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Abstract:
Differences between tropical and temperate cave communities are an important topic in the actual biospeleological thinking. Among the most striking differences is the paucity of terrestrial troglobites in tropical caves. This fact may depend on the higher energy input into tropical caves which lessens the selection pressures for energy-economizing troglobite adaptations. Consequently evolutionary rates would be slowed in tropical caves and, in a date group, troglobites would appear later in such caves than in temperate ones with lower energy input. In order to investigate this point the authors studied the degree of adaptation to the cave environment in two species of Mexican Ptomaphagus which, being phylogenetically related, probably descend from the same epigean ancestor. Among these species the first one, P. troglomexicanus Peck, lives in a typical temperate cave (i.e. cold, high altitude cave, with scarce food supply) in the Sierra de Guatemala (Tamaulipas), the other one, P. spelaeus (Bilimek), populates tropical caves (i.e. warm, lowland cave, with rich food supply) in the State of Guerrero. In addition a comparison is made with P. pius Seidlitz, an epigean species from southern Europe. The results show a striking difference between P. troglomexicanus on a side and the other two species. Differences chiefly concern morphological features such as relative antenna length, structural complexity (i.e. the number of sensilla) of the antenna chemioreceptor organs in the 70, 90, 100 segments, degree of reduction of eye, wing and pigmentation and physiological ones such as the length of the life cycle. The possible causes of these differences are discussed. According to the authors these differences appear due to the different selection pressures acting in the two types of caves. In addition a comparison between the "tropical cave" species, P. spelaeus, with the epigean one, P. pius, does not point out the differences that one could expect by the diverse ecology of these species. These observations support the idea that evolutionary rates in cavernicoles are strongly affected by the ecology of the cave, mainly depending on the degree of energy input, and are poorly consistent with the hypothesis that mutations affecting degenerative processes are selectively neutral.