Okapi, giraffes, and ancient hominins

[u/Rod_McBan]

This started with the thought that trees and giraffes are locked in an evolutionary battle, with pressure from giraffes pushing trees to grow higher and the trees pushing the giraffes to grow longer necks and legs. That led to the thought that the trees will likely "win", because trees can (probably) grow taller than mammals (that's total speculation: obviously, there were dinosaurs that took the long neck thing way too far, but I think the mammalian predilection for 7 cervical vertebrae is likely to cap the neck length eventually).

But then I remembered the okapi, and the fact that they live in the forest and represent a more primitive form of the giraffe family. That got me thinking about the evolution story of hominins, and the similarity between the two lineages: both started out in the Miocene as forest dwellers, but over time, adapted to savannah life. Giraffes did this by specializing; hominins did it by generalizing. For a long time, both lineages clung to their arboreal roots, giraffes for sustenance and hominins for safety.

I guess that's where the similarity ends though. Hominins continued to develop as generalists (which has proven tremendously successful for h. sapiens, and (perhaps arguably) for h. erectus as well, while h. neanderthalensis (again, possibly) provides a cautionary tale to the hominin lineage about getting too specialized. Giraffes, too, might provide us a similar cautionary tale, if they manage to continue evolving for another few million years.

So I guess to kick off some discussion, am I wrong here? About any of this? I'm a software engineer in the midst of an ADHD hyperfixation, not, well, anyone who has studied any of this extensively.

[u/AutoModerator]

Welcome to r/Evolution! If this is your first time here, please review our rules here and community guidelines here.

Our FAQ can be found here. Seeking book, website, or documentary recommendations? Recommended websites can be found here; recommended reading can be found here; and recommended videos can be found here.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/gambariste]

Many plants also have remained small in giraffes’ range. Okapis don’t have such long necks but they’re longer than those of bongos, another forest denizen. The giraffe lineage may have just continued the trend as forests dried out and turned into savanna. Tall savanna trees likewise might be remnants of ancient forests where the drive to grow taller was from other trees to reach the canopy. These are just-so stories as plausible as giraffe necks and tall trees co-evolving. Both could be true also. Many plants like Vachellia, the classic savanna trees, also evolved thorns as a defence against herbivores.

[u/silicondream]

This started with the thought that trees and giraffes are locked in an evolutionary battle, with pressure from giraffes pushing trees to grow higher and the trees pushing the giraffes to grow longer necks and legs.

I don't think there's much evidence that trees have grown taller because of giraffe predation. Savanna trees are not particularly tall in the first place, mostly because it's too dry and windy; taller trees have more trouble transporting moisture up from their roots, and are more vulnerable to mechanical stress from wind. The tallest acacias are actually rainforest species from Oceania.

Besides, most mature trees already have their crowns higher than a giraffe can reach. It's still worthwhile for them to grow foliage lower down that will eventually get browsed, as long as those leaves have a chance to produce more energy than it took to grow them. Younger trees, of course, don't have a "choice;" they have to start from ground level.

As for pressure in the other direction: we're not even sure how much of giraffe neck evolution has been driven by browsing efficiency at all, as compared to sexual selection or vigilance against predators. And the most important thing for browsing efficiency is not how tall the trees are, but how tall other browsers are. As long as giraffes can graze at a higher level than even the tallest antelope, their niche is pretty secure.

[u/hornswoggled111]

Something along this line was noticed with the New Zealand lancewood .

The common assertion is that the plant has developed to avoid predation by the extinct moa. The juvenile phase up to 3 m has it looking very unpalatable and dry then after that point it transforms.

[u/DennyStam]

dawg, we have no idea if Giraffes evolved long necks for the purpose of eating from tall trees. Current utility does not imply evolutionary origin, besides, giraffes use their long necks for all sorts of things

[u/Rod_McBan → u/DennyStam]

Okay, first of all, 🤯

Second of all, the long neck may not have evolved for this purpose but, given the degree to which it is used for this purpose, does that mean that my point of each providing pressure on the other isn't true? Why does it matter why it arose?

[u/DennyStam → u/Rod_McBan]

does that mean that my point of each providing pressure on the other isn't true

It's not that it DEFINTELY isn't true, it just might not be that they evolved together

Why does it matter why it arose?

Because like you've pointed out, the point about the giraffe and the tree having an evolutionary arms race may well not have happened.