One day a few years ago, Alfred was sitting by himself, eating some fruit and generally minding his own business. If Alfred knew he was being watched, he didn’t seem to mind – it wasn’t the first time he’d been shadowed like this. As he enjoyed his snack, those following him crouched in their hiding spot a short distance away, taking notes, recording footage, observing how he held the fruit and precisely how he ate it. They stayed with him all day, and the next, and the one after that: about five days in all. If he rested, they rested. If he moved, they moved, too. And during these curious five days, whenever Alfred took a moment to relieve his bowels, this prompted a quiet flurry of activity by those surveilling him. The location was noted with great interest, as well as the date and the hour. As Alfred moved away, out came the collection bag. Alfred, you see, is a Bornean orangutan, and his observers are part of the Tuanan Orangutan Research Program based in Gunung Palung National Park on the Indonesian island of Borneo. Their goal is to study the behaviour and ecology of critically endangered orangutans like Alfred and determine, among other things, whether these orangutans are effectively gardeners. The answer, as it turns out, involves the rather interesting mathematics of orangutan poo.
Boston University researcher Andrea Blackburn wanted to know just how good Bornean orangutans are at dispersing seeds. Over the course of more than a year, she and her colleagues followed dozens of orangutans, including Alfred, for up to five days each. It was an exercise in patience and the efficacy of bug-repellent, and it resulted in the collection of 733 faecal samples. According to Blackburn, around 70 per cent of those samples contained intact seeds. Sometimes they found thousands of tiny seeds in a sample, other times it was one or two big seeds, says Blackburn. ‘There was this pretty big range of diversity in terms of the seeds that we found.’ In short, over a five-day period, each orangutan dispersed an average of around thirty seeds representing up to nine genera of plants. On average, the seeds were dispersed 450 metres from their parent trees, with some ending up more than 2 kilometres away.
Importantly, not only were the seeds still able to germinate, but in most cases they were also better at germinating than the same species of seeds that had not taken a ride through an orangutan’s digestive tract. The plants that seemed to benefit the most from this gut treatment were the fruiting canopy trees Alangium and Tetramerista glabra.
In a sense the orangutans do act like gardeners, says Blackburn. ‘In terms of a conservation perspective, each individual is pretty important,’ she tells me. Orangutans are the largest arboreal frugivores and they swallow seeds of many different sizes. In addition, says Blackburn, ‘Orangutans can open these really big fruits that often have hard husks, like durians and other fruits, that other smaller frugivores and smaller primates cannot get into. So I think they’re dispersing those seeds – even if they’re not necessarily swallowing them, they might be just dropping the seeds and other animals can come and move them or interact with them.’
With the help of orangutans, these canopy trees are able to expand their ‘seed shadow’, which is the range in which a plant’s seeds are dispersed and will germinate. At the same time, while the seeds are temporarily inside an orangutan, they cannot be predated upon, and therefore destroyed, by other animals.
Orangutans are not unique in their role as unwitting gardeners, which is an important part of the evolutionary strategy of seed plants. Andrew Rozefelds points out that seed dispersal by animals can be critical to plant survival. Seeds that stay close to a long-lived parent tree will find themselves immediately in competition for resources, he says, and if they can’t move, they’re stuck. So key animal species play a vital role by moving seeds around within a habitat. ‘By moving a seed away from a host tree,’ says Rozefelds, ‘it’s got a good chance of surviving somewhere else.’
Studies have shown that up to 90 per cent of seeds in tropical regions are dispersed by animals, largely via ingestion. This process, called endozoochory, plays a major role in seed dispersal globally. If you have ever eaten a wild blackberry or blueberry while wandering through a woodland, there’s a good chance it was grown from a seed that had, at some point, passed through the fundament of a wild animal.
Berries are curiously popular across a wide swathe of the animal kingdom, from birds and rodents to deer, bears, even wolves. In a 2018 study of eight wolf packs in Minnesota, animal ecologist Joseph Bump and his colleagues found that blueberries comprised up to 83 per cent of the wolves’ mid-summer diet. Meanwhile, other studies show that in a season where sarsaparilla berries, cherries, raspberries, juneberries, blueberries and more are in abundance, a single black bear, or Ursus americanus, can eat as many as 30,000 berries in a day. Brown bears are similarly prolific when it comes to consuming fruits and dispersing seeds, with studies showing that Ursus arctos scats (poo) often contain tens of thousands of viable seeds.
In 2018, ecologists Laurie Harrer and Taal Levi from Oregon State University studied the eating habits of brown and black bears in south-eastern Alaska and discovered these bears had a staggering fondness for the berries of the devil’s club plant, a woody shrub native to the coniferous, old-growth forests of North America’s Pacific Northwest. The bears were observed consuming around thirty berries per second, which equates to just over 100,000 berries per hour of foraging. Based on this, the researchers estimated the bears were dispersing around 200,000 seeds per square kilometre per hour. These animals are well-known seed dispersers, and, much like Blackburn’s findings with orangutans, gut treatment seems to have a germination-enhancing effect on many of the seeds they ingest.
Vertebrates, especially mammals and birds, are major seed dispersers in a wide range of habitats. The list is long, varied and sometimes a bit weird, and what follows is by no means exhaustive. To begin with, there are shorebirds that disperse seeds of flowering plants and fruit trees to distant islands; migratory swans that spread pondweed seeds across continents; and mallards in Hungary that carry the seeds of elderberries, figs and bittersweet nightshade to the edge of the Black Sea. There are bluebirds that disperse myrtle seeds across South Carolina, and African and Asian hornbills that disperse a wide variety of plant species. Bats, which account for roughly 20 per cent of all mammals, are prolific seed dispersers. African elephants, which quite enjoy a nice fig or the fruit of the baobab tree, can disperse seeds up to 65 kilometres from where they feasted. As suggested by the actions of bears and wolves, some carnivores are surprisingly proficient at seed dispersal. Coyotes, it should be noted, seem to have a taste for hackberries and persimmons. There are also many seed dispersers in the Mustelidae family, which includes weasels, racoons, otters, martens and wolverines – carnivores all.
One of the oldest living family of terrestrial carnivores, the Viverridae, the most well known of which are civets, are pretty good at it, too. Asian palm civets are native to Indonesia, and have a catlike appearance and something of a coffee habit. They enjoy eating ripe coffee cherries, and the undigested coffee beans end up in their droppings. This should be a win-win: the coffee plant widens its seed shadow and the civet gets a nice fruity snack. But their mutualism is increasingly being interrupted due to the rise in popularity of kopi luwak, a drink made from the coffee beans found in civet dung. Costing up to $100 a cup, it’s the most expensive coffee in the world. Sadly, this luxury status has led to the capture and caging of many civets, and, as some experts argue, all for coffee that doesn’t even taste that good anyway.
Herbivores, as you might expect, are excellent seed dispersers and vitally important to many ecosystems, especially the grazing ungulates: horses, giraffes, hippopotami, tapirs, all the bovines, and pretty much any other mammal with hooves. Lowland tapirs (Tapirus terrestris) consume more than 350 plant species and are particularly good at dispersing their seeds in the Amazon. Even reptiles get in on the seed-dispersal act, from tiny skinks to enormous, plodding tortoises. Seed scientists Si-Chong Chen and Angela Moles have noted that Galápagos tortoises not only swallow seeds but can hold them in their guts for up to thirty-three days. As Chen and Moles explain in an article in Australasian Science from March 2016, tortoises might move slowly, but they can cover a lot of ground in just over a month. As a result any seeds they’ve consumed can be spread quite far.
Chen is a researcher at the Royal Botanic Gardens, Kew in the United Kingdom, while Moles heads The Big Ecology Lab at the University of New South Wales, and together they compiled a global database of more than 13,000 animal–seed interactions across all vertebrates. The majority of animal species on the list were mammals and birds, plus a number of reptiles, and there were a few entries that surprised even Chen and Moles, such as armadillos, aardvarks, and, most curiously, some species of fish.
Yes, fish.
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The Pantanal in South America is the largest tropical wetland in the world. Spanning more than 200,000 square kilometres, this seasonal flood plain sprawls across the central western edge of Brazil and crosses into Bolivia and Paraguay. In an article she wrote for the World Wide Fund for Nature (WWF) in 2010, the late Brazilian journalist and conservationist Geralda Magela likened the Pantanal to ‘a huge soup plate that slowly fills up with water and overflows in the rainy season, gradually empties during the dry season and then starts to fill up all over again’.
The Pantanal’s enormous ecosystem is home to the highest concentration of wildlife on the South American continent. And the water available via a vast network of streams, rivers and lakes is central to the survival of thousands of plants species on which that wildlife depends. Some, such as the native tucum palm, release their fruits during the wet season, often in the midst of flooding. As terrestrial animals move to higher ground, fish take over many of their seed-dispersal duties. During this time, pacu (Piaractus mesopotamicus), a species of freshwater fish with disturbingly human-like teeth, feed on fruit and nuts that fall into the floodwaters. The seeds are often swallowed in the process and later deposited upstream. Ecologist Mauro Galetti from Brazil’s Universidade Estadual Paulista, along with his colleagues, discovered that the tucum palm relies heavily on pacu for this service. Moreover, as they report in the journal Biotropica, the pacu acts as the primary seed disperser for many species of fruit-bearing plants found in the Pantanal. In fact, at least forty-three species, representing a significant amount of the Pantanal’s tree diversity, rely on fish for seed dispersal.
Then there is the strange case of a seed-dispersing amphibian. In 1986, researchers were studying a coastal ecosystem on the Brazilian coast, not far from Rio de Janeiro, which involved collecting a local species of tree frog, Xenohyla truncata. One morning, while transporting some of these frogs to the laboratory, one of the researchers noticed something unusual: there appeared to be fruit seeds in the frogs’ droppings. Amphibians, including tree frogs, are largely insectivorous, relying largely on a diet of insects. Many amphibians even branch out into more general carnivore territory to eat spiders, worms and other invertebrates, and some will even dine on the occasional lizard or mouse. As a rule, though, amphibians don’t go for fruit. But here were these Xenohyla truncata frogs pooping out fruit seeds. The team collected more frogs and found that they were indeed frugivorous – they regularly supplemented their diet with the small fruits of five species of local native plants, then shed the seeds around their immediate ecosystem.
Not all animal-dispersed seeds are ‘gut treated’, of course. Many simply adhere to an animal as it brushes past, enabling it to be transported long beyond the time limits imposed by a few healthy bowel movements. A seed snagging on a bird’s feathers could wind up anywhere within the same forest or in another continent entirely. In a 2006 study, Spanish researchers Pablo Manzano and Juan E Malo placed four species of herb seeds on the wool of merino sheep, which then travelled along an ancient transhumance route spanning several hundred kilometres from northern Spain towards Extremadura near the Portuguese border. A significant number of the seeds made it to the other end, with many more presumably dispersed along the way.
But lest we give vertebrates all the glory, it is important to realise that invertebrates also play a major role in this ecosystem engineering. The act of seed dispersal by ants is called myrmecochory and it contributes to the dispersal of at least 11,000 species of angiosperms – or, to put it another way, 4.5 per cent of all flowering plants. Chen tells me that when animals ingest seeds and disperse them elsewhere ‘it’s not for good intentions – they want the food’. She explains that even when it comes to ants, the seed must provide a reward. For this reason, seeds dispersed by ants usually possess a structure called an elaiosome. Under a microscope, it looks almost like an unnecessary flourish, an extra bit of fluff or an errant appendage, but elaiosomes are rich in fats and proteins, making them incredibly enticing to the ants. Because they are either sticky or structured such that they act like handles ants can grasp onto, elaiosomes make it easier for ants to transport the seeds to their nests. Although the seed is lipid-rich, the ants may only eat the elaiosome and then discard the seed, says Chen. But it’s still viable and because the soil around ant nests tends to be rich in organic matter and nutrients, it enjoys a better chance of germination.
Ants are the most studied of invertebrate seed dispersers, but by no means are they alone. In Asia, hornets disperse the seeds of the flowering Stemona tuberosa, and in New Zealand there are native grasshoppers called weta that eat fruit and defecate seeds in much the same way small mammals do. Indeed, some weta are about the size of a mouse and weigh as much as 70 grams. Their unsettling hugeness is how they got their name. ‘Weta’ derives from the Māori word wētāpunga, which means ‘the god of ugly things’, although these days the grasshoppers are more often described as gentle giants. Earthworms can be helpful, too, by dispersing seeds not horizontally, like other seed dispersers, but vertically. The worms remove seeds from the surface and scatter them deeper into soil where they have access to nutrients and are afforded protection from seed predators.
This raises an important point: not all seed eaters are good seed dispersers. While many birds do indeed disperse seeds far and wide, birds and quite a few small mammals and insects are seed predators. This means they tend to digest the seeds they eat, for the fats, carbohydrates and proteins within, and the seed is destroyed in the process. That’s not a good outcome for the plant, and so some plants have evolved seeds with harder coats, or sharp, barbed fruits, or found other ways to dissuade predators.
Now, a tough exterior might afford some protection, and even enable dormancy, but it presents an entirely different logistical problem for the plant: how does a tiny plant embryo break out of something like that? It is, so to speak, a tough nut to crack. Many plants arrived at the elegant, albeit smelly, solution of temporarily hiding their seeds in the digestive system of a different animal. For the seed, there are multiple benefits: (1) the seed predator can’t find it; (2) the animal provides seed-dispersal services by moving about; and (3) the digestive enzymes and acids can breach the seed coat just enough so as to make germination a whole lot easier when the seed emerges at the other end.
Oh, and (4) instant fertiliser.
And so, seeds did not just evolve to take advantage of gusts of air, or to tumble easily down the slope of a hill, or float on a stream or an ocean current or survive a fire. Many seeds are the way they are now due to millions of years of interactions with animals. Some became incredibly tough, or were protected by sharp, pain-inducing fruits and cones and other off-putting coverings, while others were packaged up in fleshy, tasty fruits, full of sugars, fats, proteins and nutrients. Plants evolved pigment molecules and aromatic compounds so they could be seen and sniffed out by seed dispersers when the seeds were properly developed and ready for spreading. Indeed, there is mounting evidence that these cues are so specific that they can relay the nutrient content of the fruit, attracting, for example, seed-dispersing birds that prefer high lipids. This is why ripe fruit looks, smells and even tastes different to unripe fruit. It’s why tiny elaiosomes are like an irresistible take-home meal complete with handles. Some seeds developed barbs and hooks to latch onto passers-by. Burrs are so effective in their hook-catch mechanism that they inspired the invention of velcro.
Animals changed the way plants present their genes to the big wide world, but mutualism, by definition, is never one-sided. Seeds changed animals, too. It was a matter of survival. Over time, a species could either take advantage of the energy and nutrients provided by available seeds or seed-containing fruits and grasses, or die out. So, through evolutionary processes, animals honed their senses and their anatomies, competing and compromising not only with the plants but also with other animals. Jaw and teeth structures changed to crush hard seed coats, and digestive systems adapted as well. So did visual systems, olfactory neurons, the structure and arrangements of tastebuds, and even memory, as well as the neurological circuitry that links incoming stimuli with reward systems – for example, the way sugar drives a spike in dopamine in our primate brains, whereas the taste of a clump of dirt does not. Sometimes this took millions of years. Sometimes it took a surprisingly short period of time. If we look closely at the right species, we can witness evolutionary arms races unfolding in real time.
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The weather was fine and warm on the morning of 17 September 1835 when a 26-year-old naturalist by the name of Charles Darwin disembarked from the HMS Beagle and stood on the dark, rocky shore of a small, volcanic island in the Pacific. He wasn’t exactly blown away by its beauty. ‘Nothing could be less inviting than the first appearance,’ he later recounted. ‘A broken field of black basaltic lava, thrown into the most rugged waves, and crossed by great fissures, is everywhere covered by stunted, sun-burnt brushwood, which shows little signs of life.’ The Beagle’s captain, Robert FitzRoy, was even less enamoured by the scene, describing it as ‘a shore fit for Pandemonium’.
They had landed on Chatham Island, now known as San Cristóbal, the eastern-most island of the Galápagos archipelago which straddles the equator some 960 kilometres west of Ecuador. The crew of the Beagle was in the process of finalising a survey of South American coastlines and its nearby islands and was preparing to venture across the Pacific.
As Galápagos was equatorial, and therefore firmly within the tropics, Darwin had expected it to be a little more, well, tropical: ‘I saw nowhere any member of the Palm family, which is the more singular as 360 miles northward, Cocos Island takes its name from the number of cocoa-nuts.’ Indeed, there were no shady palm trees here or lush ferns. Instead, the islands presented Darwin with a series of alien landscapes, which he variously described as ‘desolate’, even ‘sterile’. ‘All the plants have a wretched, weedy appearance,’ he wrote. ‘I did not see one beautiful flower.’ There were no frogs, either, which he found puzzling, although he did observe some giant, lumbering tortoises and a variety of lizards. Captain FitzRoy took a particular dislike to the ‘hideous’ iguanas, of which he said ‘few animals are uglier’. Darwin found them intriguing but nonetheless referred to them as ‘imps of darkness’. Darwin also noticed – offhandedly, at first – that the islands were home to a large number of ‘dull-coloured birds’.
Darwin spent five weeks on those harsh, sunburned islands, observing and collecting specimens, and in so doing he had his earliest glimpses into something truly remarkable. ‘The natural history of these islands is eminently curious, and well deserves attention,’ he would later write. ‘We seem to be brought somewhat near to that great fact – that mystery of mysteries – the first appearance of new beings on this earth.’ The archipelago that had seemed at first so uninviting would irrevocably change the way Darwin viewed all living things. Those wretched weeds, those imps of darkness, those tortoises and, especially, those dull-coloured birds would provide the foundation on which Darwin built his famous theory of species evolution by natural selection.
As Darwin explored one island and then another, he observed a great variety of birds, which he identified as mockingbirds, wrens, warblers and blackbirds, as well as a number of finches. Regarding these birds – all but the mockingbirds – Darwin noticed that their beaks varied significantly, ranging from small and needle-like to large and rounded. As psychologist and science historian Frank Sulloway explained in a 1982 article in The Journal of the History of Biology, these observations did not trigger any epiphanies about speciation at the time because Darwin truly thought the birds belonged to entirely different genera and, in some cases, entirely different families. It was only after Darwin returned to London that the proverbial penny dropped.
Soon after his arrival back home, Darwin donated his specimen collection to the Zoological Society of London, upon which the young illustrator and ornithologist John Gould inspected them closely. What he found surprised even Darwin. There were finches, yes, but the warblers, the wrens and those blackbirds were nothing of the sort. They were finches, too. Darwin had collected fourteen species of birds belonging to one genus. They were unique to the Galápagos archipelago, and, it seemed, many were unique to individual islands. Long after the Beagle pulled up anchor in the Galápagos, Darwin reflected on the diversity of those finches – and all those remarkably different beaks. He was at once puzzled and enthralled: ‘Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.’
One might, indeed. In this statement, written years before he published On the Origin of Species, you can almost feel the subtle yet seismic shift in Darwin’s thinking. At the time, this was a radical idea. Was he right? Absolutely.
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For almost half a century, Peter and Rosemary Grant of Princeton University have been closely monitoring populations of medium ground finches (Geospiza fortis) on the Galápagos island of Daphne Major, making annual trips to study the birds’ physical traits – weight, body size, beak size and shape – and gathering data on their primary food sources, mating behaviours, and birth and death rates. What they have found further supports Darwin’s theory that natural selection brought about by environmental pressures can lead to the development of new species. It also shows, among other things, just how well seeds can influence the evolution of animals. It is in seeds that we can see those ‘different ends’ Darwin wondered about.
Mitochondrial DNA dating indicates that the fourteen species of ‘Darwin’s finches’ in the Galápagos, and one species that made it as far as Cocos Island (Isla del Coco) to the north, all evolved from a single common ancestor within the past 1.5 million years. Also, they’re not really finches. That common ancestor, which so ambitiously flew to the Galápagos from the South American mainland all those years ago, appears to have belonged to a family of small ‘finch-like’ birds called tanagers. That bird’s descendants then moved from island to island, adapting to a variety of habitats and food sources – and the arrival of new islands. Geologically speaking, the Galápagos islands are still babies. They only began to form less than five million years ago and they’re still in the process of forming today. Consequently, the older and the newer islands present vastly different habitats.
Moreover, the Galápagos’s location, situated precisely at the equator on the eastern edge of the Pacific, adds further evolutionary pressures in the form of weather extremes. The archipelago is exquisitely vulnerable to the El Niño Southern Oscillation, but the effects are precisely the opposite to those experienced in Australia. Due to prevailing trade winds and warm water currents, El Niño years that bring drought conditions in Australia bring heavy rain falls to the Galápagos and the western coast of South America. Conversely, La Niña rains in Australia often mean protracted drought on the other side of the Pacific.
Medium ground finches are largely seed eaters. Their beaks are blunter than the finer beaks of the green warbler finch (Certhidea olivacea), and whereas green warbler finches use their beaks almost like sharp tweezers for catching insects, the medium ground finches use their more robust beaks for breaking seeds. Even within medium ground finches, there are variations in beak size, just as variations in height and body shape exist among humans. Those with beaks at the smaller end of the spectrum tend to subsist on small, soft seeds, many of which are deposited by wind on the rocky landscape. The finches hop all over the terrain to find them, their smaller, pointed beaks suited to exploring all the nooks and crannies. The medium ground finches with larger beaks don’t bother as much with small seeds, but they have no problem munching on the seeds of the Tribulus cistoides, contained within a large, hard, spiny casing called a mericarp. Technically it’s a fruit, but it looks like pain for lunch. From the plant’s point of view, the hard casing and sharp spines are meant to dissuade seed predation, which works for most everything except the large-beaked G. fortis, which breaks the seeds open with a forceful crushing action. In their first decade of observing finches on Daphne Major, the Grants witnessed just how important this beak–seed relationship was.
For years, medium ground finches with small beaks had done quite well scouring the island for small seeds, but that all changed in 1977 with the arrival of a La Niña event. The resulting drought was merciless. No rain fell on the Galápagos for eighteen months. Many of the plants died, especially those that produced small seeds. The only remaining seeds available were those produced by the Tribulus cistoides plant, which is drought-tolerant, but G. fortis with small beaks couldn’t tackle the large, spiny seeds. As a result, around 80 per cent of the medium ground finches died during that drought. Very few smaller-beaked birds endured, but most of the finches that survived tended to have large, strong beaks which enabled them to access the tougher seeds. These birds then passed their genes on to the next generation. The Grants found that the average beak size of G. fortis birds on Daphne Major was significantly larger after the drought than it had been before.
‘This was an evolutionary response to a natural selection event for the reason that it’s the large birds that could eat the large, hard Tribulus seeds,’ explained Rosemary Grant in a lecture on the evolution of Darwin’s finches that she presented at Cornell University in 2018. Grant went on to recount that, a few years after that fateful La Niña, it all changed again. In late 1982, an El Niño event arrived in the Galápagos, and the archipelago spent much of 1983 drenched by heavy, prolonged rainfalls. Over a metre of rain fell that year, estimated to have been the most severe rainfall event in the region in 400 years. The effect on the vegetation on Daphne Major was profound. Diminutive seed-producing plants grew in abundance, yielding large crops of small, soft seeds. Meanwhile, Tribulus cistoides plants were completely overrun, smothered by grasses, herbaceous plants and wet weather–loving vines. ‘It completely changed the island from a large, hard seed producer to a small seed producer,’ said Grant.
The wet season is usually when the finches breed. Indeed, the first heavy rainfall of the season seems to make these little birds enthusiastically libidinous. In 1983 they bred like crazy. What’s more, there were now plenty of plants producing small, soft seeds. ‘When the drought came two years later, this time it was the smaller birds who had the selective advantage and it was them who survived,’ explained Rosemary Grant. By impacting the production of certain seed shapes and sizes, weather events were providing evolutionary nudges in this isolated population of birds. And not over millions of years but twice in less than a decade. The Grants were seeing evolution by natural selection take place in real time.
To find out how the beak sizes within the finch population could respond to seed structure and availability so rapidly, the Grants and their collaborators have since undertaken a number of genetic analyses. Among the many interesting findings, their colleague Leif Andersson from Uppsala University in Sweden showed that a gene called ALX1 plays a big role in the beak size of these finches. The ALX1 gene makes the ALX1 protein, which is a transcription factor. Transcription factors play an important part in gene regulation by acting as a kind of on–off switch for a whole bunch of genes. There are many different transcription factors controlling different groups of genes, but it turns out that ALX1 regulates the activity of craniofacial genes. In the finches, variations in this gene contributed either to blunter or more pointed beaks. Whichever shaped beak afforded survival in certain environmental conditions, those finches were able to then pass the associated variations on to their offspring. Intriguingly, ALX1 isn’t specific to finches. It’s a master regulator of craniofacial development in all vertebrates, including humans. We need it for the normal development of our heads and faces. Mutations in the ALX1 gene that cause its loss of function result in severe cleft palate in infants.
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So it is that seeds changed animals and animals changed seeds, and the evidence is all around us, in ants and squirrels, and in finches that aren’t really finches. Many animals are generalists, subsisting on a variety of plants and seeds, but sometimes the pairings are so exquisitely specific that the abundance or absence of a particular plant or fruit or seed is enough to literally change the shape of a species and profoundly alter the course of its population. It can work well, with some close mutualisms persisting for eons. Other times, another animal species arrives, or another tastier fruit, and the field crowds with competitors and other options. But every now and then, one member of a close mutualistic pair disappears altogether. We can still see the echoes of animal–seed relationships that no longer exist.
In 1876, Darwin’s contemporary and fellow evolutionist Alfred Russel Wallace observed, ‘We live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared.’ Around sixty-six million years ago a wayward asteroid, or perhaps a fragment of a comet, slammed into Earth and triggered a mass extinction event that ended the reign of the dinosaurs. And yet, life persisted. Millions of years later, the world was once again populated with big vertebrates. There were giant reptiles, including giant tortoises and massive lizards. There were giant birds, too, such as the 3-metre-tall, 1000-kilogram elephant bird of Madagascar, and the giant teratorn (Argentavis magnificens), an enormous condor-like creature with a 7-metre wingspan which is the lead contender for the largest bird to have ever lived. Then, of course, there were the giant mammals. There were mastodons and mammoths, and elephant-like gomphotheres. Put in terms of modern-day geography, there were sabre-tooth cats roaming Canada and giant deer in Ireland, as well as giant sloths in Mexico. By way of further example, if you think Australia is home to strange animals now, you should have seen it 100,000 years ago. There were marsupial ‘lions’ (Thylacoleo carnifex), giant kangaroos (Procoptodon goliah), and giant wombats (Diprotodon optatum) that weighed around 3 tonnes, not to mention a koala-like animal that was, if not exactly ‘giant’, then certainly far more imposing than the cuddly ones we know and love today. The megalania, a 500-kilogram relative of the Komodo dragon lizard, also lived on the Australian continent back then, as did massive emu-like birds.
But sometime between 60,000 and 10,000 years ago, many of these megafauna went extinct. This happened at different times and in many different parts of the world, and in some places the first such extinctions began as early as 125,000 years ago. Although the factors contributing to these extinctions vary, evidence increasingly suggests that environmental changes coupled with predation by humans was often responsible. We still have blue whales, elephants and a few species of giant turtles, but Wallace was right: today’s megafauna are a shadow of what came before.
For seeds that use animals for their gut-dispersal services, size tends to be proportional to the size of the animal: small seeds for small animals, large seeds for large ones. There are exceptions, of course, as Si-Chong Chen and Angela Moles have pointed out. Ungulates, though often large, are quite adept at dispersing small seeds of grasses that get ‘hoovered up’ during grazing. And then there are those big brown bears and all those tiny berries. But in places such as the tropics, where seeds rely heavily on animal dispersal, size matters. It would be impossible for an animal with the stature of a shrew to swallow and disperse avocado seeds. It would be incredibly difficult for many animals to comfortably pass such a thing. I certainly wouldn’t want to.
The avocado is what ecologists call an evolutionary anachronism. Science writer Connie Barlow describes them more hauntingly as ‘the ghosts of evolution’ in her book of the same name. These ‘ghosts’ are all around us, she explains, and avocados are a perfect example. Evidence suggests that the avocado, which evolved in Mesoamerica (modern-day Central America) around ten million years ago, was eaten and dispersed by giant sloths and other megafauna that roamed the Americas at the time. All the fats, proteins and carbohydrates that avocados are often praised for were originally meant for a van-sized, 4-tonne lestodon in the Neocene. Other evidence suggests that giant ground sloths also fed on pawpaw fruits, that bulbous osage oranges were eaten by mastodons, and that the large hanging fruits of the cocoa tree were a favourite of the gomphotheres that roamed North America millions of years ago. Next time you enjoy chocolate, take a moment to thank those extinct gomphotheres for getting the ball rolling.
That cocoa trees did not go extinct when gomphotheres did is likely due to opportunistic seed dispersers. Indeed, seed dispersal of evolutionary anachronisms persisted when other animals stepped in – a fruit bat here, a parrot there. They might not have been the ideal partners, but it was enough to stave off extinction. For a while, anyway.
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Palaeobotanist Andrew Rozefelds is fascinated by the diversity of life and myriad strategies for survival, especially in rainforests, but he worries about the loss of today’s key seed dispersers: the ones that are still alive but could easily, without care, become evolutionary ghosts. This isn’t some hypothetical. He talks to me about cassowaries, the large, long-legged, flightless birds that belong to the same family as ostriches and emus. They reach heights of 1.8 metres, weigh upwards of 70 kilograms, have a tough keratinous protrusion atop their heads, and enormous scaly claws, and look very much like something that belongs on the set of Jurassic World. Of course, all modern birds descend from a lineage of avian dinosaurs that survived the mass extinction at the end of the Cretaceous. Indeed, there is a theory that the ability to peck on seeds is precisely what enabled them to survive the prolonged ‘impact winter’ that brought the Cretaceous to its ultimate end. But although all birds are considered living dinosaurs, cassowaries seem to bear the most striking family resemblance to their Mesozoic forbears.
‘Cassowaries are cool!’ says Rozefelds with a smile, and he’s not just talking about their appearance. He explains that cassowaries’ role in maintaining rainforest ecosystems is deeply underappreciated. They’re prolific seed dispersers, consuming the fruits and defecating the seeds of more than 230 plant species. Through their role as rainforest gardeners, they operate as keystone species – species upon which many other species, both plant and animal, survive. Yet their populations are declining. In Australia, for example, cassowaries once roamed almost the entirety of tropical northern Queensland, but today there are only 4000 left in three isolated populations, thanks to increasingly fragmented habitats. With isolation comes genetic bottlenecks and a higher risk of disease. At the moment, though, they mostly succumb to dog attacks, car strikes, and egg predation by feral pigs. This is not good, says Rozefelds, not for the cassowaries and not for the rainforests they quietly engineer. ‘If we lost cassowaries, I suspect there’s a good chance it would impact everything.’
Andrea Blackburn has similar worries about the orangutans on Borneo and nearby Sumatra, which are keystone species in their ecosystems and also critically endangered. The orangutans on these two islands are the only ones left in the wild, she tells me, adding that they face ‘all sorts of threats – deforestation, fires, hunting and poaching’. By and large, though, the biggest problem is habitat loss, she says.
Orangutans are not the only primates we need to learn more about, especially in terms of their gardening capabilities. Primates, after all, make up a substantial amount of frugivore activity in the tropics. Seed-dispersal activity has been observed in chimpanzees, gibbons, macaques, bonobos, baboons and more. Western lowland gorillas (Gorilla g. gorilla) in the Central African country of Gabon disperse the seeds of at least 117 different plants. In the dense primary forests of Colombia, brown spider monkeys disperse huge numbers of seeds from a variety of species and in so doing engineer ecosystems in which many other species – from jaguars to sloths – can live. On rare occasions, brown spider monkeys with white fur have been spotted in those forests, and it isn’t a good sign. Dubbed ‘ghost monkeys’, their pale appearance isn’t due to albinism but rather is a genetic glitch that likely signifies inbreeding. This can happen when a population dwindles to the edge of vanishing. There are now so few brown spider monkeys left in the wild, they are classed as of the most endangered primates on the planet. We already have enough ghosts of evolution. We don’t need new ones.
There is one primate with a very different story, though. More than once in its evolutionary journey, it was in danger of flickering out of existence. But that was a long time ago, and it has since proliferated to such an extent that it impacts every ecosystem on the planet. That primate is Homo sapiens. Humans. Us. As it turns out, the way we’ve interacted with seeds has quite a lot to do with how the world got to be the way it is. What we do with seeds from here on out will profoundly affect far more than you may realise.