Evolution of Communication - Volume 2, Issue 2, 1998

Language comprehension in the great apes has been investigated through a variety of paradigms. This experiment employed a match-to-sample computer task to investigate the current language comprehension of three chimpanzees (Pan troglodytes) raised in different language environments but with a similar symbol system (lexigrams). Each of these animals still uses the lexigram keyboard system on a daily basis but none of the animals are the focus of ongoing ape language research programs. Six testing conditions were employed utilizing photographs, lexigrams and spoken English. The results indicated that all apes retained knowledge of at least some of the symbols that they had previously learned, and they each learned differing numbers of new lexigrams that were never taught to them. This indicates that rearing history is important not only in initial symbol acquisition in apes, but also in extended recall of the symbols, particularly when those symbols are used infrequently later in life. Differences in the current ages of these apes requires that such a conclusion be made tentatively, and suggests the need to continue the examination of symbol vocabulary size at various times throughout the life of each ape.

Vocalizations of sexually receptive females in two primate species, the sooty mangabey (Cercocebus torquatus atys) and the pigtail macaque (Macaca nemestrina), were compared with respect to the acoustical features of calls as well as the reproductive and social factors that were associated with calling behavior. Sixty-two bouts of calling were recorded from 18 different pigtail macaque females (mean number of bouts per individual-3.48, SD = 2.17, range 1-7) over a six month period; 19.4% occurred during copulation with males, 25.8% were recorded within 30 seconds after copulation has ceased, while the majority, 54.8%, were not associated with mating. The mangabey group yielded 52 bouts from 18 different females (mean number of bouts per individual = 2.89, SD = 2.47, range 1-10) over a comparable period of time; all 52 mangabey bouts were recorded from females during copulation with males. Calls in both species were highly stereotyped and were not acoustically similar to other vocalizations in the species' repertoire; the acoustical structure of the calls of both species, with most energy distributed at relatively low frequencies, suggests adaptations for propagation over distance. Dominance rank of the caller was associated with significant variation in calling by estrus females of both species. There was a strong relationship between rank and bout length in the pigtail females, with higher-ranking females having shorter bouts; the rate of delivery for higher-ranking females was also significantly more variable than it was for lower-ranking females. For the mangabeys, lower-ranking females had significantly higher rates of delivery than did higher-ranking ones. These findings are consistent with the hypothesis that these features of calling relate to a caller's motivational state and that higher levels of sexual motivation are required by lower-ranking females before they show proceptive behavior or mating. The possibility that female-female competition may have a significant effect on important features of calling should be considered in studies that attempt to evaluate the functional significance of these vocalizations.

Here we discuss different factors that could influence the development of vocal and/or seismic communicative channels in subterranean rodents. We suggest that: 1) Highly social subterranean rodents that do not leave their burrows use essentially vocal signals in the vibrational channel; 2) Solitary and almost permanently fossorial species use vocal signals in short range and seismic signals in long range communication; 3) Other solitary species that leave the burrow system more frequently and that retain good visual capabilities are constrained to use vocal communication only. Also we suggest that seismic communication probably derives from digging activities and, consequently, developed after the acquisition of the subterranean way of life. The first three statements are based on a previously proposed relationship between visual capabilities, hearing capabilities, time spent outside the burrows, social organization and type of vibrational signals used by the species. The fourth statement is based in the correlation found between digging and transporting tools and thumping tools, that are the same across the literature on pertinent genera. Some thumping techniques unique to subterranean animals lead us to propose an evolutionary sequence leading from digging to thumping.

Based on the molecular clock model of evolution, molecular phylogenies represent reconstructions of the evolutionary process with a time scale. From these, inferences can be drawn about the evolution of other characters, including behaviour patterns. Mapping particular vocalization types in the Felidae (cats) on a published molecular phylogeny of this mammal family reveals that the distribution of these behavioural characters is fully congruent with it. Thence a time frame for the evolution of these vocalizations can be inferred, indicating large differences in their evolutionary age. Phylogenetic stasis for several million years in particular vocalization types refutes the hypothesis that behavioural characters are generally more susceptible to evolutionary change than morphological ones.