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Sensitivity of Syrian Hamsters (Mesocricetus auratus) to Amplitudes and Rates of Photoperiodic Change Typical of the Tropics

1993; SAGE Publishing; Volume: 8; Issue: 4 Linguagem: Inglês

10.1177/074873049300800405

1552-4531

Paul D. Heideman, F. H. Bronson,

Animal Ecology and Behavior Studies

Empirical data suggest that reproductive photoresponsiveness occurs in some populations of mammals above 13° of latitude, but may be absent in populations from 0° to 10° of latitude. The present experiments examined the degree to which the low amplitude of change in photoperiod in the tropics constrains mammals from using daylength as a seasonal cue. The Syrian hamster, a temperate-zone species, was studied because of its well-documented ability to respond to small changes in photoperiod, and because of the absence of an alternative robustly responding species from the tropics. We subjected adult male hamsters to photoperiods that mimicked the amplitude and rate of photoperiod change of 30°, 20°, 10°, and 5° of latitude, but centered around an estimate of their critical daylength. For comparison, a fifth group was subjected to an abrupt change in daylength of a magnitude equal to the total annual variation occurring at 30°. The two groups experiencing the gradually changing daylengths of 30° and 20° showed less within-group synchrony during testicular regression; in other dimensions of the annual testis cycle, including the degree of synchrony exhibited during recrudescence, they reacted similarly to the hamsters given the abrupt change in daylength. Some of the hamsters exposed to the gradually changing daylengths of 10° responded to this challenge, as did a few in the 5° treatment-in both cases, with poor within-group synchrony and a submaximal decrease in testis size. In an abbreviated second experiment, hamsters given abrupt decreases in daylength of magnitudes equal to those of the 10° and 5° groups responded slightly more frequently, and with maximal decreases in testis size. This suggests that mammals may not be constrained absolutely by an inability to respond to changes in photoperiod at 5° to 10° latitude. Seasonally breeding populations of mammals in the deep tropics that do not use photoperiod to regulate reproduction may use nonphotoperiodic cues because they offer a higher signal-to-noise ratio than do tropical changes in photoperiod.