The evolution of mammalian brain size.

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.

A test of the lateral semicircular canal correlation to head posture, diet and other biological traits in "ungulate" mammals.

For over a century, researchers have assumed that the plane of the lateral semicircular canal of the inner ear lies parallel to the horizon when the head is at rest, and used this assumption to reconstruct head posture in extinct species. Although this hypothesis has been repeatedly questioned, it has never been tested on a large sample size and at a broad taxonomic scale in mammals. This study presents a comprehensive test of this hypothesis in over one hundred "ungulate" species. Using CT scanning and manual segmentation, the orientation of the skull was reconstructed as if the lateral semicircular canal of the bony labyrinth was aligned horizontally. This reconstructed cranial orientation was statistically compared to the actual head posture of the corresponding species using a dataset of 10,000 photographs and phylogenetic regression analysis. A statistically significant correlation between the reconstructed cranial orientation and head posture is found, although the plane of the lateral semicircular canal departs significantly from horizontal. We thus caution against the use of the lateral semicircular canal as a proxy to infer precisely the horizontal plane on dry skulls and in extinct species. Diet (browsing or grazing) and head-butting behaviour are significantly correlated to the orientation of the lateral semicircular canal, but not to the actual head posture. Head posture and the orientation of the lateral semicircular canal are both strongly correlated with phylogenetic history.

[Rest and activity states in the Commerson's dolphin (Cephalorhynchus commersonii)].

The unihemispheric slow-wave sleep, the ability to sleep during swimming with one open eye and the absence of paradoxical sleep in the form of it is observed in all terrestrial mammals are unique features of sleep in cetaceans. Visual observations supplement electrophysiological studies and allow obtaining novel data about sleep of cetaceans. In the present study we examined behavior of 3 adult Commerson's dolphins Cephalorhynchus commersonii which were housed in the oceanarium Sea-World (San Diego, USA). The behavior of the dolphins can be subdivided into 5 swimming types: 1) active swimming marked by variable speed and irregular trajectory of movement (on average for 3 dolphins 35.1 +/- 2.7% of the 24-h period) was scored as active wakefulness; 2) circular swimming was divided into slow and fast swimming and occupied, on average, 44.4 +/- 3.8 and 9.7 +/- 0.8% of the 24-h period, respectively; while in circular swimming, dolphins swam from 1 to 6 circles on one respiration pause; 3) quiet chaotic swimming (3.9 +/- 1.2%) that occurred at the bottom and was not accompanied by signs of activity; 4) floating, and 5) slow swimming at the surface (4.1 +/- 0.5 and 2.8 +/- 0.4%), respectively; the latter two swimming types were accompanied by frequent respiration (hyperventilation). We suggest that sleep in Commerson's dolphins occurred predominantly during the circular and quiet swimming. From time to time the dolphins slowed down their speeds and even stopped for several seconds. Such episodes appeared to be the deepest sleep episodes. In all dolphins muscle jerks as well erections in the male were observed. Jerks and erections occurred during the circular and quiet chaotic swimming. Similar to other studied small cetaceans, Commerson's dolphins are in a state of almost uninterrupted swimming during 24 h per day and they sleep during swimming. Some muscle jerks that we observed in the dolphins in this study might have been episodes of paradoxical sleep.

Primate-like retinotectal decussation in an echolocating megabat, Rousettus aegyptiacus.

The current study was designed to reveal the retinotectal pathway in the brain of the echolocating megabat Rousettus aegyptiacus. The retinotectal pathway of other species of megabats shows the primate-like pattern of decussation in the retina; however, it has been reported that the echolocating Rousettus did not share this feature. To test this prior result we injected fluorescent dextran tract tracers into the right (fluororuby) and left (fluoroemerald) superior colliculi of three adult Rousettus. After a 2-week survival period the animals were killed, fixed via transcardial perfusion, and the retinas whole mounted and examined under fluorescent excitation to reveal the pattern of retrograde transport. Red and green labeled retinotectal ganglion cells were found in side-by-side patches on either side of a vertical decussation line in the temporal retina of all six retinas. The Rousettus examined thus exhibited the same pattern of retinal decussation as reported previously for other megabats and primates, but unlike that seen in other mammals. The current result indicates that the prior study appears to have suffered technical problems leading to an incorrect conclusion. The results of our study indicate that, as may be expected, all megabats share the derived retinotectal pathway once thought to be the exclusive domain of primates. The current study provides additional support for the diphyletic origin of the Chiroptera and aligns the megabats phylogenetically as a sister group to primates.

Observations on the giraffe central nervous system related to the corticospinal tract, motor cortex and spinal cord: what difference does a long neck make?

The mammalian corticospinal tract is known to contain axons that travel from the cerebral cortex to various levels of the spinal cord and its main function is thought to be the mediation of voluntary movement. The current study describes neuroanatomy related to the corticospinal tract of the giraffe. This animal presents a specific morphology that may present challenges to this neural pathway in terms of the metabolism required for correct functioning and maintenance of potentially very long axons. The spinal cord of the giraffe can be up to 2.6 m long and forms the conus medullaris at the level of the sacral vertebrae. Primary motor cortex was found in a location typical of that of other ungulates, and the cytoarchitectonic appearance of this cortical area was similar to that previously reported for sheep, despite the potential distance that the axons emanating from the layer 5 gigantopyramidal neurons must travel. A typically mammalian dorsal striatopallidal complex was transected by a strongly coalesced internal capsule passing through to the pons and forming clearly identifiable but somewhat flattened (in a dorsoventral plane) pyramidal tracts. These tracts terminated in a spinal cord that exhibited no unique anatomical features related to its length. Our results, at least at the level of organization investigated herein, show that the corticospinal tract of the giraffe resembled that of a typical ungulate.

Living on the edge: Daily, seasonal and annual body temperature patterns of Arabian oryx in Saudi Arabia.

Heterothermy, the ability to allow body temperature (Tb) to fluctuate, has been proposed as an adaptive mechanism that enables large ungulates to cope with the high environmental temperatures and lack of free water experienced in arid environments. By storing heat during the daytime and dissipating it during the night, arid-adapted ungulates may reduce evaporative water loss and conserve water. Adaptive heterothermy in large ungulates should be particularly pronounced in hot environments with severely limited access to free water. In the current study we investigated the effects of environmental temperature (ambient, Ta and soil, Ts) and water stress on the Tb of wild, free-ranging Arabian oryx (Oryx leucoryx) in two different sites in Saudi Arabia, Mahazat as-Sayd (MS) and Uruq Bani Ma'arid (UBM). Using implanted data loggers wet took continuous Tb readings every 10 minutes for an entire calendar year and determined the Tb amplitude as well as the heterothermy index (HI). Both differed significantly between sites but contrary to our expectations they were greater in MS despite its lower environmental temperatures and higher rainfall. This may be partially attributable to a higher activity in an unfamiliar environment for translocated animals in UBM. As expected Tb amplitude and HI were greatest during summer. Only minor sex differences were apparent that may be attributable to sex-specific investment into reproduction (e.g. male-male competition) during rut. Our results suggest that the degree of heterothermy is not only driven by extrinsic factors (e.g. environmental temperatures and water availability), but may also be affected by intrinsic factors (e.g. sex and/or behaviour).

Palaeoneurological clues to the evolution of defining mammalian soft tissue traits.

A rich fossil record chronicles the distant origins of mammals, but the evolution of defining soft tissue characters of extant mammals, such as mammary glands and hairs is difficult to interpret because soft tissue does not readily fossilize. As many soft tissue features are derived from dermic structures, their evolution is linked to that of the nervous syutem, and palaeoneurology offers opportunities to find bony correlates of these soft tissue features. Here, a CT scan study of 29 fossil skulls shows that non-mammaliaform Prozostrodontia display a retracted, fully ossified, and non-ramified infraorbital canal for the infraorbital nerve, unlike more basal therapsids. The presence of a true infraorbital canal in Prozostrodontia suggests that a motile rhinarium and maxillary vibrissae were present. Also the complete ossification of the parietal fontanelle (resulting in the loss of the parietal foramen) and the development of the cerebellum in Probainognathia may be pleiotropically linked to the appearance of mammary glands and having body hair coverage since these traits are all controlled by the same homeogene, Msx2, in mice. These suggest that defining soft tissue characters of mammals were already present in their forerunners some 240 to 246 mya.

Visual thalamocortical projections in the flying fox: parallel pathways to striate and extrastriate areas.

We studied thalamic projections to the visual cortex in flying foxes, animals that share neural features believed to resemble those present in the brains of early primates. Neurones labeled by injections of fluorescent tracers in striate and extrastriate cortices were charted relative to the architectural boundaries of thalamic nuclei. Three main findings are reported: First, there are parallel lateral geniculate nucleus (LGN) projections to striate and extrastriate cortices. Second, the pulvinar complex is expansive, and contains multiple subdivisions. Third, across the visual thalamus, the location of cells labeled after visual cortex injections changes systematically, with caudal visual areas receiving their strongest projections from the most lateral thalamic nuclei, and rostral areas receiving strong projections from medial nuclei. We identified three architectural layers in the LGN, and three subdivisions of the pulvinar complex. The outer LGN layer contained the largest cells, and had strong projections to the areas V1, V2 and V3. Neurones in the intermediate LGN layer were intermediate in size, and projected to V1 and, less densely, to V2. The layer nearest to the origin of the optic radiation contained the smallest cells, and projected not only to V1, V2 and V3, but also, weakly, to the occipitotemporal area (OT, which is similar to primate middle temporal area) and the occipitoparietal area (OP, a "third tier" area located near the dorsal midline). V1, V2 and V3 received strong projections from the lateral and intermediate subdivisions of the pulvinar complex, while OP and OT received their main thalamic input from the intermediate and medial subdivisions of the pulvinar complex. These results suggest parallels with the carnivore visual system, and indicate that the restriction of the projections of the large- and intermediate-sized LGN layers to V1, observed in present-day primates, evolved from a more generalized mammalian condition.

Specialization in pyramidal cell structure in the sensory-motor cortex of the vervet monkey (Cercopethicus pygerythrus).

Recent studies have revealed systematic differences in the pyramidal cell structure between functionally related cortical areas of primates. Trends for a parallel in pyramidal cell structure and functional complexity have been reported in visual, somatosensory, motor, cingulate and prefrontal cortex in the macaque monkey cortex. These specializations in structure have been interpreted as being fundamental in determining cellular and systems function, endowing circuits in these different cortical areas with different computational power. In the present study we extend our initial finding of systematic specialization of pyramidal cell structure in sensory-motor cortex in the macaque monkey [Cereb Cortex 12 (2002) 1071] to the vervet monkey. More specifically, we investigated pyramidal cell structure in somatosensory and motor areas 1/2, 5, 7, 4 and 6. Neurones in fixed, flat-mounted, cortical slices were injected intracellularly with Lucifer Yellow and processed for a light-stable 3,3'-diaminobenzidine reaction product. The size of, number of branches in, and spine density of the basal dendritic arbors varied systematically such that there was a trend for increasing complexity in arbor structure with progression through 1/2, 5 and 7. In addition, cells in area 6 were larger, more branched, and more spinous than those in area 4.

Multiple somatosensory areas in the anterior parietal cortex of the California ground squirrel (Spermophilus beecheyii).

Multiunit electrophysiological recording techniques were used to explore the somatosensory cortex of the California ground squirrel (Spermophilus beecheyii). Cortex rostral and caudal to the primary somatosensory area (SI) contained neurons that responded to stimulation of deep receptors and to muscle and joint manipulation. The region of cortex rostral to SI was termed the rostral field (R) because of possible homologies with a similar field described in other mammals. Cortex caudal to SI had neurons that responded to stimulation of deep receptors and has been termed the parietal medial area (PM), as in previous investigations in squirrels. Like SI, both R and PM contained a complete or almost complete representation of the body surface, although the receptive field size for clusters of neurons in these regions was somewhat larger than those for clusters of neurons in SI. Electrophysiological recording results were correlated with histologically processed tissue that had been sectioned tangentially. Although SI was clearly identified as a myelin-dense region, both R and PM stained much less densely for myelin. Our results indicate that as in a number of other mammals including monotremes, marsupials, carnivores, and primates, the anterior parietal cortex of the California ground squirrel contains multiple representations of the sensory epithelium. This work, as well as a growing body of studies of somatosensory cortex organization in a variety of mammals, indicates that anterior parietal fields other than SI existed early in mammalian evolution, and were present in the common ancestor of all mammals.

Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex.

Details of the representation of body regions innervated by the trigeminal nerve were elucidated in monkey cerebral cortex. Microelectrode recording was used to generate somatosensory maps in the posterior bank of the central sulcus and on the exposed cortical surface lateral to the lateral tip of the central sulcus in Macaca nemestrina. The area innervated by the contralateral trigeminal nerve is represented in an 8-mm mediolateral extent of area 3b lateral to the representation of the hand. Lateral to this, still within area 3b, there is an expanded representation of ipsilateral intra-oral structures measuring 6 mm in mediolateral extent. Both representations fill area 3b anteroposteriorly. The ipsilateral representation forms approximately 40% of the trigeminal representation, consistent with the amount of the ventroposterior medial (VPM) thalamic nucleus devoted to representation of ipsilateral intra-oral structures. Comparison of the present results with maps of the face representation in other species of monkey shows a consistent somatotopy of the face between species; size variations are mainly related to the enlarged ipsi- and contralateral representations of the cheek pouches in macaques. The general somatotopy of the trigeminal representation in monkeys is consistent with that in other mammalian species.

Representation of the face and intraoral structures in area 3b of the squirrel monkey (Saimiri sciureus) somatosensory cortex, with special reference to the ipsilateral representation.

Studies of the representation of the trigeminal nerve in the thalamus and cerebral cortex of mammals have revealed representations of both contra- and ipsilateral intraoral structures. However, the relative extent of both representations is subject to considerable species variation. The present study employed microelectrode mapping and anatomical tracing to investigate the location and extent of the ipsilateral representation in area 3b of the somatosensory cortex of squirrel monkeys. A small region, approximately 2 mm2, was found to be responsive to stimulation of ipsilateral intraoral structures. This region was located on the anteromedial border of area 3b, surrounded by the representation of the contralateral roof of the mouth. This region corresponded to areas of intense anterograde labeling following injections placed in the ventromedial portion of the ventral posterior medial nucleus of the thalamus at the only sites where neural responses could be elicited by stimulation of ipsilateral intraoral structures. The amount of thalamus and cortex given over to the ipsilateral representation in the squirrel monkey is small compared with that of the macaque monkey. This difference may be related to the lack of cheek pouches in the squirrel monkey, and therefore a different strategy for eating. The representation of the contralateral lower lip in area 3b was split by the representation of the contralateral upper lip. This split representation is in agreement with previous studies of the trigeminal representation in area 3b of the macaque monkey and may be a general feature of the representation of the trigeminal nerve in area 3b of primate cerebral cortex.

Adaptive responses of monkey somatosensory cortex to peripheral and central deafferentation.

This study deals with two kinds of activity-dependent phenomena in the somatosensory cortex of adult monkeys, both of which may be related: (1) mutability of representational maps, as defined electrophysiologically; (2) alterations in expression of genes important in the inhibitory and excitatory neurotransmitter systems. Area 3b of the cerebral cortex was mapped physiologically and mRNA levels or numbers of immunocytochemically stained neurons quantified after disrupting afferent input peripherally by section of peripheral nerves, or centrally by making lesions of increasing size in the somatosensory thalamus. Survival times ranged from a few weeks to many months. Mapping studies after peripheral nerve lesions replicated results of previous studies in showing the contraction of representations deprived of sensory input and expansion of adjacent representations. However, these changes in representational maps were in most cases unaccompanied by significant alterations in gene expression for calcium calmodulin-dependent protein kinase isoforms, for glutamic acid decarboxylase, GABA(A) receptor subunits, GABA(B) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or N-methyl-D-aspartate (NMDA) receptor subunits. Mapping studies after lesions in the ventral posterior lateral nucleus (VPL) of the thalamus revealed no changes in cortical representations of the hand or fingers until >15% of the thalamic representation was destroyed, and only slight changes until approximately 45% of the representation was destroyed, at which point the cortical representation of the finger at the center of a lesion began to shrink. Lesions destroying >60% of VPL resulted in silencing of the hand representation. Although all lesions were associated with a loss of parvalbumin-immunoreactive thalamocortical fiber terminations, and of cytochrome oxidase staining in a focal zone of area 3b, no changes in gene expression could be detected in the affected zone until >40-50% of VPL was destroyed, and even after that changes in mRNA levels or in numbers of GABA-immunoreactive neurons in the affected zone were remarkably small. The results of these studies differ markedly from the robust changes in gene expression detectable in the visual cortex of monkeys deprived of vision in one eye. The results confirm the view that divergence of the afferent somatosensory pathways from periphery to cerebral cortex is sufficiently great that many fibers can be lost before neuronal activity is totally silenced in area 3b. This divergence is capable of maintaining a high degree of cortical function in the face of diminishing inputs from the periphery and is probably an important element in promoting representational plasticity in response to altered patterns of afferent input.

Sleep in the platypus.

We have conducted the first study of sleep in the platypus Ornithorhynchus anatinus. Periods of quiet sleep, characterized by raised arousal thresholds, elevated electroencephalogram amplitude and motor and autonomic quiescence, occupied 6-8 h/day. The platypus also had rapid eye movement sleep as defined by atonia with rapid eye movements, twitching and the electrocardiogram pattern of rapid eye movement. However, this state occurred while the electroencephalogram was moderate or high in voltage, as in non-rapid eye movement sleep in adult and marsupial mammals. This suggests that the low-voltage electroencephalogram is a more recently evolved feature of mammalian rapid eye movement sleep. Rapid eye movement sleep occupied 5.8-8 h/day in the platypus, more than in any other animal. Our findings indicate that rapid eye movement sleep may have been present in large amounts in the first mammals and suggest that it may have evolved in pre-mammalian reptiles.

Plasticity of the somatosensory cortical map in macaque monkeys after chronic partial amputation of a digit.

Activity-dependent changes in cortical representational maps have been reported in many studies of adult mammals. Limits in extent of change have been attributed to limited divergence in the thalamocortical projection. However, studies have commonly been restricted to animals surviving less than a year following relatively modest peripheral sensory perturbations. After extensive deafferentation and long-term survival, more extensive changes, seemingly beyond the limits of thalamocortical divergence, have been reported. We report changes in the somatotopic map in area 3b of an adult macaque monkey, in which part of the index finger of one hand had been amputated two years previously. The representation of the remaining stump occupied the whole region of area 3b normally devoted to the representation of the entire digit. The skin surrounding the stump appeared to have been hyperinnervated by axons severed during the amputation. The hyperinnervation of remaining skin may have reactivated neurons of the somatosensory system silenced by the amputation, leading to the recovery of a cortical map but with a modified organization.

Nerve terminals of mucous gland electroreceptors in the platypus (Ornithorhynchus anatinus).

Platypus mucous gland electroreceptors differ from electroreceptors described for fish in that they lack an associated specialized sensory cell. Thus a bare nerve terminal is used to detect electrical stimuli, and also to generate local and action potentials. Previous studies have identified these terminals (an average of 16 per mucous gland), but had not shown whether the terminals have direct contact with the duct of the mucous gland. This poses the problem of how the electrical stimulus reaches the nerve terminals. This study demonstrates the portions of the nerve terminals responsible for electroreception, and shows how these portions use the surrounding epidermal tissue to overcome the combined problems of lacking a sensory cell and making physical contact with the conducting medium in the duct of the gland. A terminal axonal filament is described which accommodates for these problems, the terminal filament provides a low-resistance pathway for the electrical stimuli, and is embedded with its proximal and distal portions in high and low resistance epidermis, respectively. Lateral interactions occur between adjacent terminal filaments via a plexus that is directed circumferentially around the duct from the proximal portion of the terminal filament. These circumferential arbors form an interconnecting ring between all 16 terminal filaments, and may be used to lower the signal-to-noise ratio of the electroreceptor and thus enhance overall sensitivity.

Microbats appear to have adult hippocampal neurogenesis, but post-capture stress causes a rapid decline in the number of neurons expressing doublecortin.

A previous study investigating potential adult hippocampal neurogenesis in microchiropteran bats failed to reveal a strong presence of this neural trait. As microchiropterans have a high field metabolic rate and a small body mass, it is possible that capture/handling stress may lead to a decrease in the detectable presence of adult hippocampal neurogenesis. Here we looked for evidence of adult hippocampal neurogenesis using immunohistochemical techniques for the endogenous marker doublecortin (DCX) in 10 species of microchiropterans euthanized and perfusion fixed at specific time points following capture. Our results reveal that when euthanized and perfused within 15 min of capture, abundant putative adult hippocampal neurogenesis could be detected using DCX immunohistochemistry. Between 15 and 30 min post-capture, the detectable levels of DCX dropped dramatically and after 30 min post-capture, immunohistochemistry for DCX could not reveal any significant evidence of putative adult hippocampal neurogenesis. Thus, as with all other mammals studied to date apart from cetaceans, bats, including both microchiropterans and megachiropterans, appear to exhibit substantial levels of adult hippocampal neurogenesis. The present study underscores the concept that, as with laboratory experiments, studies conducted on wild-caught animals need to be cognizant of the fact that acute stress (capture/handling) may induce major changes in the appearance of specific neural traits.

Questioning the interpretations of behavioral observations of cetaceans: is there really support for a special intellectual status for this mammalian order?

This review evaluates and contextualizes the behavioral studies undertaken on cetaceans in terms of the relationship of these behaviors to special levels of intelligence associated with these marine mammals and the evolution of their relatively and absolutely large brain size. Many believe that the large size of the cetacean brain and reported behaviors indicate the need to create a special status for these animals in terms of their intellect, positing that they are second to humans in terms of general intelligence. Cetacean brains became relatively large approximately 32millionyearsago, at the Archaeocete-Neocete faunal transition, and have since remained stable in relative size. The behaviors reported for modern cetaceans are thought to parallel those of great apes, to the exclusion of other mammals. By creating an autocatalytic model of cetacean brain evolution, the behaviors thought to be indicative of sophisticated cognitive processes can be assessed as to their potential involvement in the evolution of larger brains in cetaceans. By contextualizing these behaviors in a broader comparative framework, and not the limited cetacean - great ape comparisons mostly used, it is evident that the behaviors used to argue for high levels of intelligence in cetaceans are found commonly across mammals and other vertebrates, and are often observed in invertebrates. This contextualization indicates that cetacean intelligence is qualitatively no different to other vertebrates. In addition, the inability of cetaceans to surpass Piaget stage 4/5 on object permanence tests and to solve an "if and only if, then" abstract task indicates the possibility that their levels of general intelligence may be less than that seen in other vertebrates. Sophisticated cognitive abilities appear to play no role in the evolution of large brain size in cetaceans, indicating that alternative theories of large brain size evolution in cetaceans should be considered in more detail.

Adult neurogenesis in a giant otter shrew (Potamogale velox).

Adult neurogenesis in mammals is typically observed in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone. We investigated adult neurogenesis in the brain of a giant otter shrew (Potamogale velox), a semi-aquatic, central African rainforest mammal of the family Tenrecidae that belongs to the superorder Afrotheria. We examined neurogenesis immunohistochemically, using the endogenous marker doublecortin (DCX), which stains neuronal precursor cells and immature neurons. Our results revealed densely packed DCX-positive cells in the entire extent of the subventricular zone from where cells migrated along the rostral migratory stream to the olfactory bulb. In the olfactory bulb, DCX-expressing cells were primarily present in the granular cell layer with radially orientated dendrites and in the glomerular layer representing periglomerular cells. In the hippocampus, DCX-positive cells were identified in the subgranular and granular layers of the dentate gyrus and strongly labelled DCX-positive processes, presumably dendrites and axons of the newly generated granular cells, were observed in the CA3 regions. In addition, DCX immunoreactive cells were present in the olfactory tubercle, the piriform cortex and the endopiriform nucleus. While DCX-positive fibres have been previously observed in the anterior commissure of the hedgehog and mole, we were able to demonstrate the presence of DCX-positive cells presumably migrating across the anterior commissure. Taken together, the giant otter shrew reveals patterns of neurogenesis similar to that seen in other mammals; however, the appearance of possible neuronal precursor cells in the anterior commissure is a novel observation.

Adult neurogenesis in eight Megachiropteran species.

The present study evaluated, using immunohistochemical methods, the presence and characteristics of proliferating and newly generated neurons in the brain of eight wild-caught adult Megachiropteran species. For the neurogenic patterns observed, direct homologies are evident in other mammalian species; however, there were several distinctions in the presence or absence of proliferating and immature neurons, and migratory streams that provide important clues regarding the use of the brain in the analysis of Chiropteran phylogenetic affinities. In all eight species studied, numerous Ki-67- and doublecortin (DCX)-immunopositive cells were identified in the subventricular zone (SVZ). These cells migrated to the olfactory bulb through a Primate-like rostral migratory stream (RMS) that is composed of dorsal and ventral substreams which merge before entering the olfactory bulb. Some cells were observed emerging from the RMS coursing caudally and dorsally to the rostral neocortex. In the dentate gyrus of all species, Ki-67- and DCX-expressing cells were observed in the granular cell layer and hilus. Similar to Primates, proliferating cells and immature neurons were identified in the SVZ of the temporal horn of Megachiropterans. These cells migrated to the rostral and caudal piriform cortex through a Primate-like temporal migratory stream. Sparsely distributed Ki-67-immunopositive, but DCX-immunonegative, cells were identified in the tectum, brainstem and cerebellum. The observations from this study add to a number of neural characteristics that phylogenetically align Megachiropterans to Primates.