Functional Morphology of the Middle Ear of Ctenomys talarum (Rodentia: Octodontidae)

1. Functional Morphology of the Middle Ear of Ctenomys talarum (Rodentia: Octodontidae) E.C., Schleich, C., Bush (2004) Functional Morphology of the Middle Ear of Ctenomys talarum (Rodentia: Octodontidae. J of Mamm, 82: 290-295.

2. Ctenomys talarum • Solitary subterranean rodent • Significant features: -enlarged middle-ear cavity -round and larger eardrum without pars flaccida -no connection between malleus and tympanic bone -partial fusion of malleus with incus -nearly flat stapedial footplate -Absence of stapedial artery

3. Continued… -reduced tensor tympani -absence of stapedial muscle • 120 and 160 g in body mass for females and males respectively • Vocalizations are moderate to low in frequency which common when living in underground environments.

5. Observations • The physical environment exerts strong effects on design of animal displays, but the influence of receiver properties on their evolution distinguishes displays from other traits. • In mammals, hearing properties of the auditory organ are related strongly to middle-ear morphology. • According to the optimality principle, physical characteristics of vocalizations should reflect adaptation to the physical environment, body size, or hearing ability.

6. The Middle Ear

7. - (1) Malleus ; - (2) Malleus ligament ; - (3) Incus ; - (4) Incus ligament; - (5) Stapes muscle (stapedius); - (6) Stapes footplate; - (7) Eardrum;- (8) Eustachian tube;- (9) Malleus muscle (tensor tympani);- (10) Nerve (chorda tympani) sectioned

8. Hypothesis • C. talarumand other subterranean species should have anatomical features that optimize low-frequency hearing.

9. Materials and Methods • Eight adult males and 14 adult females captured in Mar de Cobo using plastic live traps set at fresh surface mounds • In lab animals were killed by cervical dislocation • Naso-occipital length and greatest zygomatic width measured with digital calipers (0.01 mm) • Bullar length, width, and depth measured • With ocular micrometer (0.06 mm) length and diameter of auditory meatus, diameter of eardrum, length of head of malleus, length of lever arm of malleus, length of incus etc.

10. Results • Enlarged middle-ear cavity • Round and large eardrum without pars flaccida • Lack of connection between malleus and tympanic bone • Partial fusion of malleus with incus • Nearly flat stapedial footplate • Reduced (tensor tympani) or absent (stapedial) middle-ear muscles • Reduced Pinna which accounts for poor sound-localization capacities of subterranean forms

11. Comparison Species • Geomys bursarius- medium to small sized, dark brown gophers • Spalax ehrenbergi - blind mole rat • Heterocephalus glaber- naked mole-rat • Clyomys and Octodon • Cryptomys hottentotus- common mole-rat

12. Comparison • Reduced size of pinna shared with Spalax ehrenbergi and Heterocephalus glaber • Round eardrum wit no evident pars flaccida occurs in C. talarum, S. ehrenbergi, G. bursarius, and Cryptomys hottentotus • Middle ear cavity of C. talarum enlarged in comparison with some surface dwellers of Caviomorpha, although approximating the size of the middle ear of Geomys bursarius • Walls of middle-ear cavity of C. talarum, Clyomys, and Octodon are cancellous bone covered with compact bone

13. Conclusion • The middle ear of C. talarum has anatomical features that are probably adaptations to keep of enhance low-frequency sounds transductions. • These modifications agree with the dominant low-frequency vocalizations of C. talarum and with the best transmission frequency in subterranean environments. • Modifications also found in several species of unrelated subterranean rodents.