In order to understand why sperm pairing has evolved in most American marsupials, the movement parameters of spermatozoa from Monodelphis domestica were analyzed after incubation in capacitating medium for 15 min, 2 h, and 24 h to induce a proportion of sperm pairs to uncouple. Motility characteristics of paired and single spermatozoa were measured in media of differing composition and viscosity by means of computer-aided semen analysis. In minimum essential medium or in RPMI 1640 medium alone, the absolute mean straight-line and curvilinear velocity values of paired spermatozoa (342 +/- 34 and 361 +/- 19 microns/sec, respectively, at 37 degrees C) were significantly greater than those of single spermatozoa (247 +/- 14 and 319 +/- 16 microns/sec), while mean lateral head displacement for paired spermatozoa (5.6 +/- 2.1 microns) was significantly less than for single spermatozoa (11.4 +/- 2.6 microns). However, when medium was made more viscous with polyvinyl pyrrolidone (0.8-82 poise) and sperm motility was calculated as a percentage of maximum attained velocity (in medium alone), there was no significant difference in straight-line or curvilinear velocity for single or paired spermatozoa in medium of the lowest viscosity (0.8 poise). In contrast, paired spermatozoa in medium of higher viscosity (above 1.92 poise) maintained straight-line velocity (e.g., 54 +/- 3% of maximum straight-line velocity in medium of 2.28 poise) while single sperm moved in tight circles and exhibited poor straight-line velocity (5 +/- 1% of maximum velocity). The data show that paired spermatozoa exhibit a significant motility advantage over single spermatozoa in a viscous medium.(ABSTRACT TRUNCATED AT 250 WORDS)