October 16, 2013 | 2
Botanists have long puzzled over a peculiarity of ancient plants called cycads: they have huge, bright, fleshy seeds displayed in enormous cones. Yet there’s nothing correspondingly ginormous to eat and — I’ll use the polite term here — disperse them. And if their presumed former big dispersers (perhaps super-sized ice age mammals, marsupials, birds — or maybe even dinosaurs?) are all now extinct, how are these plants managing to persist without them? Are cycad seeds relicts that will ultimately doom their parents?
As it turns out, being spread by extinct giants may not preclude using part of the same reproductive strategy to — if not thrive — survive today. And the secret to that success may be a plant form of socializing.
Cycads are plants of great antiquity that once dominated large parts of Earth. They evolved in the Permian at the end of the Paleozoic some 250 million years ago. They look a bit like palm trees with a tuft of leaves emerging from the tip of a uniformly thick stalk, but their leathery, strappy leaflets are arranged like the vanes of a feather instead of the fingers of a hand. They so dominated Earth during the Mesozoic — what you probably know as the Age of Dinosaurs — that it has also been called (mostly by botanists, I’m sure) the Age of Cycads and Dinosaurs.
Today many fewer cycads grow on Earth, and those that do tend to populate the tropics and sub-tropics. Even though their seeds look like fruit (actually, they look a bit like mutant corn kernels), they fleshy parts are not made from a flower’s ripened ovary as true fruits are. Instead, they soft part is a fleshy seed coat — one of the three basic parts of a seed. Seeds — a pivotal innovation in the evolution of plants — package an embryonic plant along with some food in a moisture-sealing shell called a coat. It’s like packing your offspring’s lunchbag and dressing it in a warm coat before sending it out the door.
But all that baggage means that seeds are 1) tasty targets and 2) much harder to disperse than feather-light microscopic spores, seeds’ predecessors. Cycads tackled both problems by lacing their seeds with toxins while packaging them in that nutritious fleshy coat called a sarcotesta. In the Australian cycad Macrozamia miquelii, the seeds exceed 5 grams, of which nearly half the weight is flesh. The huge orange seeds are displayed tantalizingly on cones bearing around 150 each. The situation seems to beg for something big to come along and swallow mouthfuls of seeds whole, digest the fleshy reward, and then pass the unharmed seeds in some distant locale.
Botanists had noticed plants in similar predicaments before; a 1982 paper described a “megafaunal dispersal syndrome” in plants with big seeds in the New World tropics, the former stomping grounds of extinct elephant-like gomphotheres and giant ground sloths. In Australia, bull-sized marsupials like Diprotodon or ground-dwelling birds like Genyornis (which was bigger than an emu) lived in close proximity to cycads as recently as 46,000 years ago.
The suite of traits that may mark plants left behind after their titanic dispersers expired includes big seeds or fruits that drop to the ground when ripe, but which then rot for want of anything to eat them. Existing dispersers like birds, whose gift of flight depends on light bodies and payloads, can ill disperse such propagules, and small mammals may be ably equipped to eat the fruit or seeds, but poorly equipped to move it. But if this is the case, how are cycads and their similarly bereft compatriots surviving without their long-lost partners?
Two scientists from the University of Queensland in Australia — John Hall and Gimme Walter — investigated this question by following the fates of one species of cycad seeds today. Just how far were they travelling from Mom, and how were they getting there? To answer this question, the did something incredibly low-tech: they super-glued 8-millimeter steel bolts to the exposed hard ends of 840 cycad seeds. They also set up a camera trap and a hair sampling tube to ID any animals that might nab seeds. Then they waited. When they came back three months later, they fired up a metal detector (delightfully named the Bounty Hunter “Tracker IV”, and manufactured, appropriately enough, by First Texas Manufacturing of El Paso) to relocate the seeds and discover how far the bolted seeds had bolted. And the answer, published in the American Journal of Botany in August, usually was: not far.
The only disperser they observed was the brushtail possum, a small marsupial. Though the possums downed cycad sarcotestae with gusto, leaving nearly no seed unstripped at T-plus 3 months, they didn’t take the seeds very far. Most seeds stayed within 1 meter (3 feet) of their mothers. No seed moved more than 5 meters. Yet staying so close to home had a cost: their data suggested that most seeds that landed within 1.5 m of their mothers died.
But these findings raise another puzzle: if dispersal by small mammals or marsupials is now more advantageous to cycads, and staying too close to mom is lethal, why wouldn’t cycad seeds evolved to be smaller, lighter and more easily carried by small animals? Since huge, flesh-covered seeds are expensive, what reason could there be for continuing to produce super-sized seeds when smaller ones would be so much cheaper and easier to disperse?
The key to this puzzle has to do with another characteristic of cycads: they love to live together. Indeed, M. miquelii lives together leaf-by-trunk in densities reaching 1,000 to 5,000 plants per hectare, and may dominate the forest floor.
If you’re a cycad, growing in groves is great for several reasons. As it turns out, interacting with other cycads is a crucial part of their lives. First, individual cycads, like most animals, are single-sexed. That is, there are boy cycads and girl cycads. A plant living alone cannot reproduce. Even living with a few may not do, because only a subset of any given male or female cycads actually make cones in a given year. So in cycads, large societies help favor what biologists like to call “reproductive success”.
Further, in spite of having no flowers, cycads depend on pollinators — insects that can ferry sperm-bearing pollen to the egg-bearing female cone. It’s a bit strange to think of non-flowering plants with pollinators because the conifers — the major group of non-flowering plants that still dominates large sections of Earth — turned to wind to get this job done long ago and has never looked back. But pollinators are not the exclusive domain of the flowering plants. Certain beetles have specialized in doing this work for cycads, and some species live nearly their entire lives in cycad cones. So if you’re a cycad, it pays to grow in a location that your pollinators are likely to be and likely to find you — and that would be where all the other cycads are.
Finally, cycads often engage in synchronized seed production — a phenomenon botanists call “mast seeding”. By synchronizing cone and seed production, they can draw more pollinators and dispersers, maximizing seed dispersal and success. The more cycads living together, the better the effect will work.
Since small mammals and birds tend to move only one or a few seeds at a time, the advantages of living together solves the riddle of the persistent large seeds: small seeds increase the odds of a seedling finding itself consigned to a life of celibate solitude. Large seeds maximize the number of seedlings sprouting in a grove with access to mates and matchmakers. Big seeds sprout in or near the colony, because they’re too heavy for small animals to move far.
Thus extinct large dispersers, Hall and Walter argue, were only a part of the cycad reproductive strategy, and losing them did not mean the end of cycads or their mighty seeds. Extinct animals’ role was to found new groves when a herd — or perhaps even a well-fed individual — took a group potty break, depositing seeds “en masse” and increasing the odds young plants grew up in good society. It’s a dynamic that cycads could have evolved in partnership with dinosaurs as far back as their heyday in the Mesozoic.
But even if mega-disperers with such capabilites — be they dinosaurs, mammals, marsupials, or birds — are absent or rare today, that doesn’t preclude the success of the other part of cycad reproductive strategy: strength in numbers. It also means that cycads and plants with “megafaunal dispersal syndrome” have always dwelt in isolated but crowded colonies. So as long as existing colonies extend or maintain their borders, cycads will persist, even if the seeds don’t fall far from the tree.
Hall J.A. & Walter G.H. (2013). Seed dispersal of the Australian cycad Macrozamia miquelii (Zamiaceae): Are cycads megafauna-dispersed “grove forming” plants?, American Journal of Botany, 100 (6) 1127-1136. DOI: 10.3732/ajb.1200115