HOOCK, H. 1986 . Kakt. and. Sukk. Band: 37 Heft (10) Seite 208-211 Die Mimese der Astrophyten

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Observations at the habitat

The mimicry of Astrophytum

Imagine a hungry Mexican donkey which is on food search at the area of Huizache in the state of San Luis Potosi. His dish supply isn't attractive at all: thorny brushwood, isolated dry leaves of grass, pungent Hechtias, unpleasant Opuntias...rocks and stones. In the need, of course he decides least on stones. He has thus possibly overlooked vegetables tasting quite passable, namely Astrophytum myriostigma Lemaire.

The Myriostigmas and all other Astrophytums are masters of disguise. They adapt visually to her surroundings so perfectly that they can be often found heavy by the cactus collector at a conscious search, too. The mimicry increases her chances of survival considerably. They are either "invisibly" for her eating enemies or aren't interesting as described because they are mistaken for inedible plants or stones (Anonymous 1955, VAUTIER 1985). Mimicry is an answer on the biotic environment factor "to be eaten". It is a part of the selection, which in the interaction with the variations, consisting of a genetic recombination and mutation, forms a biological evolution (OSCHE 1979, MAYR 1985). Evolutionary effective enemies of the Astrophytums were and are the eating enemies. Today its are essentially grazing animals of the people like goats, sheep and donkey which even eat strongly spined Opuntias in dry seasons. Different locations in Mexico are so intensive walked over, that the plants are already trampled down and destroyed by the animals purely mechanically. The most perfect mimicry of course is also ineffective in these cases. At these locations many plants have injuries by bites, some regenerate by shoots, others stand as condemned persons on retrieval for years. The selection process by eating must be many millennia old otherwise the almost unbelievably perfect visual adaptations of the Astrophytums to her surroundings wouldn't be explicable. Next to this passive protection by disguise Astrophytum ornatum (De Candolle) Weber and partly Astrophytum capricorne var. niveum (Kayser) Okumura do posess the active and development historical older enemy defence by spines. Grown up Astrophytum ornatum with the up to ten centimetres long, pungent spines are avoided even by goats. They are often solitaire and over a long distance recognizable at the slopes of the Venados valley in Hidalgo. The visual hide-and-seek of the Astrophytums finds certain limits in her size however. Very old and large pieces are seldom in free locations but most found between thorny shrubs, Hechtias and Ocotillos in very inapproachable positions, except the mentioned Ornatum. It seems as if the selection pressure would have caused the evolutionary development against well visible, large Astrophytums for a small growth and a fast flourishing ability. Astrophytum asterias (Zuccarini) Lemaire with few centimetres of size and by retreat into the ground practical without silhouette in the surroundings, is considered as the highest developed species of the genus.
The Hechtias and agaves play a quite special roll in the biotopes of the Astrophytums as accompanying plants, with the exception at Astrophytum asterias. Obviously they take over a certain mother function for the young seedlings by offering them protection before beeing eaten and burning heat (CASTELLA 1959). Hechtias don't reach the age of Astrophytums. They leave a framework made of black, carbon rich substance if they die. This should be the critical time for many Astrophytums. Doesn't fit the disguise as in the case of the naked Astrophytum capricorne v. niveum, which one make use of exactly this appearance of dead Hechtias or agaves by somatolyse, they stand visibly and free. Unspined copies then surely are soon victims of selection. It shall not remain unmentioned that the accompanying plants serve also other cactuses in the same biotopes. Leuchtenbergia principis Hooker imitates grass in size form and colour with the warts so exactly at the location Miquihuana in the state Tamaulipas, that it can be mixed up easily (PURPUS 1911). Feigning an inedible, dangerous opponent is primarily known in the animal kingdom as a so-called mimicry. The harmless hornet dreamer for example, a butterfly, looks like the hornet so exactly, that the birds mistake for it and avoid him as a food source (FRISCH 1953).

Astrophytum myriostigma and Astrophytum coahuilense (Moeller) Kayser essentially use the body form for the Allomimese (PURPUS 1911, figure 1). They copy the rubble and the stones of the surroundings with the ball form and the ribs. However, the optics gets perfect in these cases only by the more or less thick hair spots. These bring brightness and colour of the plants exactly on the rock colour one (KRÄHENBUHL 1974, 1975). Either rain doesn't harm. Then the colour of the Myriostigmas and Coahuilenses is grown dark like the wet rock. The naked Myriostigma grows typically in or on the margin of just as green Hechtias. One find all these variants at mixed locations like in San Antonio, Tamaulipas: little hairy spotted specimens into the Hechtias to thick white spotted in it rock. The function of the characteristic tufts of hair of Astrophytum has often been discussed. The isolated consideration only with respect to protection against the sun and as a water mounting device must lead to wrong judgment (SADOYSKY SCHÜTZ 1979). The process of the mimicry shows that the tufts of hair play an important roll in all ways with regard to the visual adaptation to the surroundings.

Number, order, form and colour of the spines are the essential factors of the mimicry at the Astrophytum capricorne. Astrophytum capricorne var. minor (Runge) Okumura with the thin, thick twisted spines imitates fine, dried out grass. Already the first description 1892 reported of this (Runge). At the habitat La Rosa the plants aren't not much higher than ten centimetres, so that the geometry of isolated tufts of grass in the rock cracks remains unchanged also with that (Klaus 1971). Between Hechtia growing copies are able to grow substantially longer, partly like gherkins. They then loose their spines at the sides and disguise as grass only in the free upper part. Astrophytum capricorne (Dietrich) Britton & Rose var. capricorne, the type plant, bears stronger spines (figure 2). With these partial also numb brushwood and leafless branch work is copied apart from skinny grass. Large variations and adaptions occur at the locations between Rinconada and Viesca in Coahuila. One almost always finds this Capricorne in the brown till reddish brown rocky area. It isn't a chance that the tufts of hair of many of these plants are coloured russetly in the new growth. At different pieces the older flakes aren't completely white but rather cream-coloured to pink either. The complete disposition is for the local tints fitted in excellently with that. Astrophytum capricorne var. senile Fric is disguised as skinny, fine grass analogous to Astrophytum capricorne var. minor. The colour of the spines at the type is grey, at the variety aureum canary yellow. This is a further highly specialized mimicry which copies even the differences of the grass colour. A similar case is with the young plants of the variety Astrophytum ornatum var. mirbelii from the area of Toliman, Hidalgo. Their spines shine just as yellow as the skinny grass of her native country.

All spined Astrophytums still use the body form and colour as main mimicry at least in her youth form besides the spines in the meaning described above. If they get larger, the body loses his function for the disguise and the spines imitate dead Hechtias, grass and dead wood perfectly. A divergent but the best effect of the visual customization has made Astrophytum asterias. It disappears as mentioned completely of the earth's surface, retires into the ground and only is visible with effort from above vertically (illus.). It joins that it is inundated by sandy water regularly in the rain. At it it holds tightly the ground with the flakes, the grained epidermis and the fury areoles particularly in the vegetation point. It takes the substance of his surroundings for disguise, so to speak (author collective 1980). If the sandy loam dries up, then rents and gaps arise between the single ribs. These continue to the outside and are very similar to the immediate surroundings. Scattered smaller stones on the ground one can hardly differ from the areoles of the plants. The vegetation point is protected in addition against insects by the washed substratum. Astrophytum asterias is through this at his locations between Cd. Victoria and Tampico in Tamaulipas so hidden that it is untraceable without exact knowledge even for specialists. The grazing animals pass by carelessly. Nothing admits to see or to eat for them. Indians had tied up to the pasture only few metres from the frequented place two donkeys without harm for Astrophytum asterias. The plants already surely frequently had similar dangers come through.

Although the fact of the mimicry is conspicuous at Astrophytum, essential aspects are still unsolved. The temporary fit in the seasonal change requires intensive location observation which is incomplete for understandable reasons till now. All journeys devoted to Mexico only supply snapshots. Unprotected is still the assumption that the rib number of Myriostigma is coupled with the mimicry, too. Four ribbed plants work approximately differently as normal five ribbed in her quality as stone.

For Astrophytum adapted to the biotope visually so highly the heyday must represent a special danger situation. The disguise is given up at the moment of flowering in favour of the attractiveness for pollinators. The colour eyesight of the eating enemies and the insects may be different. One must nevertheless assume that the brightly yellow flowers make the plants visible dangerously for some days. Perhaps this can occasionally declare damages at well protected locations. The mimetic behave is in the service of surviving. It must find his limits and open new evolutionary perspectives simultaneously at the time of propagation.

Literature:

ANONYM, (1955): Astrophytum myriostigma L. Bishops Cap Cactus, Saguaroland Bulletin, Desert Botanical Garden of Arizona :113-114

AUTORENKOLLEKTIV, (1980): Das moderne Tierlexikon, Bertelsmann (7): 137, (11)21

CASTELLA, M. (1959): Relacion de una Excursion a Cd. Victoria y sus alrededores, Cactaceas y Suculentas Mexicanas 5(1) :17-20

FRISCH, K. (1953): Biologie, Bayer. Schulbuchverlag : 58,64, 67, 68, 70

KLAUS, W. (1971): Astrophytum capricorne var. minor Runge & Quehl am Standort in Mexiko, Kakt. and. Sukk. 22 (9) 1 68 - 1 74

KRÄHENBÜHL, F. (1974): Meister der Tarnung, Kakt. and. Sukk. 25(1): 5-8

KRÄHENBÜHL,F. (1975): Eine besondere Bischofsmütze: Astrophytum coahuilense (Moeller), Kayser, Kakt. and. Sukk. 26 (9): 206-209;

MAYR, E. (1985): Natürliche Auslese, Naturwissenschaften (72): 231-236

OSCHE, G. (1979): Evolution, Herder Freiburg, Basel, Wien 52-59

Purpus, J. A. (1911): Standorte und Standortverhältnisse einiger Kakteen, Monatsschrift für Kakteenkunde 21 (6) 83-84

ROWLEY, G. (1958): Astrophytum asterias Lem., Nat. Cact. Succ. J. 13 (1): 7-8

RUNGE, C., QUEHL, L. (1892): Echinocactus capricornis Dietr. var. minor Runge, Monatsschrift für Kakteenkunde 2(6) :82
SADOVSKY, O.,SCHÜTZ, B. (1979): Die Gattung Astrophytum, Flora-Verlag Titisee-Neustadt : 32-33

VAUTIER, M. (1985): Kakteen, Harenberg Kommunikation .49

Photos:

Astrophytum asterias,
Astrophytum capricorne,
Astrophytum myriostigma,
Astrophytum ornatum,
Leuchtenbergia principis

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