Rickye S. Heffner

The Role of the Corticospinal Tract in the Evolution of Human Digital Dexterity

A morphometric analysis of the corticospinal tracts relation to digital dexterity was performed on 21 species theoretically related to mans ancestral lineage. The results indicate that the Primate line is not unique among mammals with respect to the cortical control of digital dexterity. A comparative analysis suggests that two changes took place early in Primate evolution: a reduction in functional distance (i.e. number of synapses) between neocortex and spinal motor neurons innervating the digits, and an extension of direct neocortical influence beyond the cervical segments of the spinal cord. A further change progressed throughout Primate evolution, from the mid-Eocene to the present, in which the overall size of the corticospinal tract increased steadily as though consolidating the cortical influence over body musculature, especially that of the digits.A morphometric analysis of the corticospinal tracts relation to digital dexterity was performed on 21 species theoretically related to mans ancestral lineage. The results indicate that the Primate line is not unique among mammals with respect to the cortical control of digital dexterity. A comparative analysis suggests that two changes took place early in Primate evolution: a reduction in functional distance (i.e. number of synapses) between neocortex and spinal motor neurons innervating the digits, and an extension of direct neocortical influence beyond the cervical segments of the spinal cord. A further change progressed throughout Primate evolution, from the mid-Eocene to the present, in which the overall size of the corticospinal tract increased steadily as though consolidating the cortical influence over body musculature, especially that of the digits.

Audiogram of the hooded Norway rat

The behavioral audiogram of the hooded Norway rat was determined for frequencies from 250 Hz to 70 kHz. The resulting audiogram is virtually identical to the albino rat audiogram obtained by Kelly and Masterton (1977), indicating that there is no detectable effect of albinism on the audiogram of the Norway rat. The two audiograms also indicate the degree of replicability that can be obtained with current behavioral techniques.

Hearing in the elephant (Elephas maximus): absolute sensitivity, frequency discrimination, and sound localization.

A young Indian elephant was tested to determine its absolute sensitivity, frequency-discrimination thresholds, and sound-localization thresholds. The elephant was found to have an audibility curve similar to that of other mammals but one that is more sensitive to low frequencies and less sensitive to high frequencies than any other mammalian audiogram including humans. The elephants sensitivity to frequency differences at low frequencies was found to equal that of humans. Finally, the elephant was found to be very accurate at localizing sounds in the azimuthal plane, with thresholds around 1 degree for broad-band noise. The elephants ability to localize pure tones suggested that it could use both binaural time- and intensity-difference cues to localize sound.

Hearing range of the domestic cat

The behavioral audiograms of two cats were determined in order to establish the upper and lower hearing limits for the cat. The hearing range of the cat for sounds of 70 dB SPL extends from 48 Hz to 85 kHz, giving it one of the broadest hearing ranges among mammals. Analysis suggests that cats evolved extended high-frequency hearing without sacrifice of low-frequency hearing.

Evolution of Sound Localization in Mammals

The ability to locate the source of a sound too brief to be either scanned or tracked using head or pinna movements is of obvious advantage to an animal. Since most brief sounds are made by other animals, the ability to localize such sounds enables an animal to approach or avoid other animals in its immediate environment. Moreover, it can be used to direct the eyes, thus bringing another sense to bear upon the source of the sound. Given the value of sound localization to the survival of an animal, it is not surprising that the need to localize sound has been implicated as a primary source of selective pressure in the evolution of mammalian hearing (Masterton et al. 1969; Masterton 1974).

Hearing and sound localization in blind mole rats (Spalax ehrenbergi)

Two blind mole rats were tested for their ability to detect and localize sound. The results indicate that blind mole rats have severely limited, and probably degenerate, auditory abilities. Although their 60-dB low-frequency hearing limit of 54 Hz is within the range for other rodents, the highest frequency they can hear at a level of 60 dB SPL is only 5.9 kHz, giving them the poorest high-frequency sensitivity yet observed in any mammal. In addition they have poor sensitivity as indicated by the fact that their lowest threshold is only 32 dB SPL (at 1 kHz). Finally, they are unable to localize brief sounds but retain a rudimentary ability to localize sounds of 0.5 s or longer. These results, combined with those of previous studies of subterranean species (i.e., blind mole rats, naked mole rats, and pocket gophers), suggest that poor auditory sensitivity, the loss of high-frequency hearing, and an inability to localize brief sounds is a degenerate state which may be characteristic of subterranean mammals. Thus it appears that an exclusive adaptation to a subterranean lifestyle (where airborne sound propagates poorly and where directional responses are limited by the tunnels) can result in vestigial auditory abilities just as the absence of light results in vestigial vision.

Behavioral hearing range of the chinchilla

The audiograms of three chinchillas were determined using pure tones ranging from 32 Hz to 45 kHz. The animals were tested with a conditioned avoidance procedure in which their heads were fixed within the sound field by requiring them to place their mouths on a water spout. At a level of 60 dB SPL the average hearing range extended from 50 Hz to 33 kHz with none of the animals able to hear 45 kHz at 89 dB. Overall, the audiogram of the chinchilla appears to resemble the human audiogram more closely than do other rodent audiograms. An analysis of ten published chinchilla audiograms indicates that those procedures which do not fix an animal within the sound field may overestimate their sensitivity.

Audiograms of five species of rodents: implications for the evolution of hearing and the perception of pitch

Behavioral audiograms were determined for five species of rodents: groundhog (Marmota monax), chipmunk (Tamias striatus), Darwins leaf-eared mouse (Phyllotis darwinii), golden hamster (Mesocricetus auratus), and Egyptian spiny mouse (Acomys cahirinus). The high-frequency hearing of these animals was found to vary inversely with interaural distance, a typical mammalian pattern. With regard to low-frequency hearing, the animals fell into two groups: those with extended low-frequency hearing (chipmunks, groundhogs, and hamsters hear below 100 Hz) and those with restricted low-frequency hearing (spiny and leaf-eared mice do not hear appreciably below 1 kHz). An analysis of mammalian hearing reveals that the distribution of low-frequency hearing limits is bimodal with the two distributions separated by a gap from 125 to 500 Hz. The correspondence of this dichotomy with studies of temporal coding raises the possibility that mammals that do not hear below 500 Hz do not use temporal encoding for the perception of pitch.

Sound localization and use of binaural cues by the gerbil (Meriones unguiculatus)

Noise-localization thresholds and the ability to localize pure tones at 60 degrees separation were determined for gerbils. The gerbils were trained using a two-choice procedure with observing response in which the gerbils made a left or right response to sounds emanating from their left or right side in order to obtain food. The average 75% correct localization threshold of 7 gerbils for a 100-ms noise burst was 27 degrees with chance performance (p greater than .01) reached at 12 degrees. The ability of 4 gerbils to localize both low- and high-frequency pure tones indicates that gerbils are able to use both phase- and intensity-difference locus cues. The frequency at which tone localization was poorest was 2.8 kHz, well below the theoretical frequency of ambiguity of the phase cue but within the frequency range at which phase locking declines in the mammalian auditory system. The sound localization ability of gerbils is typical of small rodents, and there is no obvious sign that it is affected by the degenerative disorder of the central auditory system which has been recently discovered in gerbils.

Hearing in domestic pigs (Sus scrofa) and goats (Capra hircus).

Behavioral audiograms were determined for three pigs and two goats. The hearing of the pigs ranged from 42 Hz to 40.5 kHz with a region of best sensitivity from 250 Hz to 16 kHz. Hearing in goats ranged from 78 Hz to 37 kHz with a well-defined point of best sensitivity at 2 kHz. Because these animals are unable to localize high-frequency tones, it seems unlikely that selective pressure to use the interaural spectral-difference cue for sound localization is behind their high-frequency hearing. Instead, we suggest that these and other hoofed mammals evolved high-frequency hearing in order to use monaural locus cues which prevent front/back locus reversals.