Visual and Olfactory Brain Systems
Barton et al. 1995 examine the correlation of size of sensory structures in the brain with each other and with ecological factors of activity schedules and diet within primates, bats and insectivores. Controlling for brain size and phylogenetic relatedness, they show several correlations of residual contrasts between visual and olfactory brain structures that are in several ways consistent with ecological lifestyles of the species in question. For example, in primates there is an overall negative correlation between olfactory and visual structures, where diurnal species have larger visual cortices and nocturnal species larger olfactory structures. Additionally, diurnal frugivore-omnivores have larger visual cortices than diurnal foliovores. In bats, there is a positive relationship between olfactory and visual structure size, though it is suggested that these two characteristics are likely involved in a trade-off with echolocation abilities. Insectivore analyses typically had low sample sizes, though trends suggest a trade off between fossorial abilities and visual structure size. This study is well done, especially considering that it was published ten years ago. It sets the stage for genetic-based and functionally-based studies of these “trade offs” within particular groups (i.e. trichromatic vision in primates).
Gilad et al. provide a comparative study of the proportions of olfactory receptor pseudogenes across 19 primate species, finding that routine trichromatic primates have a higher proportion of pseudogenes than dichromatic (or allelic trichromatic) primates, suggesting decreased reliance on the olfactory sense relative to vision in trichromatic primates. This high proportion of pseudogenes association was also consistent for the howler monkey, the only new-world monkey to acquire routine trichromatic vision, an independent event from the evolution of trichromacy in old-world primates.
So, we see a decrease in utility of OR genes coincident with the evolution of trichromatic vision in primates, although Gilad et al. cannot show that this is due to a direct relationship between olfaction and vision. Here’s an idea—maybe can we test the relationship between olfactory and visual systems in other ways, looking at utility via the signature of neural connectivity across individuals in a population. Many (most? all?) new world monkeys have “allelic trichromacy” where the optic gene on the X-chromosome is polymorphic for sensitivity to medium and long wavelengths; as a result, female heterozygous individuals are trichromatic and female homozygous individuals and males are not. Assuming there is some advantage to trichromacy, and if the neural sensitivity in sensory systems are labile within individuals, maybe we could expect to see increased neural sensitivities in the visual brain regions with a corresponding decrease in olfactory neural sensitivity/connectivity for female heterozygous individuals compared to other dichromatic individuals in a population. ???
posted by Jill Jankowski at 1:46 AM
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Gillad et. al. 2004
The authors were interested if the Olfactory receptor (OR) pseudogenes was common among primates, if not, figuring out when the proportion of OR pseudogenes increased. They used the same degenerate primers to sequence 100 OR genes from 18 nonhuman primates. The found that the deterioration of the olfactory repertoire occurred in all apes, old new world monkey and, independently, howler monkey. Interestingly the reduction in olfactory “abilities” coincided with the arose of full trichromatic vision. After reading this results I was wonder what is the connection of reducing olfactory capacity and increasing visual ones. As the authors mention in their last paragraph, I was think in this two possibilities, these tradeoff may be related with either food selection or mate choice. I agree more with the mate choice one, although previous studies suggest that this is not the case. I also was wonder if the reduction of the OR and the arose of full trichromatic vision, is not just a simple energy or functional arrangement like the one we saw with brain and gut size.
Barton et. al. 1995
In general the authors of this paper explore the relation between visual and olfactory brain systems with some behavioral and ecological characteristics, in primates, bats and insectivores. They found that in all the groups included on the study each sensory structure was correlated with the size of the rest of the brain. Their results were clear in primates compared with the other groups, they found that olfactory bulbs are larger in nocturnal primates, where as striate visual cortex is larger in diurnal ones. They made the argument that frugivorous-omnivores species may relay more heavily on vision, specially color vision, than do folivores. This argument is not supported by the finding in the previous paper (Gillad et. al 2004). They also show that frugivorous bats have larger olfactory bulbs and visual abilities, compared to non-frugivorous. This also agree with the increase of echolocation abilities on insectivorous bats. The insectivores part was not very clear, but the message that I got at the end was that aquatic lineages have reduced the olfactory abilities. My opinion is that the authors are making stories base on small sample size and ecological factor which are poorly know for most of these groups, and at the end is hard to see the main message. Moreover, the ecological story can be told from the mating system point of view.
Sensory systems are important for an animal’s navigation of its environment. For this reason, Barton et al. (1995) argue that ecological factors influence differential size of brain structures. Using data from primates, bats, and insectivores, the authors found negative correlations between visual and olfactory structures in primates and insectivores (not significant) and a positive correlation in these structures in bats. Negative correlations suggest that specializations in one structure may trade off specialization in others. Ecological factors that have influenced the coevolution/differential evolution of these structures include: activity timing, diet, and habitat.
If there are trade-offs between brain structures such that developing one structure necessitates a reduction in the other, how can we use this information to infer evolutionary patterns? In the last sentence in the abstract, Barton et al. suggest that we can use evolutionary associations between brain structures as an alternative explanation for correlates in brain size which have previously been attributed to selection on intelligence. What?
In contradiction to some of the findings by Barton et al., Gilad et al. found that apes, Old World monkeys, and Howler monkeys show an increase in the number of olfactory receptor pseudogenes compared to New World monkeys. Apes, OWM, and Howler monkeys have trichromatic vision suggesting that there is a trade-off between vision and olfaction.
It’s puzzling that Howler monkeys, a folivore, would evolve trichromatic vision where other NWM have not. Is it possible that some other ecological factor (Barton et al.) selected for trichromatic vision in Howler monkeys? What is up with the genetic mode of determination for trichromatic vision in NWMs? Does this suggest the importance of sexual selection in shaping NWM vision?
Gilad et al. performed a study in which they compared the proportion of olfactory receptor (OR) pseudogenes in 19 primate species. They randomly chose subsets of 100 OR genes from each species, and found that apes, old world monkeys, and the howler monkey had a much higher proportion of pseudogenes than new world monkeys and the lemur. This high proportion of pseudogenes also corresponds with full trichromatic vision. Interestingly, the old world monkeys and apes developed trichromatic vision independently of the howler monkey. These results suggest that the acquisition of trichromatic vision made smell less vital of a sense.
Barton et al. also studied the relationship between visual and olfactory receptors in the brain. They studied the correlation between the size of olfactory and visual strucures in primates, bats, and insectivores. They also compared the size of these brain structures amongst the diurnal and nocturnal animals in these groups, among frugivores and folivorous animals, and among aquatic animals and fossorial animals. They found that activitity timing, diet, and habit all had effects on sensory system size. Correlation between visual and olfactory structures in primates was found to be negative in primates, negative in insectivores but non-significant, and postivive in bats. Both nocturnal primates and nocturnal insectivores were found to have larger olfactory structures than their diurnal counterparts. In diurnal primates, larger visual cortexes were found. Frugivores has larger visual structures in diurnal animals and larger olfactory structures in nocturnal animals. Also, olfactory structures were found to be smaller in aquatic lineages,and visual structures were smaller in fossorial lineages.
Both of these studies suggest that animals are capable of enlarging the sensory structures through evolution which are most important to their niche, while shrinking the size of less important sensory structures. These studies make me wonder if this trade off is found in all brain structures or just sensory structures.
In examining primates, bats, and insectivores, Barton and colleagues find that brain regions vary in size due to different ecological pressures. Overall, nocturnal species had greater olfactory structure size than their diurnal counterparts. The trade-off being a more diminutive visual sensory system. Further, diurnal species had increased levels of cones, which is sensible due to color vision requiring higher levels of light. It follows that nocturnal frugivores would be more reliant on other senses (olfaction).
Correspondingly, the researchers find that nocturnal frugivores have greater olfactory sense structures. I do not know quite what to make of decreased olfactory structures in aquatic insectivores. Intuitively it seems olfaction is hindered in water, but the aquatic lifestyle does not seem to accentuate any other sense system with the exception of sound which travels faster in water. Surely it must be even harder to see underwater than on land, as most wavelengths of light are absorbed by forty feet beneath sea-level.
The relationship between retina and visual cortex size seems paradoxical. If animals at night have larger eyes, it follows that they would have more cells in the eye, and thus need greater neural connectedness, etc. Yet, we find that diurnal animals, which have smaller eyes (think a monkey versus an owl), have larger visual cortices than their nocturnal counterparts.
The Gilad et. al. paper brings some evidence in favor of the trade-offs proposed by Barton. These researches find that degradation of genes (pseudogenes) increased going from mice, to non-human apes, to humans. It is startling to read that around 60 percent of humans olfactory receptor genes contain at least one coding region disruption. It would be interesting to see OR data for pseudogene prevalence in the Canis Genus and Chondrichthyes Class. Looking to find where the proportion on OR pseudogenes increased, they examined 19 primate species. Ultimately, New world Howler monkeys were the aberration, and incidentally had Full Trichromatic vision. This is surmised as a possible alleviating factor that enabled a decrease in olfactory capability.
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