Saliva, Spit and Slobber
“Beware, we spit”. This is what it says in front of the llama enclosures of many zoos. But llamas are not the only ones that have perfected this kind of activity. The small Australian town of Manjimup hosts annual spitting championships; the South African “rinkhals cobra” (Hemachatus haemachatus) spits venom over distances of just over 2 metres into the eyes of its foes; archer fish are famous spitters and knock down insects from leaves above the water, and the match-stick long, wormlike members of animals known as Onychophora (e.g., Peripatus spp.) eject sticky fluid from oral glands to pin down and immobilize prey of beetles, spiders and slaters that they then consume. A similar method is employed by the “spitting spiders” of the genus Scytodes: they don’t snare their prey in webs, but when located with their extremely sensitive touch receptors on their legs, they spit some gooey liquid up to 2 cm over their prey. The liquid hardens, pins down and immobilizes the prey.
Salivary glands are developed in many animals: spiders, insects, molluscs (but not in bivalves) and most vertebrates other than fish. The main function of the saliva is to lubricate food and render it swallowable, which explains why fish and aquatic mammals like walruses, seals and whales don’t need any saliva. But in animals that salivate, often enzymes are admixed to the saliva. That of humans and elephants is particularly well endowed with alpha-amylase, which helps to break down starch (and a reminder of the fact that our ancestors were vegetarians). Enzymes to attack proteins are rare in spittle and amongst the vertebrates only known from snakes and lamprey. Enzymes involved in the breakdown of fats, known as lipases, are virtually absent from the oral secretions of all vertebrates, but have been reported from water-scorpions (actually insects of the order Hemiptera).
The saliva of blood-sucking animals contains anti-coagulants to prevent clotting, while that of frogs and especially that of the chameleon operates with the toughest and most adhesive super glue any animal’s mouth is able to produce. The venomousness of another reptile’s saliva (that of the Indonesian’s Komodo dragon), incidentally, stems from the various bacteria allowed to grow in it but not as in the American Gila monster Heloderma suspectum, from the big lizard itself. Poisons are part of the saliva of many animals including that of the octopus (the blue-ringed species Hapalochlaena spp. in particular) and that of the deadly coral reef cone shell Conus geographus. Salivary compounds of other marine snails are characterized by extreme acidity, strong enough to corrode the calcareous shells of mussels, but in many mammals saliva is mildly alkaline and has an antiseptic effect. An analysis of the spit of a Peripatus through mass spectrometry by Dr A. Wilkens of New Zealand revealed barbiturate-like substances, which together with other additions could be responsible for the rapid demise and resignation of prey when st(r)uck down with a successful spit-shot.
If all this sounds a little too slippery to your taste, try a Chinese specialty: bird nest soup. What that has to do with our topic of spit, you wonder? Well, during their breeding season birds of the genus Collocalia glue their nests to cliff walls with secretions from enlarged salivary glands and with a bit of oriental culinary magic, Chinese cooks had found a way of turning this bird-spit into a lot of cash (via the famous and a little bit slimy bird nest soup). Does the thought make your mouth water? If so, spare a thought for Ivan Pavlov, he received a Nobel prize for his work on slobbering, salivating dogs that were thinking of food.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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