International Journal of Comparative Psychology

Over the past 55-60 million years cetacean (dolphin, whale, and porpoise) brains have become hyperexpanded so that modern cetacean encephalization levels are second only to modern humans. At the same time, brain expansion proceeded along very different lines than in other large-brained mammals so that substantial differences between modern cetacean brains and other mammalian brains exist at every level of brain organization. Perhaps the most profound difference between cetacean and other mammalian brains is in the architecture of the neocortex. Cetaceans possess a unique underlying neocortical organizational scheme that is particularly intriguing in light of the fact that cetaceans exhibit cognitive and behavioral complexity at least on a par with our closest phylogenetic relatives, the great apes. The neurobiological complexity underlying these cognitive capacities may involve the extreme multiplication of vertical structural units in the cetacean neocortex.

Comparative studies have yielded substantial insight into the functional relationships between the brain and behavior in birds. There are, however, important limitations to this method and problems can arise in the interpretation of the results. I use as an example, a test of whether interspecific brood parasitism is correlated with relatively smaller brains in the cuckoos and allies (Cuculiformes). Both conventional and phylogenetically based comparative statistics were used in conjunction with three alternative phylogenetic trees of the species examined. The comparisons between brood parasitism and relative brain size yielded mixed results, depending upon both the statistical method and the phylogeny employed. Although this could indicate that the evolution of interspecific brood parasitism is not related to relative brain size, the limitations of the comparative method in conjunction with the mixed results make it impossible to determine this with any certainty. The fact that different phylogenetic relationships yielded different results highlights the importance of phylogenetic relationships in assessing brain-behavior relationships. The continued use of phylogenetically based comparative methods should therefore be done cautiously, particularly with respect to interpretation of the results as the outcome may be as dependent upon the phylogeny as it is on the data itself.

A cladistic approach was used to reconstruct the probable changes from the basic sensorimotor system of early mammals to the much more complex system of humans and other anthropoid primates. At the cortical level, early mammals had as few as 4-5 somatosensory areas and possibly no separate motor areas. Early primates already had 7-8 motor areas, and additional higher-order somatosensory areas in lateral and posterior parietal cortex. Anthropoid primates are further distinguished by more serial processing, 4 distinct fields in anterior parietal cortex, and more areas in posterior parietal cortex.

A review of birthing in marsupials shows that there are at least three distinct methods. In the opossums (Didelphidae), possums, and kangaroos (Phalangeroidea), the expelled newborns crawl from the urogenital sinus to the pouch. In the bandicoots (Peramelidae), the expelled newborn remain attached to the placenta via the umbilical cord while they swim from the urogenital sinus to the pouch. In the carnivorous Dasyuridae, the newborn are expelled in a column of viscous fluid in which they “swim up” to the tunnel between the urogenital sinus and the pouch and then move to the pouch. Some of the recent anatomical studies, on the relative development of the neural system in newborn marsupials and on the behaviours of the newborns within the three birthing methods, have reawakened interest in the mechanisms that might be used to find the pouch. The motor patterns occurring in the newborn marsupials have many similarities to the motor patterns that appear in eutherian embryos at these same developmental stages. Studies that correlate the motor behaviours with the sensory and neural development of newborn marsupials could have important benefits for the understanding of the early organization of behaviour in mammals in general.

The mammalian neocortex varies greatly in size and internal organization across species. However it is often difficult to attribute specific cognitive abilities to corresponding cortical specializations. Here mammals with different sensory specializations are compared with their less, or differently, specialized relatives in order to identify trends in mammalian cortical evolution associated with increased behavioral abilities and sensory processing. In addition, some of the features of small versus large brains are considered in the context of evolution. The enlargement of cortex, changes to the organization of cortical areas, and the subdivision of cortex into additional areas, are seen as important trends correlated with the ability to process greater volumes of complex sensory information. Recent advances in the ability to manipulate gene expression during development suggest some of the mechanisms that have produced these changes. These mechanisms include alterations to a sensory surface (retina, cochlea, and skin) that affect neocortical maps through a cascade of inductive influences during development and more dramatic changes in brain organization that may result from duplication and subsequent specialization of cortical areas.

Play is rare in the Animal Kingdom, but relatively common in the larger brained vertebrate taxa. Comparisons at the level of classes, orders, and, in some cases, families, suggest that larger brained taxa are more likely to contain playful species. However, at the species level, such relationships generally disappear. In some well documented mammalian taxa, such as Rodentia, it is clear that there are species which do not play at all, some where the play is quite complex and some showing all grades in between. Comparative methods are used here to supplement proximal analyses of the content of one particular form of play, play fighting, so as to identify the neurobehavioral mechanisms that are needed in rodents to evolve complex play from simpler antecedents. At least five independent neural mechanisms are shown to be necessary to produce the most complex example of play fighting in rodents. The identification of such levels of control provides a new method for systematizing the diversity of play present in mammals. Furthermore, this approach sets the stage for re-evaluating the relationship between brain size and play. That is, the issue can be reconceptualized in terms of whether species with larger brains are more likely to have a greater number of control mechanisms. It is not that larger brained species are more likely to play, but rather, that when they do play, the content of their play is more flexible. Suitable comparative data sets are needed to test these possibilities.