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Basically I'm curious as to why when it comes to mammalian predators that some like say bears and canines tend to have longer muzzles than those like big cats.

aside from their ancestors/evolutionary line and such, what advantages or disadvantages do short muzzles and long muzzles have against each other and which is better?

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  • $\begingroup$ Welcome to Biology.SE! We encourage you to do some research on your own and then, informed by what you have learned, ask any questions you still have (ideally with references to reliable sources). Note particularly that "which is better" is probably the wrong question — I suggest starting with the assumption that varying traits (e.g. different muzzle shapes) have costs and benefits with optima that depend on the circumstances. ——— Please also take the tour and then go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. Thanks! 😊 $\endgroup$ – tyersome Dec 29 '19 at 1:34
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This paper, The biomechanics of foraging determines face length among kangaroos and their relatives might provide some insight, but very likely misses a lot of factors that would apply, e.g., to wolves vs cougars. In the case of carnivores or omnivores, hunting strategies, prey capture methods, and even tearing & chewing techniques are likely to interact synergistically with muzzle length via the Baldwin effect (Wikipedia). Another paper The Baldwin Effect and Genetic Assimilation compares the Baldwin Effect to the Waddington effect, which can have similar results.

Edit: In response to the suggestion by @Willk and @tyersome, I've added the following explanation:

The Baldwin Effect is widely accepted among evolutionary biologists. The Waddington Effect is not as widely accepted and is sometimes (incorrectly) interpreted as a form of Lamarckism.

In essence, the Baldwin Effect works like this:

  1. a population is subjected to a new environmental challenge or opportunity
  2. a few individuals learn a new strategy that overcomes the challenge or exploits the opportunity
  3. natural genetic variation makes some individuals in the population more capable of learning the new strategy
  4. Individuals whose genetic makeup makes them more capable of learning the new strategy have a fitness advantage and produce more descendants, so over time the proportion of individuals in the population who can are genetically equipped to learn and use the new strategy, increases.
  5. Individuals whose genetic makeup gives them a physiological advantage in regard to using the new strategy will have a further fitness advantage, and will consequently produce more descendants than others in the population.

Let's say the environmental challenge for two different kinds of carnivore (let's call them Bogs and Dats) is to catch Mophers. Both Bogs and Dats initially have the same medium-to-short muzzles. Some Bog individuals figure out that they can dig Mophers out of their burrows, and some Dat individuals figure out that they can catch Mophers at night when the Mophers leave their burrows. Both strategies are successful. Some Bogs happen to have longer muzzles than their cousins, and find it turns out that longer muzzles work synergistically with the digging strategy, allowing Bogs to stick their noses into the Mopher burrows to grab escaping Mophers. The resulting fitness advantage results in an increase of the long-muzzle trait in further generations of Bogs. Note that in this scenario it is the adaptive behavioral strategy that creates selective pressure that favors a particular genetic adaptation.

Dats on the other hand, because of their nocturnal hunting strategy, benefit from improved night vision; and long muzzles don't provide any fitness advantage to Dats because Dats don't dig Mophers from their burrows. As long as Bogs and Dats don't hybridize, they will most likely end up with long and short muzzles respectively.

The Waddington effect, also called “Genetic Assimilation”, is somewhat more direct:

  1. An environmental stress causes a proportion of a population to develop one or more abnormal traits, by interfering with embryological development.
  2. If there is a selective pressure in the environment that favors some subset of those traits, individuals whose genetic makeup makes them more likely to develop that subset of traits, those individuals are likely to produce more descendants than other members of the population.
  3. If being “more likely to develop” that subset of traits results from a weakening of genetically determined development controls that would otherwise prevent development of that subset of traits, then the subset of traits can eventually become the normal phenotype.

An explanation of how the Waddington effect might influence muzzle length requires a bit more hand-waving, largely because there is no evident environmental factor that would cause perturbations in the embryological development of, e.g., dogs. However, I'd assert that an abrupt release of a strong selective pressure can have a closely analogous result. Wolves really have to look and act a lot like wolves in order to survive in the wild (partly because of their survival strategies), so they are subject to strong selective pressure. Domestic dogs, on the other hand, are subject to much weaker selective pressure. Moreover, when a variant shows up (a pug, for instance which has a very short muzzle), it is often considered “cute” and is bred separately to fix the trait in a subpopulation. Although the "release of selective pressure" mechanism isn't quite what Waddington contemplated, the process is closely analogous.

Note that the Waddington effect and the Baldwin effect are not mutually exclusive. Relaxation of selective pressure that reduces fitness of variants, and environmental factors that cause disturbances in embryonic development (a la Waddington) both produce a rich source of variants upon which the Baldwin effect depends.

However, It is very difficult to prove that any specific phenotypic trait in an existing species was caused by a particular selective pressure. At best we can only make plausibility arguments supported by models and rather sparse environmental & ecological data from the geological record, from analyzing genotypes, etc. In the case of canines vs felines (which split off from a common ancestor perhaps 42 million years ago), we can really only guess what the environmental challenges & opportunities might have been, and what behavioral strategies may have been adopted by different individuals and populations.

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    $\begingroup$ Curious to know why this was downvoted. $\endgroup$ – S. McGrew Dec 29 '19 at 2:14
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    $\begingroup$ Because that is what happens on the Biology Stack. Fleshing out the Baldwin effect so save me the wikipedia click would be nice. Upvote from me, though. $\endgroup$ – Willk Dec 29 '19 at 15:24
  • $\begingroup$ I think your answer deserves better than anonymous downvoting, but I would guess that the down voters felt like that this wasn't a good answer. I don't have time right now to read the links you've given, but a superficial scan of the Wikipedia article on the Baldwin effect left me unconvinced of its appropriateness in an answer to this question. If you elaborate on the Baldwin effect and its relevance, I would expect you to get a more favorable response. $\endgroup$ – tyersome Dec 29 '19 at 21:52
  • $\begingroup$ Thanks for that advice. I'm not looking for favorable responses per se; just want to be helpful to the OP. I will elaborate on how the Baldwin effect and Waddington Effect might be relevant. $\endgroup$ – S. McGrew Dec 29 '19 at 23:43
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    $\begingroup$ The problem you have is it does not answer the question, which is what functional aspects of muzzle length favor different relative lengths. I get that you are saying it is difficult to answer but you need to say that explicitly, basically you need to say your not answering becasue it can't be answered (which is wrong but hey its your answer) $\endgroup$ – John Dec 31 '19 at 18:30
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Its mostly due to different forms of predation.

Longer jaws are deeper and can reach farther around a prey item, it also increases the chances of catching a moving target in the jaws becasue the area the jaw covers is larger, all things being equal it should always be favored but things are rarely equal, everything has a cost. It is common to see longer snouts in animals that hunt primarily with the jaws. There two ways to do this which depends on how big the pery to are after is, for predators going after relatively small prey they favor long thin snouts, is really noticable in aquatic predators like gharials, it makes for a very fast bite with a wide attack range. Alternatively if they are going after large prey they need to balance strength with speed so you see long, but not as long, more robust snouts. Think alligators, hyena, wolves. Whereas cats who attack with their forepaws can use a smaller head and jaw and favor a more precise bite, becasue they are moving with the prey becasue they have grabbed it, the arms are absorbing most of the stresses. Cats favor grabbing and strangulation for which a small short jaws works fine. Energy and material saved here can be used elsewhere. A longer snout would be weaker unless they add a lot more muscle to stabilize it. In terms of resources it is a lot of cost for little gain, whereas in wolves it is worth the cost becasue they need that strength anyway, becasue they jaw is where all the stress of the struggling prey is being transmitted.

You see the same thing in dinosaurs, predators that can attack with the claws have weaker bites/smaller heads. In the case of dinosaurs however snout length varies less because the small heads favor slashing bites, which favors a longer snout (thus a longer cutting row of teeth) there heads get narrower and weaker more than shorter. Mammals can't use this strategy because they don't get replacement teeth, and they would quickly not have a tooth in their head, so instead you see strangulation favored instead.

Of course an animal can try to balance a range of strategies and there are all kinds of specialized forms like saber teeth, venoms, ect. Of course this is also simplified a lot of other things effect snout length as well, things like climate (long snouts cause less heat/water loss through breathing), sense of smell, or even vision.

https://www.cambridge.org/core/journals/paleobiology/article/dinosaurian-and-mammalian-predators-compared/BE3492DA7E0A103C6F6E77B9611D86A9

https://link.springer.com/chapter/10.1007/978-1-4615-0161-9_14

https://www.cambridge.org/core/journals/paleobiology/article/constraint-and-adaptation-in-the-evolution-of-carnivoran-skull-shape/77B1591461066F9D0E30F309F6C495C6

https://www.sciencedirect.com/science/article/pii/0034568770900757

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