Acta Theriologica 43 (4): 337-348,1998. PL ISSN 0001-7051 Modulating factors of the energetic effectiveness of huddling in small mammals Mauricio CANALS, Mario ROSENMANN, F. Fernando NOVOA and Francisco BOZINOVIC* Canals M., Rosenmann M., Novoa F. F. and Bozinovic F. 1998. Modulating factors of the energetic effectiveness of huddling in small mammals. Acta Theriologica 43: 337-348. Huddling is effective in decreasing metabolic rate permitting energy saving. However, this decrease varies among different species depending on physical, physio- logical and behavioral characteristics of the huddled individuals. Following a general model we analyzed the effects of ambient temperature, thermal conductance and ontogeny on the huddling effectiveness (energy saving level from huddling behaviour) in white mice Mus musculus. Also, we studied the effects of thermal conductance by using the Sigmodontine Abrothrix andinus as a model organism. To put our results in a general context we analyzed literature data of huddling of several species of rodents at different temperatures. No effects of temperature and thermal conductance was detected. However, based on literature data, we found that at temperatures lower or near thermoneutrality the huddling effectiveness decrease. Also, the huddling effectiveness depends on the stage of development. Temperature probably affects the intensity of huddling, while changes in huddling effectiveness at early stages of development are likely consequences of structural (morphological) changes during the ontogeny. In this sense, it appears that the capacity to change the body form is and individual structural constraint which is extended to the huddling group. Departamento de Ciencias Biológicas Animales, Facultad de Ciencias Veterinarias, Universidad de Chile, Casilla 2, Correo 15, Santiago, Chile (MC); Departamento de Ciencias Ecológicas, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile (MC, MR, FFN); Departamento de Ecología, Facultad de Ciencias Biológicas, P. Universidad Católica de Chile, Casilla 114D, Santiago, Chile (FB) Key words: huddling, energy saving, temperature, ontogeny, thermal conductance Introduction Crowding of small rodents may be a result of predation pressures and temporal changes in availability of food and space (Alexander 1974, Wittemberg 1981). However, these conditions are not sufficient to explain all grouping situations because additional physical aspects of the environment such as temperature and *To whom all correspondence should be addressed: Departamento de Ecología, Facultad de Ciencias Biológicas P. Universidad Católica de Chile, Casilla 114-D, Santiago, Chile, fax: 562 - 5552212, e-mail: fbozinov@genes.bio.puc.el [337]