Individual behavior differences related to induced tumor growth in the female Syrian hamster: two studies

The relationship of individual behavioral differences to induced tumor growth and to Natural Killer (NK) cell activity was investigated in the Syrian hamster in the context of a long-term natural stressor (group social interaction). In the first sub-experiment, 90-day-old females had blood drawn for NK cell assays. Animals were then placed in groups of ten, and two samples of behavior were obtained on videotape. All hamsters were injected midback subcutaneously with malignant melanoma, developed tumors, and were sacrificed 35 days later. Eight reliable behaviors were coded from the videotapes and were factor analyzed, yielding two factors: "Activation" and "Inactivity." The first factor of "Activation" was correlated p less than .06, two-tailed) with autopsy tumor size. The experiment was repeated using females aged 210 days. Factor analysis generated two factors virtually identical to those previously found. The factor of "Activation" was again correlated with autopsy tumor size (p less than .05, one-tailed). For older animals only, the second factor of "Inactivity/dominance" was associated with higher NK cell activity for one of the two samples. A second study was conducted in order to replicate the factor structure of the first two sub-experiments, as well as the relationships of behavioral variables to tumor outcome variables. However, the design differed mainly in taking four rather than two samples of behavior; and in using as an outcome measure time to detection of palpable tumor, rather than autopsy tumor size. The factor analysis derived very similar factors. The second factor of "Inactivity" was correlated significantly (p less than .007, two-tailed) with tumor latency, which is congruent with the previous results. Overall, the results showed that: (a) individual behaviors in hamsters can be rated reliably, and cluster repeatedly across independent studies into certain patterns; and (b) the pattern interpreted as "Activation" was associated with poorer tumor outcome relative to the pattern of "Inactivity-(dominance)."