Factor	Effect	References
Intrinsic
Age	No difference in 50-kHz ultrasonic vocalizations (USVs) between rats ages 24-44 and 89-92 days old	18, 29, 42
	Decrease in 50-kHz USVs for rats from 71 days old to 148 days old	29
	17 day old males made many more 50-kHz USVs than 7-9 month old males	43
	Rats tickled as both juveniles and adults showed the most positive results compared to rats only tickled as juveniles or adults.	10
Sex	Male rats produced more 50-kHz USVs at 44 and 71 days old, but females produced more 50-kHz USVs at 148 days old	29
	Male rats produced more 50-kHz USVs than females at 44 and 58 days.	43
	No sex differences in 50-kHz USVs in 96 juvenile Long-Evans rats or 52 juvenile Wistar rats	42, 44
	Female juvenile Wistar rats produced more 22-kHz USVs than males	44
Inter-individual differences	Rats that uttered more 50-kHz USVs showed decreased approach time to self-administered tickling, increased positive judgement bias after tickling, and increased hippocampal cell proliferation after tickling	27, 37, 45
	More playful rats produced more 50-kHz USVs	43
Strain	Not reported, but anecdotal evidence suggests rats from different strains and breeders differ in their response to tickling. Rat lines diverged when selected upon high vs. low 50-KHz USVs rate in response to tickling.	22, 26, 42, 46
Previous experience of tickling	Increased production of 50-kHz ultrasonic USVs	12
Extrinsic
Bedding	Rats produced more 50-kHz USVs when tickled on familiar bedding vs no bedding	33
Stress	Tickling rats 1 hour, but not 23 hours, after restraint stress reduced 50-kHz USVs production	47, 48
Timing	Tickling rats before injection was more effective in increasing 50-KHz USVs than tickling after injection	12
Sex of handler	No effect found	Panksepp, J. 2014, Unpublished data
Familiarity of rat with handler	Rats produced more 50-kHz USVs and spent more time near a familiar compared to an unfamiliar tickler	11
Handler tickling experience / vigor of tickling by handler	Affected production of 50-kHz USVs but not preference and approach behavior	Cloutier, S. 2011, Unpublished data
Duration of pins by handler / frequency of alternation between dorsal contacts and pins	Lower frequency of nibbles (interpreted as play solicitation) was directed to a hand imposing 15-s pins (restraint) than a hand giving tickles with multiple dorsal contacts and pins per 15 s	11
Odorants (e.g., perfume, shampoo, shaving cream) used by handler	Not reported
Social group size	Individual housing increased positive responses to tickling compared to group housing in most experiments. Tickling group-housed rats with cage mates in the home cage reduced this difference	7, 22, 27, 36
Light intensity	Tickling at 1000 lux reduced 50-kHz USVs compared to 25 lux.	43
Noise	Not reported
Cage cleaning before tickling	Induced production of 22-kHz USVs in some rats	12
Time of day	No effect on 50-kHz USVs production when performed at the beginning or end of the light period. Tickling rats at the beginning/end of the light period vs middle of light or dark period has not yet been compared	Cloutier, S. 2011, Unpublished data