Saving face: Salamanders show why it's important to know thy enemies (from Anne-Marie Hodge's blog)

by Noah Gray in Nothing's Shocking

In the past I have focused on reviewing newly published literature, but I have decided to periodically take a look back at some “classic” zoology papers. Defining “classic” is difficult, I am using the slightly arbitrary criteria that the paper has been cited over 100 times, in addition to the very arbitrary and slightly self-centered criteria that the paper was published before the year of my birth, 1986. So, without further adieu, I bring you discussion of a fascinating paper 1981 paper by Robert G. Jaeger, who has now spent over 40 years studying the ecology of red-backed salamanders.Amphibians are a fascinating group of vertebrates with an amazing diversity of life history strategies. Frogs (Anura) are the best known and most diverse amphibian clade, and salamanders (Caudata) are often overlooked as rather prototypical little vertebrates. Four legs, a tail, rather mundane…or not. In reality this group displays amazing diversity in morphology, ecology, and behavior. Within Caudata, the largest family of salamanders is the Plethodontidae. This family consists of over 350 species species. They range anywhere from 3 to 30 cm long, and have a wide variety of life history strategies as well: some lay eggs in the water and go through an aquatic larval stage, while others are terrestrial, lay their eggs on land and skip the larval stage altogether.Another of their characteristic features: they don’t have lungs. Or gills. How, you ask, does an animal breath without lungs or gills? Salamanders exchange gases through blood vessels in their skin, a strategy known as cutaneous respiration that is (hopefully) a common-knowledge characteristic of amphibians. Although “breathing through the skin” is not unique to plethodontids, the total loss of the lungs is considered a key diagnostic feature of this clade. The exact mechanism of how this evolved (adaptations to cool streams, or other ecological conditions? allowing room the hyobranchial structure for extended tongue protrusion?) is apparently the subject of much controversy.Plethodontids have another extremely interesting anatomical feature, and it is the main topic of this post. They have nasolabial grooves, which stretch from each nostril to the lip of the upper jaw. These may seem like just a structural feature, but they are actually part of a sophisticated chemosensory system. The salamanders press their snouts against things as they forage, which causes fluid to move up the grooves into the nostrils and nasal chambers, and in the process if passes over the vomeronasal organ. This organ (aka Jacobson’s Organ), most popularly recognized in snakes’ tongue-flicking behavior, is actually common among the vertebrates (including humans). This allows salamanders to both track prey and detect pheromones from other members of their species, key for mating success.So, the sensory physiology of plethodontids is extremely interesting. These are the only salamanders that have such a large morphological structure devoted to both inter- and intra-specific chemical detection. But what is even more fascinating is how their behavior is linked to this physiology. This species is very territorial, and resident males will attack and bite any other males that intrude upon their home range. A study on aggression in the red-backed salamander ( Plethodon cinereus ) revealed an specific and severe risk incurred during fights between males over territory.Bites to the body can scare males away and bites to the tail can cause autotomization, but these problems are of little concern compared to what can happen if bites are aimed at the face. Males appear to attempt to direct their attacks at their opponent’s snouts, specifically the parts of the face that house the nasolabial grooves. Damage to the snout can diminish the sensory capabilities facilitated by the grooves, and this has dire consequences for the unlucky males. In his 1981 study, Jaeger found that undamaged males caught an average of 18.6 flies in a 2 hour period, while individuals with scarred snouts caught only 5.8. This has huge implications for a male’s survival, reproductive success, and ability to defend his own territories.Obviously there is a huge risk in engaging in territorial battles. If you are constantly bickering with your neighbor over boundary disputes, the probability of recieving a nasty bite to the snout is relatively high. So what is a self-respecting male salamander to do? It is thought that this situation has led to something called “dear enemy recognition.” This basically means that the salamanders detect and recognize the pheromone signals of “familiar” individuals that hold adjacent territories. These neighbors are always close, so it would be a waste of energy to attack them every time they are detected nearby. Salamanders recognize the signals of neighbors, and can also recognize a new, unfamiliar individual as an “intruder” worthy of attack. This allows them to prioritize their fighting efforts and minimize their number of aggressive encounters, and thus their risk of receiving crippling bites to their precious chemosensory organs.The moral of this story: “know thine enemies,” and be careful where you stick your nasolabial grooves.(This post is an revisited/edited version of a write-up of this paper from my previous blog, Pondering Pikaia)
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