Despite carrying the responsibility for the survival of a species, some fruits and seeds don’t seem to function in their current natural environment. How can this be when evolution through natural selection has shaped them over millions of years? The answer is like something from an Arthur Conan Doyle novel...

Seed of the Month - Dermatophyllum secundiflorum

The hard-nosed scientists among you might have found my last blog on why the seeds of Ravenala madagascariensis are blue already somewhat hair-raising, given that there is no published direct scientific evidence, yet, to back up my story. Well, here comes another daring observation and the hard-evidence junkies among you had better fasten your seat belts.

Texas mountain laurel flowers (Photo by Betty Alex)

Boring fruits, flashy seeds

The Texas mountain laurel (Dermatophyllum secundiflorum, until recently called Sophora secundiflora or Calia secundiflora), is a shrub from the southwestern United States and Mexico. It has some rather boring looking, large brown fibrous-woody fruits (‘pods’) that neither get eaten by any creature nor ever open to release their seeds, and yet, they contain extremely hard, beautiful shiny red seeds which seem perfectly adapted to attract birds for dispersal, if only the birds could get to see them. It’s a fact of life that fruits must ‘make sense’ within their evolutionary background (i.e. achieve the dispersal of their seeds in their co-adapted environment), otherwise they would not exist. Therefore, I conclude that the indehiscent fruits (meaning the fruits do not split open) of the Texas mountain laurel are typical ‘megafauna’ fruits, i.e. fruits that are adapted for dispersal by large (greater than 50 kg) mammals.

Fruits of the Texas mountain laurel (Photo by Wolfgang Stuppy)

To the taste of big beasts

The biggest terrestrial animals alive today are the elephant, rhinoceros, hippopotamus and giraffe. It is therefore no coincidence that the megafauna dispersal syndrome is best expressed in Africa and Asia where these animals still occur. In fact, ruminants such as giraffes and antelopes and non-ruminants such as elephants and rhinos are among the most important seed dispersers in the African savannah.

The fruits of many legumes are specifically adapted to attract these animals in that they are large, indehiscent, with a brown, leathery husk and often with a distinct smell that attracts even cattle. Since herbivores (plant eaters) are colour-blind, the brown fruits are visually inconspicuous but rich in digestible carbohydrates and protein; they contain extremely hard, smooth seeds that can withstand the grinding of strong molars. The fruits can remain on the tree but are often dropped to the ground as soon as they are ripe to provide easy access for their large terrestrial dispersers.

Seeds of the Texas mountain laurel (Photo by Wolfgang Stuppy)

This description pretty much matches the fruits of the Texas mountain laurel which happens to also be a legume. The only problem is that North America hasn’t really got much of a megafauna other than, for example, the native pronghorn and introduced cattle and horses.

Pronghorn in Texas (Photo by Wolfgang Stuppy)

Seeds from a lost world

However, until 13,000 years ago, towards the end of the Pleistocene epoch (1.8 million to 11,550 years ago) when the last Ice Age drew to a close, North America boasted a megafauna far richer than Africa’s today. The ancient menagerie of potential megafauna seed dispersers featured several species of native wild horses, camels and tapirs alongside fantastic creatures such as gomphotheres (four-tusked elephant-like creatures), mastodons and woolly mammoths weighing up to ten tons, giant ground sloths, the largest the size of a modern elephant, glyptodonts (giant armadillo-relatives the size of a small car), giant short-faced bears nearly twice the size of a grizzly, giant bisons, giant peccaries, giant beavers and giant tortoises. By the way, the pronghorn is a relic of this ancient megafauna.

'Megafauna' (Photos by Wolfgang Stuppy)

 
Returning to the fruits of the Texas Mountain Laurel, having no takers among the animals around today, I suggest that they are adapted to be eaten by one or several members of North America’s extinct megafauna. The rock-hard, shiny red seeds then stuck out from these animals’ dung piles where birds could spot them.

Looks juicy but.....

Red seeds of the Texas mountain laurel (Photo by Ellen Woods)

This is somewhat unsavoury, but mistaking them for something like a juicy red berry, at least some inexperienced birds would be tempted to pick up the seeds and fly away with them to a safer place (e.g. a tree branch) where they would try to eat them. However, the hard red seed coat is just a con, mimicking something fleshy and edible. The frustrated birds would then simply discard the seeds – dispersal achieved!

Nowadays, with their co-adapted dispersers long gone, the fruits of the Texas mountain laurel only reveal their seeds after the fruit wall has rotted away on the ground where the seeds might then be spotted by ground-dwelling birds like turkeys. Native Americans probably helped with the dispersal of the seeds, too, using them as beads and, although all parts of the plant are poisonous, as a hallucinogenic drug before they discovered the peyote.

Acknowledgements:

Thanks to Michael Eason (Alpine, Texas) for sharing his first-hand knowledge about the Texas mountain laurel and to Betty Alex (Terlingua, Texas) for contributing the image of a flowering specimen.

- Wolfgang -
 

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Related Links

• The Millennium Seed Bank Partnership in the USA
• More about seed dispersal
• Seed Image Gallery
• Pleistocene epoch 

 

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