Seed Dispersal by Way of Tree Climbing Goats

Goats are surprisingly good climbers. Given the opportunity, they’ll climb just about anything, including each other. So what’s stopping them from climbing a tree, especially if there is something up there they can eat? And so they do. Tree climbing goats are such a fascinating sight, they even have their own calendar. But the story doesn’t end there. The goats find food in the trees, entertaining humans as they go; meanwhile, the trees have a reliable partner in the goats, who inadvertently help disperse the tree’s seeds.

In general, goats don’t need to climb trees to find food. Goats aren’t known to be picky eaters, and there is usually plenty for them to eat at ground level. However, in arid climates where food can become limited, ascending trees to eat foliage and fruits is a matter of survival. This is the case in southwestern Morocco, where goats can be found in the tops of argan trees every autumn gorging on the fruits of this desert tree.

goats in Argania spinosa via wikimedia commons

Argan (Argania spinosa) is a relatively short tree with a sprawling canopy and thorny branches. It is the only species in its genus and is endemic to parts of Morocco and neighboring Algeria. The tree is economically important to the area due to the oil-rich seeds found within its bitter fruits. Argan oil has a variety of culinary uses and is also used medicinally and in cosmetics. To get to the oil, goats are often employed in harvesting the fruits. The goats retrieve the fruits from the tops of the trees and consume their fleshy outer layer. The hard, seed-containing pits are expelled, collected, and cracked open to get to the seeds.

This is where a team of researchers from Europe come in. There has been some confusion as to how the pits are expelled, with some reports claiming that they pass through the goats digestive track and are deposited in their manure. This is a common way for the seeds of many other plant species to be dispersed, and is carried out not only by goats and other ruminants, but also by a wide variety of mammals, as well as birds and even reptiles. However, considering the average size of the pits (22 mm long x 15 mm wide), the researchers thought this to be unlikely.

fruits of Argania spinosa via wikimedia commons

Others reported that the seeds were spat out in the goats’ cud while they ruminated. Goats, like other ruminants, have stomachs composed of multiple compartments, the first of which being the rumen. Partially digested food, known as cud, is sent back into the mouth from the rumen for further chewing and may be spat out or swallowed again. Goats are known to ruminate in the same location that they defecate, which results in confusion as to when and how certain seeds, like those of the argan tree, are deposited.

By feeding various fruits to a group of goats, the researchers were able to test the hypothesis that seeds could be regurgitated and spat from the cud and that this is a viable method of seed dispersal. The researchers reported that larger seeds were more commonly spat out than smaller seeds, but that “almost any seed could be ejected during, mastication, spat from the cud, digested, or defecated.” The viability of spat out seeds was tested, and over 70% of them were found to be viable.

pits and seeds of Argania spinosa via wikimedia commons

This discovery suggests that seed dispersal via spitting by ruminants could be a common occurrence – possibly far more common than previously considered. The researchers postulate that studies that have only considered seeds dispersed in manure “may have underestimated an important fraction of the total number of dispersed seeds” and that the seeds spat from the cud likely represent different species from those commonly dispersed in dung. In addition, the seeds of some species don’t survive the digestive tract of ruminants, so “spitting from the cud may represent their only, or at least their main, dispersal mechanism.”

This study surrounding the argan trees was followed up by the same group of researchers with a literature review that was published last month. The review looked into all available studies that mentioned seed dispersal via regurgitation by ruminants. While they considered over 1000 papers, only 40 published studies were found to be relevant for the review. From these studies, they determined that the seeds of 48 plant species (representing 21 different families) are dispersed by being spat from a ruminant’s cud, and that most of these plant species are trees and shrubs whose fruits contain large seeds. Also of note is that ruminants across the globe are doing this – representatives from 18 different genera were mentioned in the studies.

ruminating goat via wikimedia commons

The researchers conclude that this is a “neglected” mechanism of seed dispersal. It’s difficult to observe, and in many cases it hasn’t even been considered. Like so many other animals, ruminants can disperse seeds in a variety of ways. Seeds can attach to their fur and be transported wherever they go. They can pass through their digestive track and end up in their dung, potentially far from where they were first consumed. And, as presented here, they can be spat out during rumination. Investigations involving all of these mechanisms and the different plant species involved will allow us to see, in a much clearer way, the role that ruminants play in the dispersal of seeds.


Idaho’s Native Milkweeds (Updated)

As David Epstein said in an interview on Longform Podcast, “Any time you write about science, somethings is going to be wrong; the problem is you don’t know what it is yet, so you better be ready to update your beliefs as you learn more.” Thanks to the newly published Guide to the Native Milkweeds of Idaho by Cecilia Lynn Kinter, lead botanist for Idaho Department of Fish and Game, I’ve been made aware of some things I got wrong in the first version of this post. I appreciate being corrected though, because I want to get things right. What follows is an updated version of the original post. The most substantial change is that there are actually five milkweed species native to Idaho rather than six. Be sure to check out Kinter’s free guide to learn more about this remarkable group of plants.


Concern for monarch butterflies has resulted in increased interest in milkweeds. Understandably so, as they are the host plants and food source for the larval stage of these migrating butterflies. But milkweeds are an impressive group of plants in their own right, and their ecological role extends far beyond a single charismatic insect. Work to save the monarch butterfly, which requires halting milkweed losses and restoring milkweed populations, will in turn provide habitat for countless other organisms. A patch of milkweed teems with life, and our pursuits to protect a single caterpillar invite us to explore that.

Asclepias – also known as the milkweeds – is a genus consisting of around 140 species, 72 of which are native to the United States and Canada. Alaska and Hawaii are the only states in the U.S. that don’t have a native species of milkweed. The ranges of some species native to the United States extend down into Mexico where there are numerous other milkweed species. Central America and South America are also home to many distinct milkweed species. Asclepias species found in southern Africa are considered by many to actually belong in the genus Gomphocarpus.

The habitats milkweeds occupy are about as diverse as the genus itself – from wetlands to prairies, from deserts to forests, and practically anywhere in between. Some species occupy disturbed and/or neglected sites like roadsides, agricultural fields, and vacant lots. For this reason they are frequently viewed as a weed; however, such populations are easily managed, and with such an important ecological role to play, they don’t deserve to be vilified in this way.

Milkweed species are not distributed across the United States evenly. Texas and Arizona are home to the highest diversity with 37 and 29 species respectively. Idaho, my home state, is on the low end with five native species. The most abundant species found in Idaho is Asclepias speciosa, commonly known as showy milkweed.

showy milkweed (Asclepias speciosa)

Showy milkweed is distributed from central U.S. westward and can be found in all western states. It occurs throughout Idaho and is easily the best place to look for monarch caterpillars. In fact, the monarch butterfly is Idaho’s state insect, thanks in part to the abundance of showy milkweed, which is frequently found growing in large colonies due to its ability to reproduce vegetatively via adventitious shoots produced on lateral roots or underground stems. Only a handful of milkweed species reproduce this way. Showy milkweed reaches up to five feet tall and has large ovate, gray-green leaves. Like all milkweed species except one (Asclepias tuberosa), its stems and leaves contain milky, latex sap. In early summer, the stems are topped with large umbrella-shaped inflorescences composed of pale pink to pink-purple flowers.

The flowers of milkweed deserve a close examination. Right away you will notice unique features not seen on most other flowers. The petals of milkweed flowers bend backwards, which would otherwise allow easy access to the flower’s sex parts if it wasn’t for a series of hoods and horns protecting them. Collectively, these hoods and horns are called the corona, which houses glands that produce abundant nectar and has a series of slits where the anthers are exposed. The pollen grains of milkweed are contained in waxy sacs called pollinia. Two pollinia are connected together by a corpusculum giving this structure a wishbone appearance. An insect visiting the flower for nectar slips its leg into the slit, and the pollen sacs become attached with the help of the corpusculum. When the insect leaves, the pollen sacs follow. Pollination is successful when the pollen sacs are inadvertently deposited on the stigmas of another flower.

Milkweed flowers are not self-fertile, so they require assistance by insects to sexually reproduce. They are not picky about who does it either, and their profuse nectar draws in all kinds of insects including bees, butterflies, moths, beetles, wasps, and ants. Certain insects – like bumble bees and other large bees – are more efficient pollinators than others. Once pollinated, seeds are formed inside a pod-like fruit called a follicle. The follicles of showy milkweed can be around 5 inches long and house dozens to hundreds of seeds. When the follicle matures, it splits open to release the seeds, which are small, brown, papery disks with a tuft of soft, white, silky hair attached. The seeds of showy milkweed go airborne in late summer.

follicles forming on showy milkweed (Asclepias speciosa)

Whorled or narrowleaf milkweed (Asclepias fascicularis) occurs across western and southern Idaho. Its distribution continues into neighboring states. It is adapted to dry locations, but can be found in a variety of habitats. Like showy milkweed, it spreads rhizomatously as well as by seed. It’s a whispy plant that reaches one to three feet tall and occasionally taller. It has long, narrow leaves and produces tight clusters of greenish-white to pink-purple flowers. Its seed pods are long and slender and its seeds are about 1/4 inch long.

flowers of narrowleaf milkweed (Asclepias fascicularis)

seeds escaping from the follicle of narrowleaf milkweed (Asclepias fascicularis)

Swamp or rose milkweed (Asclepias incarnata) is more common east of Idaho, but occurs occasionally in southwestern Idaho. As its common name suggests, it prefers moist soils and is found in wetlands, wet meadows, and along streambanks. It can spread rhizomatously, but generally doesn’t spread very far. It reaches up to four feet tall, has deep green, lance-shaped leaves, and produces attractive, fragrant, pink to mauve, dome-shaped flower heads at the tops of its stems. Its seed pods are narrow and around 3 inches long.

swamp milkweed (Asclepias incarnata)

Asclepias cryptoceras ssp. davisii, or Davis’s milkweed, is a low-growing, drought-adapted, diminutive species that occurs in southwestern Idaho. It has round or oval-shaped leaves and produces flowers on a short stalk. The flowers have white or cream-colored petals and pink-purple hoods. The range of Asclepias cryptoceras – commonly known as pallid milkweed or jewel milkweed – extends beyond Idaho’s borders into Oregon and Nevada, creeping north into Washington and south into California. Another subspecies – cryptoceras – can be found in Nevada, Utah, and their bordering states.

Davis’s milkweed (Asclepias cryptoceras ssp. davisii)

The final species is rare in Idaho, as Idaho sits at the top of its native range. Asclepias asperula ssp. asperula, or spider milkweed, has a single documented location in Franklin County (southeastern Idaho). Keep your eyes peeled though, because this plant may occur elsewhere, either in Franklin County or neighboring counties. It grows up to two feet tall with an upright or sprawling habit and produces clusters of white to green-yellow flowers with maroon highlights. Its common name comes from the crab spiders frequently found hunting in its flower heads.

A sixth species, horsetail milkweed (Asclepias subverticillata), has been falsely reported in Idaho. Collections previously labeled as A. subverticillata have been determined to actually be the similar looking A. fascicularis.

Party Time for Puncture Vine, Year Two

Party Time for Puncture Vine, Year Two

A noxious weed brought Boiseans together for the second year in a row. After a successful inaugural year, the Boise Goathead Fest returned to downtown Boise, Idaho on the 2nd and 3rd of August. The festival’s namesake comes from a particularly destructive weed whose spiky fruits are notorious for puncturing bike tires. Known commonly as goathead or puncture vine, Tribulus terrestris is abundant in the Treasure Valley and the bane of area bicyclists. Organized by the Boise Bicycle Project and other bike-centric non-profits, Boise Goathead Fest is a celebration of bike culture, as well as an opportunity to spread awareness about this problematic plant.

goathead-themed art

For the two months leading up to Goathead Fest, Treasure Valley residents were encouraged to pull as many goatheads as they could get their hands on. Those who took on the challenge were rewarded with tokens to be redeemed at the Fest for drinks or ice cream. Trophies and prizes were also presented to those who pulled the most goatheads over the two month period. This effort resulted in thousands of pounds of goatheads being removed from the area, saving bicyclists from countless flat tires and slimming down puncture vine’s extensive seed bank.

scale for weighing goathead collections

After two months of collecting goatheads, it was time to celebrate. The two day long party consisted of live music and DJs, a huge bike parade around downtown Boise (for which participants were encouraged to decorate their bikes and wear costumes), and a variety of other bike-themed and non bike-themed activities. Goathead education continued during the Fest with the help of folks from Ada County, City of Boise, Idaho Botanical Garden, and others.

A peak inside the Ada County Weed, Pest, and Mosquito Abatement education and outreach trailer

Goathead coloring page created by Wendy of Idaho Botanical Garden

Puncture vine pennants created by Anna of Idaho Botanical Garden

With all of this attention and awareness focused on a single noxious weed, might it be possible to eliminate it from our community and free ourselves from ruined rides and trashed tires? The seeds of puncture vine are relatively short lived, and as an annual plant, seeds are its only method of reproduction. Even if we can’t altogether eliminate it, we could certainly see a dramatic reduction in its abundance and distribution. Perhaps in the future we will spend less time pulling it and more time celebrating its rarity, reflecting back on the time when punctures permeated our pedal-powered lives. Whatever the result, puncture vine has brought our community together once again. If such a loathsome weed can bring people together in celebration like this, perhaps it’s not entirely bad.

me with goathead balloon

See Also: Boise Weekly – Goathead’s the Burr, Community’s the Word