Figure 1
(A-B): Averaged equine MT (A) and cortisol (B) rhythms under conditions of light dark (LD 13.5:10.5) and constant darkness (DD). The barn LD cycle is depicted above each graph: white bars represent light in LD and subjective day in DD; black bars and internal shading represent darkness in LD and subjective night in DD (CT14-24). Sampling began at ZT/CT0 in LD and ended at CT22 in DD after 32 h in continuous darkness. Hormone data are presented as mean ± SE for six mares (n = 6). CT0 represents 0700 h; CT2 0900 h, etc. (A) MT remained low during hours of light (L) in LD but not during the corresponding times (subjective day, CT2-CT10) in DD. A 24-h MT rhythm is evident under LD conditions, but not under DD (p < 0.0001). *, ** denote significant difference (p < .05, p < .01) at specific time points (Bonferoni post hoc tests). (B) In contrast, cortisol showed similar 24-h patterns in LD and DD.
Figure 2
(A-D): Individual equine MT (A-C) and cortisol (D) time series throughout the experimental LD and constant dark (DD) conditions described for Figure 1. Due to substantial individual differences in peak MT levels expressed in the first hours of darkness individual MT data were normalized and expressed as a percentage of the ZT16-ZT22 mean (set to 100%). The resulting plots (B, C) illustrate the two different temporal patterns discussed in the text: continuously high levels in B contrasting with eventual MT declines in C. Panel D illustrates the substantial individual and ultradian variation in blood cortisol. Other conventions are the same as in Figure 1.
Table 1
Significance (p) values from 24-hour Cosine fits to melatonin (MEL) and cortisol (Cort) time series for individual mares during LD and DD and for corresponding group means (12 points/fit).
| 24-h fits | Mel LD | Mel DD | Cort LD | Cort DD |
|---|---|---|---|---|
| Mare 1 | .013 | .01 | .6 | .07 |
| Mare 2 | .0038 | .1 | .21 | .37 |
| Mare 3 | .002 | .83 | .2875 | .2858 |
| Mare 4 | .036 | .07 | .7436 | .7473 |
| Mare 5 | .0016 | .22 | .53 | .11 |
| Mare 6 | .0015 | .016 | .2 | .002 |
| Group Mean (n = 6) | .00026 | .51 | .076 | .005 |