Category: Senses
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An organism's senses connect it to the outside world and are essential in allowing it to react adequately to environmental stimuli. As the structure of sensory organs is limited by the physical properties of the signals they are supposed to pick up (e.g. light, sound), it is not surprising that they represent prime examples of evolutionary convergence.
With respect to vision, classic cases of convergence include the camera eye (which has not only been invented by cephalopods and vertebrates but also by several other groups) and the compound eye (famously found in arthropods as well as in other invertebrates). Colour vision has evolved multiple times in insects and vertebrates, but remarkably is also found in mantid shrimps, highly effective predators with complex compound eyes. Vertebrates are primitively tetrachromatic (having four different types of cone cells with different light absorption spectra), but most mammals are dichromatic, which might reflect a transition to a nocturnal way of life. In the largely diurnal primates, however, trichromatic vision has re-evolved, allowing them to detect coloured fruit and leaves. In many animals, both vertebrate and invertebrate, vision extends into the ultraviolet part of the electromagnetic spectrum. For example, ultraviolet plumage ornaments are often involved in bird courtship, kestrels prey on voles by following their UV-reflecting urine trails and numerous insects use UV marks on flower petals to find nectar.
Although to date there is no direct evidence for infrared vision, a number of animals have independently evolved systems of infrared detection (that in a number of respects are closely analogous to the eye) based on heat. Many snakes (e.g. pythons, pit vipers and rattlesnakes), vampire bats and insects such as bedbugs detect infrared emitted as heat from warm-blooded prey, whereas several beetles as well as hemipterans are attracted to the heat of forest fires (they lay their eggs in burnt wood). The general ability to sense thermal radiation has mainly been studied in insects and mammals. In some butterflies, for example, the antennae are equipped with thermoreceptors that seem to have some similarities to the pit receptors of snakes.
A sense of balance is essential in a three-dimensional world, and an almost universal, but convergent, method to detect changes in orientation is with the help of statoliths. These are small grains attached to fine hairs, the movement of which then triggers nerve impulses. Statoliths are employed in cephalopods, cnidarian jellyfish, crustaceans and other invertebrates, whereas in vertebrates, balance is achieved by using the semi-circular canals of the ear. Remarkably, a strikingly similar canal system operates in some crabs.
Hearing has evolved independently in a number of groups, notably in the insects and vertebrates. Many animals are able to hear very high frequency (ultrasound) or very low frequency sounds (infrasound) that lie well outside the range of human hearing. Most famous amongst those sensitive to ultrasound are the bats, which use echolocation for prey detection and navigation. But echolocation has also evolved in cetaceans (whales and dolphins), other mammals such as shrews and tenrecs, as well as some birds. In addition, a species of frog is capable of ultrasonic communication. Low frequency sounds travel partly by air and partly through the ground, which is the basis for seismic communication. It entails quite long distance transmissions (e.g. in elephants) or shorter distances (e.g. in the many insects that sense vibrations).
Other pressure-sensitive systems also show convergence. Fascinating examples include the lateral-line system, a specialised system for pressure detection under water, and mechanosensory systems based on small corpuscular touch receptors (often linked to seismic communication). The lateral-line system is best known in aquatic vertebrates (mostly fish as well as some amphibians), but analogues have evolved independently in some cephalopods, a few aquatic mammals and, in an intriguingly separate way, in some penaeid shrimps. Mechanosensory systems in different groups of animals employ different, and convergent, types of touch receptors, such as Herbst corpuscles in the bills of birds and equivalent Pacinian corpuscles in mammals. Equally noteworthy are the tactile Eimer's organs in moles and convergent push-rods in monotreme mammals. Closely associated with the mechanosensory system of monotremes is their electrosensory system. Electroreception, i.e. the ability to perceive weak electric fields, has also been demonstrated in fish (where it evolved at least twice) and amphibians, relying on different types of receptors and different brain regions for processing the electric signals.
Many features of olfaction strongly suggest that there really is an optimal solution to detecting smells. The common design of olfactory receptors has evolved independently many times and olfactory processing circuits in the brain are convergent between insects and mammals. Pheromone recognition, widespread in vertebrates and insects, employs closely analogous, but convergent, systems. Olfaction furthermore exemplifies that a reduction in sensory capacities can be convergent, too. Apes and humans show a loss of olfactory genes, which is likely to be linked to the development of acute vision, as do whales, which lost their terrestrial olfactory capacities upon returning to the oceans. Although the capacity to taste is clearly very ancient, it still provides interesting insights into convergence. A bitter modality has independently evolved in humans and chimpanzees, and in a striking example of molecular convergence, the gustatory proteins of arthropods are effectively identical in structure to those of other animals, but clearly have a completely different origin.
| Topic title | Teaser text | Availability |
|---|---|---|
| Light producing chemicals: how to make bioluminescence | The most remarkable luciferin in terms of its distribution is known as�coelenterazine. This nitrogen-ring based molecule is found in nine separate groups, ranging from radiolarians to fish. | Available |
| Vibrational communication in mammals | Kangaroo rats drum their foot on the ground upon encountering a snake. Why? Read on for this and many other fascinating examples of vibrational communication in mammals… | Available |
| Vibrational communication in insects and spiders | Some spiders have evolved a most remarkable method of capturing other spiders – they imitate the vibrations of insects caught in their victim’s web. And this is only one of numerous intriguing examples of vibrational communication in arthropods… | Available |
| Electrolocation and electrocommunication in weakly electric fish | Fish have eyes, but they live in a much more complex sensory world, where even electricity plays a surprising - and convergent - role. | Available |
| Echolocation in bats | How can bats navigate in total darkness amongst trees and branches, but still locate a tiny, fluttering insect with extraordinary acuity? All made possible through echolocation, an astonishing sensory mechanism… | Available |
| Monochromacy in mammals | Underwater environments are dominated by blue light. Ironically, whales and seals cannot see blue, because they have independently lost their short-wavelength opsins. | Available |
| Ear structural modification in iguanids | n/a | Unavailable |
| Ultraviolet (UV) vision in insects and vertebrates | n/a | Unavailable |
| Vibrational communication in animals | What on earth could an elephant or treehoppers have in common with a seismometer? | Available |
| Pressure sensitivity and the tactile sense (excluding the lateral line) | The star-nosed mole is famous for, well, its nose, but do you have any idea what these peculiar 'tentacles' are for? The answer is rather touching and, of course, convergent... | Available |
| Lateral line system in fish and other animals | Some cavefish are completely blind, so how do they manage to navigate through their environment with astonishing ease? | Available |
| Echolocation in birds: oilbirds and swiftlets | The best known example of echolocating birds are the South American oilbirds (Steatornis caripensis), so called because their flesh yields abundant oil. | Available |
| Thermal sensing in mammals and insects | Insects and mammals have a group of ion channels (known as TRPs or Transient Receptor Potential channels) that are very similar and assumed to have a single origin. | Unavailable |
| Crabs: insights into convergence | You might think of crabs mainly as food, but this group is also highly instructive in terms of convergence… | Available |
| Bats: Insights into convergence | Bats show a fascinating array of convergences, from echolocation to flight to nectar feeding. Vampire bats can even detect infrared radiation, while others might be able to see into the ultraviolet end of the spectrum. | Available |
| Echolocation in toothed whales and ground-dwelling mammals | Given the extraordinary powers of echolocation in bats, it is not surprising that this group has received the most attention. However, they are not the only mammals to have evolved echolocation. Who invented sonar millions of years before the Navy? | Available |
| Trichromatic vision in mammals | Who has not enjoyed the splash of colour in a market: gorgeous red peppers, the green of basil and what on earth are these purple vegetables over there? All thanks to trichromatic vision, another story of convergence. | Available |
| Olfaction: insights into convergence | Although olfaction is very widespread, there is abundant evidence for repeated convergence of key features, strongly suggesting that there really is an optimal solution to detecting smells. | Available |
| Infrared detection in insects | Whilst infrared detection is probably best known in the snakes (where it has evolved twice), in point of fact in terms of convergence the insects provide by far the most striking example. | Available |
| Infrared detection in snakes | Warm-blooded rodents watch out! There are heat-sensing predators on the prowl... | Available |
| Infrared detection in animals | Some snakes are famous for 'seeing' infrared, but did you know that their heat-sensing abilities are rivalled by some beetles that can detect forest fires over considerable distances? | Available |
| Taste in arthropods and mammals | The ability to taste is obviously an essential component in the life of any animal, both to assess the potential quality of food, its nutrient capacities and also to detect toxins or other dangers. | Available |
| Loss of olfactory capacity in primates and cetaceans | It is widely thought that reduced olfactory capacity in apes is linked to the development of acute vision, especially trichromacy. | Available |
| Corneal nipple arrays in insect eyes | Anti-reflection coating? Not only on mobile phone displays, but also on insect eyes... | Available |
| Compound eyes in ark clams | Read on if you want to know more about bivalves with burglar alarms… | Available |
| Camera eyes in gastropod molluscs | The fast-moving cephalopod molluscs are famous for their camera eyes, but why on earth have gastropod snails, which are not exactly known for their speed, evolved this superb visual organ at least four times? | Available |
| Scanning eyes in molluscs and arthropods | Some sea snails have a linear retina. What a hopeless arrangement, to see the world through just a narrow slit! Not quite, because they have come up with a rather intriguing trick to extend their visual field - and it's a trick too good to use only once. | Available |
| Telephoto eyes in animals | Pursued by the paparazzi? Watch out for those animals equipped with telephoto lenses... | Available |
| Camera-like eyes in arthropods | Arthropods are famous for their compound eyes, but some groups have had a fair crack at evolving the optically superior camera eye… | Available |
| Sabre-toothed cats and marsupials | Marsupials with giant fangs? Yes, not all of the extinct sabre-toothed cats were actually cats… | Available |
| Electric fish: insights into convergence | Ever seen an electric eel in an aquarium? Don’t dare putting your hand in the tank... | Available |
| Butterflies and moths: insights into convergence | Some moths feed on the secretions from the tear-ducts of mammals, and some moths in Madagascar have evolved this independently, but instead of mammals they frequent birds. | Unavailable |
| Strepsipterans: convergent halteres and eyes | Strepsipteran females spend their whole life inside a wasp. The males are rather more exciting, particularly in terms of convergence… | Available |
| Senses and cognition in flies (dipterans) | Flies show intriguing convergences in various sensory modalities, notably in the senses of taste and hearing. | Unavailable |
| Beetles: insights into convergence | The beetles are probably the most diverse animal group on earth, so it is not at all surprising that they provide many fascinating insights into convergence. | Available |
| Sharks and rays (elasmobranchs): insights into convergence | In terms of sensory evolution the elasmobranchs are of particular interest, because independently of other fish and even some mammals (e.g. duck-billed platypus) they have evolved electrosensory systems. | Unavailable |
| Pheromones in arthropods | Not surprisingly this is a rich area of insights into evolutionary convergence because if an animal, such as a spider, can independently evolve the pheromone then a sexual lure is turned into a metaphorical honey trap. | Unavailable |
| Crustaceans: insights into convergence | Whilst predominantly marine, quite a number of crustaceans have invaded freshwater habitats and even more interestingly a few demonstrate terrestrialization, effectively freeing themselves from their aquatic ancestry. | Available |
| Elephants: senses, intelligence and social structure | There is evidence that elephants are sensitive to seismic communication, with the large pads of the feet and the trunk tip capable of picking up vibrations transmitted through the ground. | Unavailable |
| Transparent tissues: eyes, bodies and reflective surfaces | Read on if you want to know about the numerous animal equivalents to the invisible man... | Available |
| Statoliths and balance in animals | An almost universal, but convergent, method to detect changes in orientation is for small grains (statoliths) to be attached to fine hairs, whose movement triggers nervous impulses. | Unavailable |
| Hearing and ears in animals | Hearing has evolved independently in a number of groups, notably in the insects and vertebrates. | Unavailable |
| Ultrasound communication in mammals and amphibians | Amphibians are adept at vocalization, and remarkably ultrasonic communication has also evolved in the Concave-eared torrent frog, from China. | Unavailable |
| Electroreception in fish, amphibians and monotremes | From an evolutionary point of view, electroreception is particularly intriguing as a sense modality that has been repeatedly lost and reinvented again. | Available |
| Camera eyes of cephalopods | The remarkable similarity between the camera eyes of cephalopods and vertebrates is one of the best-known examples of evolutionary convergence. | Available |
| Octopus and other cephalopods: convergence with vertebrates | What could be more different from us than the alien-like octopus? Hold on. Look it in the eye and think again. | Available |
| Camera eyes in vertebrates, cephalopods and other animals | Camera eyes are superb optical devices, so it is not surprising that they have evolved several times. But why, of all animals, in the brainless jellyfish? Or for that matter in a slow-moving snail? | Available |
