+
+
 // if optional 'attacker' is provided, only select form armours which they
 // can reach.
 object_t *getrandomarmour(lifeform_t *lf, lifeform_t *attacker) {
@@ -21757,6 +21774,9 @@ void setbodytype(race_t *r, enum BODYTYPE bt) {
 			addbodypart(r, BP_LEGS, NULL);
 			addbodypart(r, BP_FEET, "talons");
 			addbodypart(r, BP_WINGS, NULL);
+			if (getracesize(r->id) <= SZ_SMALL) {
+				addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
+			}
 			break;
 		case BT_HUMANOID:
 			for (i = BP_WEAPON; i <= BP_LEFTFINGER; i++) {
@@ -21770,6 +21790,7 @@ void setbodytype(race_t *r, enum BODYTYPE bt) {
 			addbodypart(r, BP_LEGS, NULL);
 			addbodypart(r, BP_WINGS, NULL);
 			addflag(r->flags, F_SEEWITHOUTEYES, B_TRUE, NA, NA, NULL );
+			addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 			break;
 		case BT_QUADRAPED:
 			addbodypart(r, BP_EYES, NULL);
@@ -21780,25 +21801,32 @@ void setbodytype(race_t *r, enum BODYTYPE bt) {
 			addbodypart(r, BP_BACKLEGS, "back legs");
 			addbodypart(r, BP_FRONTLEGS, "front legs");
 			addbodypart(r, BP_FEET, "paws");
+			if (getracesize(r->id) <= SZ_SMALL) {
+				addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
+			}
 			break;
 		case BT_FISH:
 			addbodypart(r, BP_EYES, NULL);
 			addbodypart(r, BP_HEAD, NULL);
 			addbodypart(r, BP_BODY, NULL);
 			addbodypart(r, BP_TAIL, NULL);
+			addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 			break;
 		case BT_SNAKE:
 			addbodypart(r, BP_EYES, NULL);
 			addbodypart(r, BP_HEAD, NULL);
 			addbodypart(r, BP_TAIL, NULL);
+			addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 			break;
 		case BT_SPIDER:
 			addbodypart(r, BP_EYES, NULL);
 			addbodypart(r, BP_HEAD, "cephalothorax");
 			addbodypart(r, BP_BODY, "abdomen");
 			addbodypart(r, BP_LEGS, NULL);
+			addflagifneeded(r->flags, F_STABILITY, B_TRUE, NA, NA, NULL);
 			break;
 	}
+
 }
 
 int setfacing(lifeform_t *lf, int dir) {

lf.h

@@ -286,6 +286,7 @@ enum POISONSEVERITY getpoisonseverity(enum POISONTYPE ptype);
 */
 enum RACECLASS getraceclass(lifeform_t *lf);
 int getracerarity(enum HABITAT hab, enum RACE rid, enum RARITY *rr);
+enum LFSIZE getracesize(enum RACE rid);
 object_t *getrandomarmour(lifeform_t *lf, lifeform_t *attacker);
 enum BEHAVIOUR getrandombehaviour(void);
 enum BODYPART getrandomcorebp(lifeform_t *lf, lifeform_t *attacker);