Endogenous activity rhythms and the reproductive response to photoperiod in the reddish grey musk shrew and eastern rock elephant shrew

Abstract

Rhythmicity in biological processes is ubiquitous to life on Earth and is commonly synchronized to one of the most fundamental components of the temporal environment, the light-dark cycle. The present thesis investigated the circadian rhythm of locomotor activity in two little-studied small African mammals that were field-captured in Limpopo Province, South Africa: the reddish-grey musk shrew (Crocidura Cyanea) and the eastern rock elephant shrew (Elephantulus myurus). Furthermore, the reproductive response to photoperiod was investigated in male C. cyanea by maintaining males on either a long-day length (L16:D8) or short day-length (L8:16D). Thirteen C. cyanea individuals were subjected to three successive light cycles, each cycle lasting approximately two weeks: An LD cycle (12 h light/12 h dark), a DD cycle (constant darkness) and a DL cycle (an inverse of the LD cycle). In C. cyaena, locomotor activity was strongly entrained by the daily light-dark cycle and light primarily suppressed activity. Under LD, C. cyanea had a mean active phase (α) of 10.8 h and 78.9% of the total activity occurred during the dark phase. The mean α increased to 13.2 h under DD and all animals expressed free-running rhythms (mean τ = 23.0 h), evident of a functional biological clock. On the DL cycle, the mean α was lowest (09.3 h) and 71.5% of the total activity occurred during the dark phase. Locomotor activity in E. myurus was investigated under similar light regimes as for C. cyanea: 13 individuals were subjected to an LD cycle, DD cycle, a second LD cycle and a DL cycle, each cycle lasting approximately 18 days. Elephantulus myurus also exhibited an internally controlled rhythm of locomotor activity entrained by the light-dark cycle and were active approximately 25% of the day. Under LD, 56.5% of all activity occurred during the light phase and 43.5% during the dark phase. Elephantulus myurus had a mean τ of 23.6 h, with significant interindividual variation. Under DL, 49.6% and 50.4% of all activity occurred during the dark and light phases, respectively. The activity profile of E. myurus was similar to that of closely related species under natural conditions. Both species, therefore, possess circadian locomotor activity rhythms that are entrained to the light-dark cycle by means of an internal clock. Crocidura cyanea is primarily nocturnal and may exhibit intermittent activity primarily during the afternoon. Elephantulus myurus is most active at dawn, least active during the afternoon and tends to display an intermediate amount of intermittent activity throughout the night. Furthermore, the results suggested that male C. cyanea is reproductively responsive to photoperiod, which may at least in part, stimulate reproductive maturation in this species. Testicular mass relative to body size, as well as testicular volume were significantly higher in males maintained on an LD compared to males on an SD. Interestingly, the seminiferous tubule diameters were larger in SD males than in LD males. Plasma testosterone concentration was unaffected by photoperiod and varied greatly amongst males within each photoperiodic treatment but did not differ significantly for males maintained on either an LD or an SD.