{"id":24085,"date":"2023-01-10T14:12:05","date_gmt":"2023-01-10T13:12:05","guid":{"rendered":"https:\/\/www.discoverwildlife.com\/?p=65445"},"modified":"2023-01-10T15:37:06","modified_gmt":"2023-01-10T14:37:06","slug":"asymmetry-what-it-is-and-why-it-exists-in-animals","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcwildlife\/rss_feed\/asymmetry-what-it-is-and-why-it-exists-in-animals\/","title":{"rendered":"Asymmetry: what it is and why it exists in animals"},"content":{"rendered":"

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By JV Chamary\n \t\t<\/p>

Published: Tuesday, 10 January 2023 at 12:00 am<\/p>

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What is asymmetry in animals?<\/h2>\n

All but the simplest animals have a body with two distinct sides \u2013 one left and one right \u2013 that are roughly mirror images, at least in the embryo. That \u2018bilateral symmetry\u2019 can even be seen in starfish, before the larvae develop into adults with multiple arms radiating from the centre (\u2018radial symmetry\u2019). But while left and right may look similar, the bodies of most creatures are only superficially symmetrical.<\/p>\n

How are animals asymmetric?<\/h2>\n

Asymmetry can appear across the whole body. One example is flatfish: a juvenile starry flounder swims vertically and has eyes on opposite sides of its head, but one eye will migrate across its skull as it matures so both eyes end up on either the left or right of a lopsided adult. The male fiddler crab, meanwhile, has one major claw \u2013 used to fight rivals and impress females \u2013 that can reach half the animal\u2019s total size.<\/p>\n

Each body part can have asymmetry too. Parts can exist in mirror-image forms that can\u2019t be superimposed on one another \u2013 like our right and left hands, for example. Known as \u2018chiral\u2019 structures, they can take either right-handed (dextral) or left-handed (sinistral) forms.<\/p>\n

What determines the direction of the asymmetry?<\/h2>\n

The direction of asymmetry can be fixed or random. Fixed asymmetries include a narwhal<\/a><\/strong>\u2019s tusk \u2013 an elongated left tooth with a left-handed spiral in 100 per cent of individuals.<\/p>\n

By contrast, studies of American lobsters show that the dominant crusher claw forms from whichever side is preferred as a juvenile, so the proportion of left to right ends up 50:50. Generally, random asymmetry is determined by chance and influenced by environmental cues, whereas fixed asymmetry is largely inherited and programmed by genes.<\/p>\n

Is asymmetry common?<\/h2>\n

Yes. Asymmetric features can be striking, but many are subtle. The vast majority of animals have fixed asymmetries that aren\u2019t visible externally. The position of the human heart (as well as the stomach and spleen) is off-centre to the left of the body, while the liver and gall bladder are to the right, for instance.<\/p>\n

Lungs typically differ in many vertebrates: humans have two lobes on the left and three on the right side; mice have one lobe on the left and four on the right; and most snakes have only one functioning lung on the right side.<\/p>\n

In rare cases, genetic mutations or accidents during development can create reversed mutants: having organs in an abnormal, mirror-image arrangement (heart on right) gives the rare condition \u2018situs inversus\u2019. It causes health problems for some individuals with the condition, but most suffer no ill effects.<\/p>\n