I already lampooned the idea of a virgin birth last year, and while folk stories abound at this time of the year because of the Solstice, we’ll stick with the Christian myth once again this year. Specifically, the part about the three kings who follow the star to find the stable of Bethlehem. This type of navigation was popular (and still is if you like to do things old-skool) and effective. In this post, I’m going to tell you about other animals who can also use celestial clues to navigate. Not very blasphemous (cf. Appendix 1), but it’s the best I could think of.
The first thing that must be said is that there’s a critical difference between how humans use the stars and sun, and how other animals use them. Humans make a map (of the constellations, etc.). There has classically been a debate about whether other animals do this, but the evidence mostly points to them using the stars/Sun/Moon as compasses. This requires a hefty amount of memorisation – memorisation so powerful, that a honeybee only needs to see a small patch of sky (e.g. between clouds) to find out what direction she’s flying in (Wehner, 1989), and the recognition is instantaneous (Sakura et al., 2012).
Putting birds in planetaria is a good experiment to see if they’re sensitive to star patterns. Emlen (1967) found that by producing artificial constellations, he could predict how his indigo buntings would change their orientation. It’s tempting to say that they got confused due to a different star map, but in fact, they orient themselves along the North-South axis of the constellations, not on the positions of the stars (Wiltschko et al., 1987) – it’s a compass, not a map; although it must be said that they do afterwards learn a map, as shown by Emlen (1972). Ants and honeybees can also memorise the patterns of constellations and stars (Collett & Collett, 2002).
I already wrote about ants. Briefly, desert ants use light polarised by the atmosphere as a compass (Wehner, 1994); this ability was first discovered in honeybees by von Frisch (1949), who also discovered that the waggle dance of bees also contains information about the sky, with the angle between the motion and the vertical being equal to the angle between the direction of the resource/new nest and the sun’s position (von Frisch, 1948) – this is also why the bee dance language is so dynamic, the sun’s position changes all the time and so the dance has to be modified every time.
The polarisation patterns are a function of the sun’s position in the sky; chromatic contrast also plays a big role, and both of these features are used to determine what direction the ant is facing and in what direction it’s supposed to go. Many other insects, especially those that have to migrate across long distances (Shashar et al., 2005), can do this as well, and all use the same apparatus. Generally, only UV, blue and green polarisation patterns are used (Wehner, 1991).
The compound eyes of these insects contain a special area called the dorsal rim area (Homberg & Paech, 2002; diagram above from same source), populated by photoreceptors with a high sensitivity to polarised and UV light. They also connect to specific areas of the lamina and medulla of the optic lobe. These areas are in turn populated by specific neurons, the polarisation-sensitive interneurons, whose firing rates are dependent on the polarisation of the light (Homberg et al., 2011) – this serves to clean the signal and remove noise. They’ve been found in representatives of the Odonata, Orthoptera, Blattodea, Hemiptera, Coleoptera, Neuroptera, Hymenoptera, Lepidoptera, and Diptera. The system has best been studied with locust and cricket brains (Pfeiffer et al., 2005), because they’re large enough to allow practical electrophysiological study. In crickets, there are three types of these neurons, attuned to firing at e-vectors oriented 10°, 60° and 130° to the head (Labhart et al., 2001). Kinoshita et al. (2007) suggest that these neurons also code for the chromatic pattern in the sky.
Orient yourself with the above diagram from Pfeiffer et al. (2005). The polarisation-sensitive interneurons in the locust are housed in a specific neuropil connected to the medulla, the signal going to the anterior optic tubercle (Homberg, 2004). From there, other neurons take the signal to the lateral accessory lobe (Homberg et al., 2003), where it is finally processed by the central complex (Heinze et al., 2009), where a “map” forms from the differences in the intensity of the activity from the incoming neurons (Heinze & Homberg, 2007). The interpreted signal is then sent back to the lateral accessory lobe and redirected to the thoracic motor centers and translated into movement (Träger & Homberg, 2011).
In ants, the ocelli can also provide information on polarisation patterns (Fent & Wehner, 1985). Whether this is also the case in other insects isn’t known, but in general, it’s worth noting that the transmission of the information from ocelli is faster than from compound eyes (Parson et al., 2006), so it shouldn’t be surprising if it turns out ocelli play a larger role than the specialised dorsal rim area.
Tiger salamanders can also use polarisation patterns (Adler, 1976). I don’t know the details of the mechanism, though. Hawryshyn (1992) suggested that fish can also use polarisation patterns, Munro & Wiltschko (1995) proposed the same for birds. However, no conclusive proof has been given so far. Interestingly, cephalopods are sensitive to polarisation patterns, but it isn’t known whether they use them for navigational purposes. Wolf spiders use celestial clues as well (Papi, 1955), getting the information from their anterior median eyes (Magni et al., 1964), but I’m not sure how widespread it is in other spider families.
The polarisation pattern produced by the moon is used by nocturnal animals (Ugolini et al., 1999), even if it is a million times dimmer than the sun’s on even the best nights (Gál et al., 2001). An example is dung beetles (Dacke et al., 2003). A big worry nowadays is that excessive light pollution is messing around with such navigational systems (Horváth et al., 2009), since human lights, when they get scattered through the atmosphere, get polarised (Kerola, 2006). Reflections on glass or water also get polarised (Horváth et al., 2009). An effect has already been demonstrated in the navigation of European moths (Nowinszky & Puskás, 2011), and more research is underway to determine how serious the problem potentially is.
The position of the Sun in the sky can be seen by all animals with eyes. And that source of information is used by all sorts of animals. We’ll stick with the arthropods. It was shown as early as Santschi (1911) that ants (he studied Lasius niger) can use the sun’s position as a compass. Cockchafers make sure to fly towards the sun after laying their eggs (Couturier & Robert, 1958). Herrnkind (1972) explains that crustaceans also use the sun for orientation – this also applies to those crabs that spend time on land, e.g. hermit crabs (Vannini & Chelazzi, 1981). Surprisingly, other underwater animals can use the sun, e.g. fish (Leggett, 1977) – I’m not sure how they deal with the effects of refraction; most likely they’re acclimatised to it from birth (just as terrastrial animals are acclimatised to the refraction through air). It’s common across all vertebrates as well – mammals use it (Lüters & Birukow, 1963), as do reptiles (Adler & Phillips, 1985), including birds (Kramer, 1957).
Using the sun as a compass is fine and all – except that sometimes, the sun isn’t visible due to clouds. So there has to be some kind of backup system. Birds use their circadian rhythm to keep them in the same rough direction – they sense that the sun moves ~15° every hour and make sure they keep that calibration (Schmidt-Koenig, 1960). This control has been investigated at the molecular level in monarch butterflies by Zhu et al. (2008).
An alternative, or complement (no generalisations can be made, it depends on the taxon), is to use the magnetic field. Salmon can do this, when they’re migrating under ice and so can’t see the sun (Quinn & Brannon, 1982).
The colour of the sky is also useful. Freshly-hatched turtles, for example, use the colour of the sky to find the horizon, which is assumed to be where the ocean is.
Theoretically, it is possible that some animals calculate their orientation from the direction and length of shadows, but this has still not been demonstrated (it would make an interesting study though, even if the results will likely be negative).
Just to round the post off, some trivia about human use of stars for navigation. Thales, the prototypical absent-minded philosopher/professor (or so he was portrayed by Plato) and all around brilliant guy (sage, engineer, statesman, geometry, astronomy, he worked in all of them), is credited by Callimachus with having discovered the use of the Little Bear as a navigation aid, and celestial navigation was considered one of the critical practical crafts to know (as we can see in the Socratic Dialogs). Pytheas of Massilia (late 4th century BC) made detailed calculations for the position of the North Star for use in navigation. Even so, no sailors dared to go beyond sight of the coast in the Mediterranean except on the best starlit nights, when the Pleiades and Sirius (mostly) were visible. Abbé de St. Pierre (17th century) named navigation as one of the most important sciences. Celestial navigation came to a high-point with the Portuguese sailors and explorers who started exploring the coast of Africa, using Polaris at night and the Sun during the day – this was a far cry from the pathetic travellings around the Mediterranean Sea, since many more latitudes had to be travelled through.
Anyway, I wish all of you a Merry Christmas/Hannukah/Solstice/Monkey/whatever it is you celebrate. You’ll get a Happy New Year’s wish at the New Year’s post.
Some of the hilarious hate-mail I got for the parthenogenesis post from overly-sensitive people who can’t take facts or a joke.
- how dare you insult our Holy Mother!!! rot in hell heathen [I like how you prefer Mary to English grammar. I'm sure she'd find it cute, except that she was probably an illiterate. Also, no way in hell (pun maybe intended?) she knew modern English.]
- I think you have a profoundly darkened view of the world. The Christmas story is a METAPHOR, not meant to be taken literally. Your post wasn’t an insult to us, although I’m sure that’s all you wanted. It was an insult t your intelligence for thinking people take the Virgin Birth as anything more than a allegory. Good day, sir. [O RLY? You may hold a high view of your fellow theists, but here are some poll results showing that they really do believe in the ridiculous crap and don't take them as metaphors. Also, my post was an educational one. If you noticed, 90% of it was about the biology of sexual reproduction, or did the mention of the Virgin Mary automatically make you blind? Also, I'm an astigmatic, but my retina functions just fine.]
- You are a disgrace how can you spew such vile nonsense on the internet. I will pray for you. [Could you just send me money instead? ← Note that punctuation mark.]
- Typical liberal leftie asshole, using science to indoctrinate everyone else into his atheistic immoral worldview. Fuck you. [I am a liberal, I am a leftie, and I am an asshole. But I have never mentioned politics in any of my posts here, and certainly not in the parthenogenesis post, my dear projector bulb friend. Also, I wasn't using science to indoctrinate, I was stating scientific facts that lead to an obvious conclusion. Also, did you really miss that 90% of the post had nothing to do with the Virgin? Finally, no word said on morality in this blog or in that post, and I haven't revealed anything about my worldview either, except maybe indirectly that I like to live in the real world, not in a make-believe fantastical world that makes some people feel better about themselves at the expense of others.]
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