Increasing age and puberty affect the hypothalamic pituitary adrenal (HPA) axis maturation, which is likely associated with an increase in environmental demands (e.g., social) and vulnerability for the onset of psychiatric conditions (e.g., depression). There is limited research as to whether such patterns are consonant in youth with autism spectrum disorder (ASD), a condition marked by social challenges, dysregulation of the HPA axis, and higher rates of depression setting the stage for enhanced vulnerability during this developmental period.

The current study interrogated diurnal cortisol by examining (1) cortisol expression longitudinally over the pubertal transition between autistic and neurotypical youth, (2) the trajectory of diurnal cortisol and the unique contributions of age vs. puberty, and (3) potential sex differences. As hypothesized, results indicate autistic compared to typically developing youth demonstrate a shallower diurnal slope and elevated evening cortisol. These differences were in the context of higher cortisol and flatter rhythms based on age and pubertal development. Also, sex-based differences emerged such that females in both groups had higher cortisol, flatter slopes, and higher evening cortisol than males. The results show that despite the trait-like stability of diurnal cortisol, HPA maturation is impacted by age, puberty, sex, as well as an ASD diagnosis.

Periodicity in nematode egg excretion may be of evolutionary origin as it can favour dispersal of the eggs in the environment. We investigated whether egg excretion by Heterakis gallinarum shows a repeatable pattern of periodicity. The faecal egg concentration and total number of eggs excreted within 4-h intervals were significantly affected by the sampling time within 1 day, but remained unaffected by the sampling day or interaction effects. By contrast, the total number of eggs excreted within 24 h did not differ among the 4 days of the study, collectively indicating repeatable egg excretion patterns. Both host feces and parasite egg excretion increased from night to late afternoon, followed by a decrease in the evening, resulting in higher egg excretion during daytime than the dark period. Feces excretion and worm fecundity showed overlapping diurnal rhythms with similarly timed phases, suggesting the existence of synchronicity between the host feces and nematode egg excretion patterns. We conclude that egg excretion by H. gallinarum is synchronized with host feces excretion and is higher during the daytime than during the dark period. This overlaps with the maximum activity of the day-active host and allows a maximal dispersal of the eggs in the environment.

The present study demonstrates the influence of the time of the day and tide on the frequency of individuals and on the social and foraging behaviour of an equatorial population of Sotalia guianensis. A population of S. guianensis from enseada do Curral (6°13′00″S 35°3′36″W), north-eastern Brazil, was investigated from October 1999 to September 2003. Social and foraging activities oscillated significantly, being less intense at high tide in comparison to other analysed tides. Socialization was more intense between 1000 and 1300 hours and foraging was more intense in between tides, than at high tide. No diurnal or tidal fluctuation was found on the frequency of individuals, which remained constant throughout the day. These results demonstrate the influence of circadian factors on an equatorial population which is highly impacted by human activities and highlights a need for protective measures.

Circadian rhythms are considered an important factor in the aetiology, expression and treatment of major affective disorders, including seasonal affective disorder (SAD). However, data on the effects of daylight length manipulation or melatonin administration are complex. It has been suggested that since diurnal and nocturnal mammals differ significantly in their physiological and behavioural responses to daylight, diurnal rodents offer a preferable model of disorders related to circadian rhythms in the diurnal human. We previously found that diurnal fat sand rats maintained under short daylight (SD), show depression-like behaviour in the forced swim test (FST). The present study was designed to test additional behaviours related to affective disorders and study the involvement of melatonin in these behaviours. Sand rats were divided into short-daylight (SD, 5 h light:19 h dark) and long-daylight (LD, 12 h light:12 h dark) groups, and received 100 μg melatonin or vehicle administration for 3 wk (5 h and 8.5 h after light onset in the LD room). Animals were then tested for reward-seeking behaviour (saccharin consumption), anxiety (elevated plus-maze), aggression (resident-intruder test), and depression-like behaviour (FST). SD or melatonin administration resulted in a depressed/anxious-like behavioural phenotype including reduced reward seeking, increased anxiety, decreased aggression and decreased activity in the FST, supporting the notion that in a diurnal animal, reduced light results in a variety of behavioural changes that may model depression and anxiety; and that melatonin may be a significant factor in these changes. We suggest that the sand rat may offer an excellent model species to explore the interactions between daylight, affective behaviour and the related underlying mechanisms.

Measurement and interpretation of acid–base status are important in clinical practice and among racing jurisdictions to determine if horses have been administered alkalinizing substances for the purpose of enhancing performance. The present study used the physicochemical approach to characterize the daily variation in plasma electrolytes and acid–base state that occurs in horses in the absence of feeding and exercise. Jugular venous blood was sampled every 1–2 h from two groups (n=4 and n=5) of Standardbred horses over a 25 h period where food and exercise were withheld. One group of horses was studied in October and one in December. The time course and magnitude of circadian responses differed between the two groups, suggesting that subtle differences in environment may manifest in acid–base status. Significant daily variation occurred in plasma weak acid concentration ([Atot]) and strong ion difference ([SID]), [Cl−], [K+], [Na+] and [lactate−], which contributed to significant changes in [H+] and TCO2. The night-time period was associated with a mild acidosis, marked by increases in plasma [H+] and decreases in TCO2, compared with the morning hours. The night-time acidosis resulted from an increased plasma [Atot] due to an increased plasma protein concentration ([PP]), and a decreased [SID] due to increases in [Cl−] and decreases in [Na+]. An increased plasma [K+] during the night-time had a mild alkalotic effect. There were no differences in pCO2. It was concluded that many equine plasma electrolyte and acid–base parameters exhibit fluctuations in the absence of feeding and exercise, and it is likely that some of these changes are due to daily variation.