The influence of abiotic factors on the onset and cessation of activity ofsemi-free Callithrix jacchus

The influence of abiotic environmental factors on the period of activity of a single group of South American common marmosets (Callithrix jacchus, Cal litrichidae, Primates) was investigated under semi-free conditions. A group of eight members had a territory consisting of a heated wooden hut with a veranda, surrounded by an open area with a few trees (ca. 3.5 m high) and t hree runways made out of roofing slats, on which feeding places and sleepin g boxes were fixed. The food supply was held constant throughout the observ ation period with respect to amount, composition and spatial distribution. From July to November 1995, the times of the onset and cessation of activit y were determined using a video camera. An electronic weather station recor ded the temperature, humidity, air pressure, wind speed, wind direction, pr ecipitation, and light intensity at intervals of 5 min. There was a linear correlation between day light length and the length of t he period of activity over a day length of 10 to 14 hr. With shorter day le ngths, the marmosets were also active during the twilight, whereas with lon ger day lengths a sleep phase during the late morning was introduced. Ambie nt temperature and humidity had also an effect on the time when activity be gan or ceased. Callithrix jacchus has one of the longest activity periods w ithin the Callitrichidae. The time of sunrise or sunset, temperature, and humidity accounted for 66.2 % of the variation in the time when activity began and 75.5% of the varianc e in the cessation of activity of the study group within the multivariate m odel. The results from the present study add to the indications that in the Calli trichidae there is a strong selection pressure for the highest possible ene rgy saving during the comparatively long phase of inactivity. Am. J. Primat ol. 47:241-253, 1999. (C) 1999 Wiley-Liss, Inc.