Saturday, December 23, 2006
An open access/free paper from the first edition of PloS ONE:
Citation: Schillaci MA (2006) Sexual Selection and the Evolution of Brain Size in Primates. PLoS ONE 1(1): e62. doi:10.1371/journal.pone.0000062
Reproductive competition among males has long been considered a powerful force in the evolution of primates. The evolution of brain size and complexity in the Order Primates has been widely regarded as the hallmark of primate evolutionary history. Despite their importance to our understanding of primate evolution, the relationship between sexual selection and the evolutionary development of brain size is not well studied. The present research examines the evolutionary relationship between brain size and two components of primate sexual selection, sperm competition and male competition for mates. Results indicate that there is not a significant relationship between relative brain size and sperm competition as measured by relative testis size in primates, suggesting sperm competition has not played an important role in the evolution of brain size in the primate order. There is, however, a significant negative evolutionary relationship between relative brain size and the level of male competition for mates. The present study shows that the largest relative brain sizes among primate species are associated with monogamous mating systems, suggesting primate monogamy may require greater social acuity and abilities of deception.
Since Darwin's 1871 publication* on the evolution of humans and sexual selection, reproductive competition among males has been considered a powerful force in the evolution of primates and other mammals. The evolution of brain size and complexity in the Order Primates is widely regarded as the hallmark of primate evolutionary history. Despite their importance to understanding primate evolution, the relationship between sexual selection and brain size evolution is not well studied. With the exception of whales, primate brain evolution is unique among mammals. For primates, the evolutionary increase in brain size is often attributed to increased social complexity. Research associating increasing brain size with increasing group size and social complexity in primates predicts brain size, specifically, the size of the neocortex, will co-evolve with mating systems exhibiting social complexity. In this context, larger brains are selected for because they confer greater reproductive fitness associated with increased social acuity or the ability to manipulate others within the group. Increases in the size of the prefrontal cortex in particular, which mediates important components of complex social behavior such as planning, working memory, memory for serial order, and language may have played an important role in human brain evolution.
Recent research on mating systems and brain size in a closely related mammal, bats, predicts a negative evolutionary relationship between levels of sperm competition as measured by relative testes mass, and the development of brain size stemming from an investment trade-off between two metabolically costly tissues. The results from that study indicated that while species with mating systems that include multiple copulations by males has no evolutionary impact on relative brain size, mating systems with multiple matings by females do influence brain size evolution. Bat species with mating systems based on female promiscuity were associated with smaller brains and larger testes, while species with mating systems based on female fidelity were associated with significantly larger brains and smaller testicles.
The present research investigates the evolutionary relationship between brain size and two components of primate sexual selection in primates 1) sperm competition as measured by relative testes size, and 2) male competition for mates estimated from the level of sexual mass dimorphism.
Continued at "Sexual Selection and the Evolution of Brain Size in Primates"
The nature of the following work will be best understood by a brief account of how it came to be written. During many years I collected notes on the origin or descent of man, without any intention of publishing on the subject, but rather with the determination not to publish, as I thought that I should thus only add to the prejudices against my views. It seemed to me sufficient to indicate, in the first edition of my Origin of Species, that by this work "light would be thrown on the origin of man and his history"; and this implies that man must be included with other organic beings in any general conclusion respecting his manner of appearance on this earth. Now the case wears a wholly different aspect. When a naturalist like Carl Vogt ventures to say in his address as President of the National Institution of Geneva (1869), "personne, en Europe au moins, n'ose plus soutenir la creation independante et de toutes pieces, des especes," it is manifest that at least a large number of naturalists must admit that species are the modified descendants of other species; and this especially holds good with the younger and rising naturalists. The greater number accept the agency of natural selection; though some urge, whether with justice the future must decide, that I have greatly overrated its importance. Of the older and honoured chiefs in natural science, many unfortunately are still opposed to evolution in every form.
In consequence of the views now adopted by most naturalists, and which will ultimately, as in every other case, be followed by others who are not scientific, I have been led to put together my notes, so as to see how far the general conclusions arrived at in my former works were applicable to man. This seemed all the more desirable, as I had never deliberately applied these views to a species taken singly. When we confine our attention to any one form, we are deprived of the weighty arguments derived from the nature of the affinities which connect together whole groups of organisms- their geographical distribution in past and present times, and their geological succession. The homological structure, embryological development, and rudimentary organs of a species remain to be considered, whether it be man or any other animal, to which our attention may be directed; but these great classes of facts afford, as it appears to me, ample and conclusive evidence in favour of the principle of gradual evolution. The strong support derived from the other arguments should, however, always be kept before the mind. (Continued)
Info on Primatology:
Primatology is the study of non-human primates. It is a diverse discipline and primatologists can be found in departments of biology, anthropology, psychology and many others. It is closely related to physical anthropology, which is the primatology of the genus Homo, especially Homo sapiens. The fields cross over in the study of the hominids, which includes all ape-like ancestors of man and the other great apes (for a list of common ancestors with other living species see The Ancestor's Tale).
Modern primatology is an extremely diverse science. It ranges from anatomical studies of primate ancestors and field studies of primates in their natural habitat, to experiments in animal psychology and ape language. It has cast an immense amount of light on basic human behaviors and ancient ancestry of these behaviors.