Why Weren't Birds Top Predators After 66Mya

[adthefossil]

Hi Folks,

Birds are close relatives of the scariest dinosaurs of all. They had the best lungs and bones on Earth. They were probably warm blooded judging by the speeds they could run at. They had great teeth. They'd specialised in predation for >10^8 years.

Mammals, by contrast, had specialised in staying out of sight. I'm not sure when the placenta evolved, but it's hard to think of any other advantage they had over therapods, and in any case the placenta is not exactly a predatory gadget. Feathers are better than fur in every way. The most efficiently running mammal today is two-legged: the kangaroo. Warm bloodedness was probably common knowledge. They even had an air force.

Some very close relatives of those top predators survived the extinction 66mya. Why did they not retake the top predator spot immediately afterwards? What did?

Any help understanding this much appreciated,

Adrian.

[ashcraft]

Some opinions only

While avian lungs are superior to mammalian lungs at oxygen extraction, that does not infer an advantage for top predator status. A Ferrari is much faster then a Ford truck, but if you can only run at interstate speeds, then the Ferrari has no real advantage. Mammal lungs work well enough for capturing prey in a modern environment.

In some instances, birds have been dominate predators. "Terror Birds" come to mind.

In other instances, I suspect it took longer for a small surviving bird to evolve into a large land-locked predator, then an omnivorous mammal, which really only had to get larger to fill the niche, and exclude other species from that niche.

Brent Ashcraft

[adthefossil]

I don't think they had speed cameras yet ;-)

By "Terror Birds" I guess you mean those South American things that got wiped out by North American dogs. It's key that dogs wiped them out, but that was much later after the dogs had had a chance to evolve. I'm thinking about *immediately* after 66mya.

Any species can radically change size more or less overnight if there's a reason to do so.

So I still don't see how these cute furry mice could have got the upper hand over such specialised predators.

Adrian.

[Raggedy Man]

I believe that your set of inferences might be hindering your ability to understand. Just because an apple is spherical and grows on a tree doesn't make it an orange.

[ashcraft]

North American dogs may or may not have had a hand in extinction of the large predatory birds, but the birds were able to move into N. America and survive quite well, despite of their competition, until quite recently. These large predatory birds existed far longer then modern dogs, which are actually in the third or fourth evolutionary expansion.

Changing forms is a rapid process only if the parts are already there. Look at a mouse and a lion body plan, really not much difference other then size. Compare that to a small seed eating bird, and how much change it has to go to become predatory. It clearly was done, but they couldn't do it as fast as surviving mammals, who filled the niches and kept the birds on the periphery.

Think of a catastrophic event that would wipe out all of the flying birds, but left a few species of bats and few species of penguins. Penguins wings are still built for flight in a fluid, so could presumably re-evolve air flight. However, could they do it fast enough to keep the bats out of the niche? Once a niche is filled, it is hard to dislodge the species. It generally only happens where you have two species competing for the same niche, where each have a large population base, and they meet at the periphery. An example would be thylacosmilus and "traditional" placental sabre tooths, when the Americas joined.

If you think as survival as either a die or live meeting, you have to realize that not all meetings would have lead to the deaths of the thylacosmilus, but most would have. Overtime, this allows theexpansion of the saber tooth, while Thylacosmilus numbers shrink, because they lose more often, and lose territory. Without a large population base though, random chance would favor a large population of poorer adapted species to win over a much smaller population of better adapted species. Losing one individual to a group of ten, is much more serious problem then losing 5 individual to a group of 100. Unless the group of ten wins at a greater rate then 80% of the time, they will all die out before the group of 100.

It could also have been that it was just random dumb luck that favored the mammals initially also.

Brent Ashcraft

Edited January 23, 2015 by ashcraft

[Ramo]

I have similar questions. I often wonder why sharks evolved into their form (for the most part) millions of years ago, and that body shape, behavior, etc hasn't really changed. I find it amazing that I find shark teeth here in Kansas that look almost identical to the teeth of sharks that are living today.

On the other hand, mammals in just the "flash of a few centuries" are now sending rockets into space!

Ramo

[Mediospirifer]

Keep in mind that "furry mice" doesn't really describe all of the Mesozoic mammals accurately. Check out this beast: Repanomamus

Repenomamus was a carnivore, 2 or 3 feet long (depending on species), and at least one was found to have died with juvenile dinosaur bones in its gut! That's a bit of a start in the top predator game, especially if the feathered competition are largely seed-eaters. Most modern flightless birds (excepting diving birds) are largely herbivorous or insectivorous (to my knowledge; I'll be happy to learn something if someone can correct this).

Also keep in mind that there is an upper limit to size for a flighted animal. Terror birds didn't have that problem, but neither did sabretooth tigers.

One last point: Running efficiency is more relevant for an herbivore (such as a kangaroo) than for a predator. Canids are good at the endurance game, but the fastest runners are cats--cheetahs. They don't run far, but if they're close enough to the (slower) prey at the start of the race, they don't need to.

Just my 2 cents!

[Rockwood]

The assumption so far seems to be that change or advancement is the goal of evolution. As Stephen Gould said so often, evolution has no preferred direction.

[Triceratops]

Often evoloution takes the least expected road.

It may have just been luck that drove mammals to the top.

[Missourian]

In a sense, they are top predators. Have you ever seen a lion or shark mess with an eagle?

[ashcraft]

The assumption so far seems to be that change or advancement is the goal of evolution. As Stephen Gould said so often, evolution has no preferred direction.

That is not quite right. Evolution has no goal, but life does-survival and reproduction. What has worked before is likely to work again. We see this time and time again through convergent evolution. Whales are shaped like bony fish like icthyosaurs like dugongs like sharks like whatever comes next. Saber teeth are another example. They have arisen several times in mammals in unrelated groups. Most groups also experiment with gigantism, from giant rodents, to giant apes, and even giant kangaroos.

As long as ecosystems function under the same sets of rules, newness is relative. My herp professor lamented that the loss of a turtle form would mean that they would never come back. True to a point, but mammals and dinosaurs have also experimented with the general shape/concept.

I in no way mean to imply that evolution is guided by intelligence, but selection forces haven't changed much, so the results don't change much.

Brent Ashcraft

[Rockwood]

All true, but the question implies that the goal of birds was to become top predators. They may have been fine with pine seeds and sardines at the time. If no traits happened to lead their evolution along the right path at the right time so be it.

Edit: The question doesn't imply a goal, but most of the answers seem to.

Edited January 24, 2015 by Rockwood

[ashcraft]

All true, but the question implies that the goal of birds was to become top predators. They may have been fine with pine seeds and sardines at the time. If no traits happened to lead their evolution along the right path at the right time so be it.

Edit: The question doesn't imply a goal, but most of the answers seem to.

We are not implying a goal, other then survival and reproduction. We are commenting on what actually happened, and why. "May have been fine with pine seeds" is implying a conscious decision. Not what you meant but possibly misconstrued.

Brent Ashcraft

[adthefossil]

Hi All,

Thanks for all your responses!

Firstly, I wasn't implying that birds "wanted" to be predators, or that evolution involves any such intentionality. As for the relationship between intelligence and evolution, I believe those words are synonyms: the very same process in different spheres, but that's another topic.

It's clear that picking seeds from tree-tops was a viable strategy that some birds took. But evolution generally exploits every viable strategy, and I still don't know why predation wasn't viable for them at that time. The idea that mammals were better at it doesn't seem very convincing to me because, picking up Brent's bats vs. penguins argument, it seems to me that the birds, not the mammals, had the right tools for the predatory spot. In other words, it looks like Brent's penguins somehow grabbed his bats' logical spot. Birds had superb one-way lungs and fast growing lightweight bones. They'd developed the first proper tooth sockets a little while earlier. They were probably warm blooded already. In fact, what did mammals of 65mya have going for them at all, other than a placenta?

Thanks for the repenomanus link though. I never heard of that one, but it was gone by 123ish. Was anything comparable around in 66?

Among terror birds, most are from the miocene. The only one old enough to be relevant is paleopsilopterus at 61-59ish, but why didn't it do better? And why did the later ones all seem to fail as well?

No biped is gonna run as fast as a modern cheetah, but cheetahs weren't around then and therapods were the fastest runners in the late Cretaceous. Not all mammal predators compete on speed anyway. Hyenas and dingos rely more on stamina and persistence. Velociraptors probably had a similar strategy. It actually works better on two legs, as the kangaroo knows.

One possible argument is that the therapods' food (big sauropods) had just gone extinct. But we all seem to agree that size can change fast, so why shouldn't a six inch raptor have gone after hedgehogs?

So I'm still confused. How could a family that had specialised in predation for 10^8 years have lost that spot to one that had merely tried to hide?

Adrian.

[Mediospirifer]

I think the problem here is your assumption that mammals were specialized for hiding. Yes, there were lots of larger predatory critters around that would happily eat them given a chance, and camoflage is a common strategy for both successfully catching prey and avoiding predation. But this is true for nearly all animals; we don't say something is specialized to hide so much as specialized to fill a particular niche, usually based on their food source: woodpeckers are specialized to eat bark beetles and grubs, cats are specialized predators, horses are grazers. Some critters are specialized for different qualities than their food source (burrowers like moles come to mind), but the only critters that I would call truly specialized for camoflage are insects. Walking sticks, leaf insects, and some katydids are amazing!

Not everything specializes, either. Rats, raccoons, humans, starlings, and pigeons are all generalists. We're good at fitting into any of several available niches and thriving there. I'm reminded of a comment in a documentary about the wildlife in New York City: "Pigeons are generalists. They're good at a lot of things. Peregrine falcons are specialists. They're very, very good at just one thing: killing pigeons".

My point with the Repenomamus link was to demonstrate that sizable predators already existed among mammals before the extinction event. While they're only known from the early Cretaceous, the fossil record is spotty enough that I would avoid making assumptions that a particular type of animal wasn't there. Mammals were around as long as dinosaurs, and I think it's reasonable to assume that (at least within the small size range that we know they occupied) there was as much variation among them as there was among the small dinos. After all, we know there were lots of small dinos, and they would have had the same "don't get eaten by T. rex" problem as the mammals.

Basically, I don't think that birds really had a significant advantage over the mammals of the day at the time, in the "top predator" race (if you can call it that when neither group is trying for that goal, or any goal beyond survival). Both groups faced the same challenges, and adapted in different ways. I'll point out again that a flying predator is limited in how large it can grow, compared to landbound critters. A golden eagle can kill a caribou calf (I've seen the video!), but it can't carry off more than a few pounds of the meat, and if it has nestlings to feed it can't stay to guard the remains. A big cat can drag its prey home to the cubs, even prey larger than itself.

You could call eagles and large owls top predators, just because nothing much messes with them. But: both groups share their habitat with larger mammalian predators that also can be called top predators, for the same reason.

I really don't know why the terror birds didn't survive. It does seem like they at least had a good chance. Maybe raccoons ate their eggs when they reached North America! Or maybe migrating humans wiped them out like the moa (and so many other large North American megafauna).

Edited January 24, 2015 by Mediospirifer

[adthefossil]

@Mediospirifer,

I take your point about size limits of flying things. That means that if mammal predators were getting big, then birds would have to either bow out or give up flying, as I believe the terror birds did. But what about the advantages that made dinos great in the first place: lung sacks, light bones, tooth sockets, feathers (for warmth as well as flight), and a very efficient two legged form. A large flightless bird with these advantages still looks like a far more formidable predator than any mammal with its easily waterlogged fur, dense bones and two-way lungs where its hard to get stale air out of the alveoli.

Brent's point about it being easy to defend a niche is important here: the birds should have been into that niche more or less overnight cos it's what their immediate relatives (raptors, t.rex, etc) were all about, while the mammals would have had no more chance after the extinction than before it. You could be right that some undiscovered big mammal predators could have existed in 67, but there's no doubt that therapods had the upper hand, and I still don't see why 66 changed that fact.

If anything, the loss of herbivorous dinos should have directed the surviving therapods to go after mammals and compound their problems.

I hear your point that eagles and lions can both be considered top predators in their own environments, but I'm specifically asking why huge flightless birds didn't keep the mammals in their former place.

There's a piece missing from this jigsaw. There has to be a concrete reason why birds couldn't do that. With that reason in place, the mammals would have plenty of time to practice chasing each other around until they could fend off some miocene terror bird. But what kept the birds out of that niche in the immediate aftermath of 66?

[Rockwood]

We are not implying a goal, other then survival and reproduction. We are commenting on what actually happened, and why. "May have been fine with pine seeds" is implying a conscious decision. Not what you meant but possibly misconstrued.

Brent Ashcraft

I'm not really arguing against anyone. I just think that random chance and conscious decisions will always be factors. Giving up flight is not a number on a die. It's a decision. If random chance doesn't result in it being a bad move at the time it will persist. Animals do tend to fill niches, but I don't believe the mutations that allow them to have to happen at the right time and place to allow it.

[donnyjoe]

Once a bird is a bird (smaller and lighter) it can't just go back to being anything like a t.rex when there are already larger and stronger land animals more suited to filling that niche.

[Rockwood]

If there was a preferred direction to evolution we would all be there.

[jpc]

My two cents....

Large carnivorous flightless birds did indeed take over pretty quickly. There is Gastornis which is known form the Paleocene and Eocene of France, and Diatyrma from the Eocene of Wyoming. My theory on all this is that the Paleocene record of land critters is pretty spotty. In North America it is pretty much limited to teeth and jaws. Europe's record features a few more skulls and such. In other places, I don't thin there is that much Paleocene terrestrial fossils. Additionally, the record favors small fossils. So, birds were the top predators in Europe pretty quickly after the dinosaurs, and probably here in North Am as well, but not visible in the fossil record. That's my thoughts.

Why they went extinct, is a different story. But you would certainly have to look at why they went extinct at different times in different places.

[siteseer]

Birds were nearly wiped out. It seems nearly all, if not all, the modern bird orders appeared in the Paleocene so they pretty much had to start over. The world was up for grabs in the first few million years of the Paleocene. The Cenozoic could have been the Age of Crocodiles instead of the Age of Mammals. There were terrestrial crocs in North America and Europe but it seems they never became common. It also seems that large ground birds like Gastornis (Diatryma) were rare, but as noted by JP, the terrestrial Paleocene is not as well-represented in the fossil record as later environments/times. What is known is that by the mid-late Paleocene there were already dog-sized and bear-sized carnivorous mammals (now-extinct groups).

The earliest known placental mammal dates back to the Early Cretaceous. For whatever reason placentals survived the K/T extinctions better than marsupials, which nearly died out, and other groups (multituberculates, monotremes, etc.).

Hi Folks,

Birds are close relatives of the scariest dinosaurs of all. They had the best lungs and bones on Earth. They were probably warm blooded judging by the speeds they could run at. They had great teeth. They'd specialised in predation for >10^8 years.

Mammals, by contrast, had specialised in staying out of sight. I'm not sure when the placenta evolved, but it's hard to think of any other advantage they had over therapods, and in any case the placenta is not exactly a predatory gadget. Feathers are better than fur in every way. The most efficiently running mammal today is two-legged: the kangaroo. Warm bloodedness was probably common knowledge. They even had an air force.

Some very close relatives of those top predators survived the extinction 66mya. Why did they not retake the top predator spot immediately afterwards? What did?

Any help understanding this much appreciated,

Adrian.

[ashcraft]

I'm not really arguing against anyone. I just think that random chance and conscious decisions will always be factors. Giving up flight is not a number on a die. It's a decision. If random chance doesn't result in it being a bad move at the time it will persist. Animals do tend to fill niches, but I don't believe the mutations that allow them to have to happen at the right time and place to allow it.

Absolutely agree, that is why WHO survives an extinction event has a tremendous impact on speciation. I believe the original poster is looking at why the group dinosaurs didn't re-estabish dominance. The problem is that the group dinosaurs was gone with all of their genetic variability because of the extinction event. What was left were a few species of dinosaurs (birds), which were no more (and probably less) diverse then the mammals that survived. This could have been as much "luck" as anything. If one of the small raptor species had survived the extinction event, the world would look very different today. This is why I think the Cretaceous extinction event was cataclysmic, causing overwhelming and random extinction, regardless of how well that species could adapt.

If an extinction event happened tomorrow, and the only survivors were panda bears and rats, who would fill the most niches? Pandas would not stand a chance, they are too specialized.

Also, birds are not superior to mammals. Their body plan does give them some advantages in some respects, but not in others. Look at bats. Not near as strong flyers as birds, and a much later entry into the contest. Yet they dominate the night skies.

The diapsids were not"dominant" in the Permian, as mammals were not in the Mesozoic. They were under the thumb of synapsids, and if not for the Permian and Triassic extinction events, who knows?

Brent Ashcraft

[Mediospirifer]

One thing mammals are good at (some groups anyway) that may have been a factor in a changing climate is hibernation. I don't know how much of a factor that may have been in the early Paleogene, but if food resources were seasonally scarce it would have provided an advantage among high-energy-metabolism landbound animals. I'm specifying landbound because flighted animals can more easily migrate to more habitable locations.

In practice, today's hibernating warm-blooded animals are mostly small, other than omnivorous bears. (I'd be happy to hear about a large carnivore that hibernates!) An animal has to be able to slow down its metabolism and tolerate a lowered body temperature to accomplish this. Many small animals store seeds and experience a semi-hibernation, during which they will wake and feed several times over the course of a winter. Birds generally live a very high-energy lifestyle, and I don't know of any that hibernate (except possible Emperor penguins--is their huddling strategy considered true hibernation?).

Winter may have been too much for land-living flightless birds, particularly long-legged and -necked species adapted for hunting. Modern flightless birds do seem to either live in the tropics, on islands, or in places with few landgoing mammalian predators.

The wiki page on flightless birds is interesting.

[Ramo]

Very interesting reading. I appreciate all who have added to this discussion. The discussion here seems to be heavy on what lived on the land, and in the air. What about the water? The sharks survived the KT unscathed, and for the most part, unchanged. Did they reach the "end" of their evolutionary ladder millions of years before us mammals even started climbing?

[adthefossil]

@jpc: It's an interesting idea that perhaps they did exist directly after 66. But we'd still need to know why they eventually failed, and that mysterious reason could probably also explain why they never got started.

Another problem is that they no longer think Gastornis/Dyatyrma was carnivorous because of some calcium isotopes. I don't know how convincing that calcium test is.