In response to the global increase in ambient temperatures, heat stress (HS) has emerged as a significant concern for livestock production. Core body temperature (CBT) has been a reliable indicator of HS in broilers; however, most research has focused on single-time-point measurements, thereby limiting our current understanding of thermoregulation in broilers. Consequently, the objective of this study was to conduct a time-dependent measurement to evaluate the effect of HS on the diurnal pattern of CBT in finishing broilers. To this end, n = 96 mixed sex chicks were divided into a thermoneutral group (CON, 21°C, 49% relative humidity (RH)) and a cyclic HS group (maximum 33°C, 53% RH, from 10:00 h to 16:00 h) from 34 to 40 days of age. Each group consisted of 8 replicates, each with 6 birds. Both environmental temperature and CBT were recorded every 5 min using a thermohygrometer and an ingested telemetry-based sensor, respectively, throughout the trial. Data were analyzed using both linear mixed models and cosinor-based rhythmometry. The results indicated that HS significantly (P < 0.01) modulated the hourly CBT of broilers for 7 consecutive h throughout the days, with the HS group recording higher CBT from 12:00 h to 16:00 h but lower CBT at 18:00 h and 19:00 h compared to the CON group. However, no significant difference was found in the hourly average of CBT between male and female broilers, irrespective of thermal treatment. Additionally, cosinor-based rhythmometry highlighted that rhythmicity parameters were significantly (P = 0.022) influenced by thermal challenge, with the HS group exhibiting a slightly higher mesor (41.19°C vs. 41.00°C), greater amplitude (0.66°C vs. 0.21°C), and an expedited acrophase (13:52 h vs. 15:25 h) compared to those of the CON group. Besides, the CON male broilers exhibited a slightly higher amplitude (P < 0.01) compared to their female counterparts. Overall, the current study demonstrated that HS disrupts the circadian rhythm of CBT in broilers by increasing their mesor and amplitude and synchronizing their acrophase to the time of the peak environmental temperature. Furthermore, broilers may be able to adapt to cyclic HS by substantially decreasing their basal CBT during cooler times of the day.
