Biological journal of the Linnean Socie& (1982), 18: 49-58.
Plant mimicry: evolutionary constraints
G. B. WILLIAMSON*
Department of Biology, University of Miami,
Coral Gables, Florida 33124, U.S.A.
Acwpted for pubhation December 198I
Because plants are sessile and their flowers and fruits are aggregated, plant mimics are less likely to be
mistaken for their models than animal mimics which are mobile and dispersed among their models.
Therefore, operator sprcies are more likely to be deceived by iinimal mimics than plant mimics. In
addition, the autonomy of plant appendages implies that warning mimicry provides less advantage to
plants than to animals because plants sufTer less from sampling by naive operators. Therefore, the
advantage of warning mimicry is much greater for animals than plants. These reasons may explain
why plant mimicry is less common than animal mimicry, based on attraction of rather than avoidance
by operator species, and limited to the class of aggressive mimicry.
KEY WORDS:-Mimic
mimicry
plant mimicry
pollination
dispersal.
CONTENTS
Introduction . . . . . . .
Constraints on plant mimicry . . .
Bypassing the sessile constraint . .
Bypassing the aggregated constraint
Mimicry within the constraints. .
Classes of plant mimicry . . . .
. . . . . . .
Summary.
Acknowledgements. . . . . .
References, . . . . . . .
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INIRODUCTION
Mimicry systems with different species of model and mimic may involve two
plants, two animals, or one plant and one animal. (Hereafter I refer to these
systems as plant mimicry, animal mimicry, and plant-animal mimicry.) Animal
mimicry systems are quite common (Wickler, 1968) and involve many different
classes of mimicry (Vane-Wright, 1976). In contrast, plant systems are rare
(Proctor & Yeo, 1973; Wiens, 1978) and may be limited to two classes of
interaction (Williamson & Black, 1981).
Here I describe the traits of plants that may have hindered the evolution ofplant
mimicry, as well as the special conditions where it appears to have evolved. Given
the heterogenity within and among complex mimicry systems, my “objective is not
so much the discovery of the universal as the accounting for differences’’ (Levins,
*Current address: Department of Botany, Louisiana State Univenity, Baton Rouge, LA 70803, U.S.A.
0024-4066/82/050049
+ 10$03.00/0
49
01982 The Linnean Society of London
50
G . B. WILLIAMSON
1968: 6). I focus on plant mimicry in opposition to animal mimicry in order to
clarify their differences, although I suggest that plant-animal mimicry systems are
intermediate in occurrence and character.
CONSTRAINTS ON PLANT MIMICRY
Generally, the mimicry process involves two signal-transmitters, the model (S,)
and the mimic (S,), and a signal receiver, the operator (R),who confuses S, for S,
(Vane-Wright, 1976). Then, “mimicry occurs when an organism or group of
organisms (the mimic) simulates signal properties of a second living organism (the
model), such that the mimic is able to take some advantage of the regular response
of a sensitive signal-receiver (the operator) towards the model, through mistaken
identity of the mimic for the model” (Vane-Wright, 1976: 50). The mistaken
identity or deception may be an innate response (Smith, 1975, 1977, 1978) or a
learned response through formation of a search image (Tinbergen, 1960) or a n
avoidance image (Clarke, 1962). A learned image is developed and maintained
through experience and reinforcement with the model, so many kinds of mimicry
have been shown to depend on the frequencies of models and mimics (Brower,
1960; Emlen, 1968; Vane-Wright, 1976). However, it is not the true frequencies of
models and mimics, but rather the operator’s contact frequencies with them that
influence the effectiveness of mimicry (Williamson & Nelson, 1972; Matthews,
1977).
Within the mimicry process three traits of terrestrial plants may hinder the
evolution of mimicry. First, plants are sessile so operators may have time to
scrutinize the model and the mimic fully and perhaps learn to distinguish them.
Additionally, the sessile condition may permit learning operators to recall locations
as well as search images; therefore, operators may remember an individual plant
and its identity as a model or mimic by location rather than by search image after
only a single experience.
A second trait that may hinder the evolution of plant mimicry is aggregation of
mimics. The pattern of most plant species is aggregated, and where only parts of
plants are the signal-transmitters such as leaves, flowers or fruits, these ‘conspecific’
signal-transmitters by necessity occur as aggregates. Therefore, the contact
experience of most learning operators may not be an alternating or even random
sequence of models and mimics, but instead a series of models followed by a series
of mimics and so on. Such spatial aggregation may facilitate the operator’s
recognition of patches of mimics as distinct from patches of models and
subsequently lead to learned reactions to each or to oscillations between search and
avoidance images (Matthews, 1977). With either result, mimetic deception may be
attenuated.
The sessile and aggregated conditions of plants together represent contraints on
the evolution of plant mimicry because operator deception is more likely to be
maximized with mobile mimics interspersed among their models.
A third relevant trait of plants is the relative autonomy of plant appendages,
such as leaves, flowers and fruits. Loss of one leaf or one flower by a plant mimic is
exactly that, whereas loss of one leg or one wing by an animal mimic may result in
further exposure to predation or death. Where deception results in avoidance by
the operator as in Miillerian mimicry, the selective advantage of mimicry may be
much greater in animals than in plants.
PLANT MIMICRY
51
All plants exhibit appendage autonomy, so this trait may represent an
unavoidable constraint to the evolution of some types of plant mimicry. However,
the sessile and aggregated traits of plants are constraints that may be circumvented
under some conditions of growth.
Bypassing the sessile constraint
Entire plants are sessile, but plants may avoid memory location by operators
through periodic production of appendages such as flowers and fruits. Thus, the
vast majority of hypothesized plant mimicry systems involve floral or seed mimicry
(Wiens, 1978) where the appendages involved are present only for short periods of
time relative to the plant’s lifespan. In contrast, foliage which is present
continuously has been hypothesized in only two cases of plant mimicry : mimicry of
host leaves by parasitic mistletoe leaves (Barlow & Wiens, 1977), and mimicry of
host leaves by PassiJIra vines (Gilbert, 1975). However, in neither system has
experimental work been done to test the mimicry hypothesis. Caution must be
exercised in proposed cases of leaf mimicry in order to differentiate mimicry from
crypsis. Vane-Wright ( 1980) distinguished the two processes as crypsis where the
herbivore fails to detect its host plant because the host blends into the visual
background of foliage or as mimicry where the herbivore avoids its host because
the host signal corresponds to the avoidance image developed by the herbivore
from negative experience with the model.
In summary, plants may circumvent the sessile appearance of some appendages
through periodic production as in the case of flowers and fruits, but not leaves.
Moreover, most invertebrate herbivores chemically locate their host plants, so
visually mediated leaf mimicry may offer little advantage (Wiens, 1978). Evidence
for chemical mimicry as an herbivory deterrent awaits further investigation.
Bypassing the aggregated constraint
While plant appendages are aggregated (ips0 facto) on whole plants, and while
dispersal limitations usually cause aggregation of plant individuals, any of the
following conditions would allow association of a plant or its parts with another
species and thereby include potential mimics (and their models).
( 1 ) Parasites or hemi-parasites (and their hosts).
(2) Vines or epiphytes (and their hosts).
( 3 ) Female flowers (and male flowers) of monoecious plants.
(4)Female flowers (and male flowers) of dioecious plants with joint dispersal of
both sexes.
(5) Harvested seeds (and crop seeds).
Parasites, hemi-parasites, vines and epiphytes may have their flowers, fruits or
foliage interspersed among their hosts’ respective parts which then may serve as
models. The effectiveness ofmimicry of the host would depend among other factors
on the host-specificity of the mimic; therefore, parasites and hemi-parasites may
represent more likely mimicry candidates than vines and epiphytes. As mentioned
above, foliar mimicry is limited by the continuous presence of leaves, so floral and
fruit mimicry may be more common. Several floral mimics have been suggested
among root parasites (Wiens, 1978).
Unisexual flowers may represent associated plant appendages where either sex
4.
52
G . B. WILLIAMSON
may serve as a model for the other. In monoecious plants, each floral sex may be in
close proximity to or interspersed with the other. In dioecious plants, the floral
sexes will occur interspersed only if the seeds of both sexes are dispersed together in
multi-seeded fruits as in the Cucurbitaceae. Several examples of floral mimicry
involving unisexual flowers have been proposed in both monoecious and dioecious
plants (Gilbert, 1975; Baker, 1976; Bawa, 1977, 1980; Schmid, 1978; Pijl, van der,
1978). In most cases female flowers are less abundant than males and offer little or
no nectar reward to nectar-seeking pollinators or no pollen to pollen-seeking
Hymenopt era.
Seed mimicry for dispersal has been found in crop weeds (Wickler, 1968; Wiens,
1978). I suggest that a distinction be drawn in this case to the process of removal
from the plant or harvesting and any subsequent dissemination. Before harvest
mimetic seeds are aggregated on a plant whereas harvested mimics may be mixed
among other species’ seeds. Mimicry may be involved in post-harvest
dissemination but probably not in the harvest (Wickler, 1968). Harvesting is
usually an indiscriminate collection of model and mimic seeds, but mimicry
functions in the post-harvest attempts to sort the model crop seeds from weed seeds
and debris. Therefore, the first step of harvesting mixes the crop models and weed
mimics and ameliorates the aggregated condition of weed seeds.
Mimicry within the constraints
The sessile and aggregated constraints may have hindered the evolution of plant
mimicry, but some systems have evolved without circumventing these two
conditions. Such cases seem limited to the following types of mimics (and their
models).
1. Insectivorous plants (and other flowers).
2. Nectarless flowers with pollinia (and other flowers).
3. Nectarless ‘multiple bang’ vines (and other Bignoniaceae).
4. Imitation arils and berries (arillate seeds and fleshy fruits).
Insectivorous plants (e.g. Dionaea, .Nepenthes, Cephalotus, Sarracenia and
Darlingtonia) often bear flower-like appendages, colours, nectar or glistening drops
that attract insects which are captured by the plants. These plants share the feature
of capturing insects who have innate floral preferences or previous experiences
with flower models, but little or no experience with the mimic. Operator
experience with the mimic is limited in this kind of mimicry since deceived
operators are eliminated.
Some nectarless flowers have been purported to be mimics of other nectarproducing flowers. The absence of nectar per se is not sufficient evidence for floral
mimicry since pollination may occur through autogamy, wind pollination or
pollen-attracted vectors (Williamson & Black, 1981). I n most cases clumps of
nectarless flowers will be avoided by learning pollinators, so floral visitation may
be so rare that mimicry results in little seed set. However, flowers bearing pollinia
share the unique trait that even very rare visitation may result in sufficient pollen
transfer to effect massive seed set. Consequently, nectarless flowers with pollinia are
probably better candidates for floral mimicry than nectarless flowers with free
pollen because even rare deception may effect large seed set. Several nectarless
orchids are suspected floral mimics (Hcinrich, 1975; Boydcn, 1980; Bierzychudek,
1981).
PLANT MIMICRY
53
The nectarless ‘multiple-bang’ vines of the Bignoniaceae are also suspected
mimics of other nectar-producing Bignoniaceae (Gentry, 1974). These vines flower
explosively several times a year, each time for only a few days. Insect visitation is
recorded only rarely at these plants and Gentry (1974) assumes it involves insects
investigating new floral resources. Again, this system is confined to operators with
little or no experience with the mimic. How the rare deceptions afford adequate
pollination is unclear.
The brightly coloured, red or red and black seeds (e.g. Erythrina, Ormosia and
Abrus) lacking an arillate or fleshy reward, are suspected mimics of seeds with
rewards for avian dispersal agents (Ridley, 1930; Pijl, van der, 1969; McKey,
1975; pers. ob.). Two facts of dispersal are apparent: birds do remove the seeds,
but the removal rate is notoriously low (McKey, 1975; pers. ob.). Concomitant
with low removal rates are hard seed coats and sturdy attachment to the treetraits that maintain seed viability and seed presentation even with slow dispersal.
McKey (1975) suggests that in undisturbed vegetation these species were quite
rare. An alternate explanation is that these seeds are aposematically coloured to
warn seed-eaters of their toxicity.
These four systems of purported plant mimicry may have evolved without
bypassing both the sessile and aggregated constraints. They share the common
feature of depending on operators who have extensive experience with, or innate
preference for, models, yet limited or no experience with the mimics. They all
require an individual operator to be deceived only rarely, so the average operator’s
contact frequency with models greatly exceeds its contact frequency with mimics.
Furthermore, the rare deception affords great benefit to these mimics.
CLASSES OF PLANT MIMICRY
The nine cases outlined above, five where both the sessile and aggregated
constraints are circumvented and four where the constraints are not circumvented,
include most of the suspected cases of plant mimicry. Other previously suggested
cases are probably crypsis as in the case of mistletoes or simply evolutionary
convergence caused by pollinator sensory systems as in the case of pollinator
syndromes. The latter case is illustrated by red tubular flowers of many
hummingbird-pollinated species (Grant, 1966) that sometimes are purported to be
Mullerian mimics (e.g. Proctor & Yeo, 1973; Brown & Kodric-Brown, 1979).
Under close inspection, flowers which initially show similar morphologies often are
found to be distinguishable by pollinators (Eisner et a f . , 1969; Williamson & Black,
1981). However, given that pollinators have innate or acquired generalized search
images, mimicry is difficult to distinguish from simple investigation of new
resources. Mimicry must involve the mistaken identity of the mimic for the model
(Vane-Wright, 1976), but experimentally testing the deception may be difficult.
Compounding the difficulty is the fact that naive operators preferentially sample
novel food that is similar to previously experienced food even where deception is
not involved (Coppinger, 1970).
Similar confusion exists between mimicry and floral convergence where
pollinators exhibit preference among flowers (e.g. Dafni & Ivri, 1981 ). Several
years ago, I noted that flowers visited by pollen-collecting Bombus cphippiatus on
Costa Rican mountaintops exhibited radially symmetrical yellow flowers. Thc
flowers included representatives of different families as well as several species in the
G . B. WILLIAMSON
54
genus Hypericum. The latter flowers showed only slight differences in size and style
structure. Furthermore one species, Hypericum strictum, was visited more frequently
in natural patches where it was associated with the pollinator-preferred Hypericum
irazuense, so mimicry was a plausible hypothesis. An experiment was designed
which included a simulated shrub with equal numbers of flowers from both
Hypericum species. The movement of bees was recorded from flower to flower and
failed to show that a bee’s present experience (i.e. which species a bee was visiting)
influenced its next floral choice. The bees simply showed preference, but they were
not deceived despite the close natural association of the two species.
Demonstration that visitation to one species increases the subsequent probability
of visitation to others is necessary but not sufficient proof for mimicry because some
pollinators may require different resources from different flowers. For example,
butterflies utilizing nectar and pollen (Gilbert, 1972) may preferentially search for
pollen resources after visiting nectar resources or vice-versa. For the moment
insufficient evidence exists to classify generalized pollination syndromes as mimicry
rings.
The nine cases of plant mimicry described here are shown in Table 1 together
with Vane-Wright’s (1976) classification that neatly divides mimicry systems
according to three dichotomous factors: ( 1) mimetic deception is advantageous
(synergic) or disadvantageous (antergic) to the model; (2) the operator’s reaction
to the model in the absence of deception, and (3) the operator’s reaction to the
mimic in the absence of deception. In the absence of deception the mimicry is
warning where the operator avoids both model and mimic, aggressive where it
avoids the mimic only, defensive where it avoids the model only, and inviting
where it approaches both model and mimic.
Remarkably, all nine cases of plant mimicry fall into the two classes of aggressive
mimicry, antergic (VII) where the model and mimic are different species and
synergic (11) where they are the same species. In contrast, Vane-Wright describes
examples of animal mimicry in at least six of the eight classes (Table 2). This
comparison suggests that plant traits that are constraints on mimicry may inhibit
the evolution of some classes of mimicry more than others.
Table 1. Highly suspected cases of plant mimicry indicated by
Wright’s ( 1976) mimicry classification
Synergic
I.
11.
+
+
111.
+, and Vane-
Antergic
IV.
V.
VI.
VII.
+
+
VIII. Cases ofplant Mimicr)
I . Parasites, or hemi-parasites
2. Vines or epiphytes
!3. Female flowers, monoecious plants
+
+
+
+
+
4.
5.
6.
7.
8.
Female Rowers, dioecious plants
Harvested seeds
Insectivorous plmts
Nectarless flowers with pollinia
Nectarless multiple bang flowers
9. Imitation arils mid berries
PLANT MIMICRY
55
Table 2. Summary of classes of plant mimicry (from Table l ) , plant-animal
indicates highly suspected or known cases
mimicry and animal mimicry where
exist
+
Synergic
I
11.
111.
+
+
+
Type of mimicry
Antergic
IV.
+
+
+
v.
VI.
+
+
+
VII.
+
+
+
+
VIII.
1 . Plant mimicry
2. Plant-animal
mimicry
mimic)
3. Plant-animal mimicry
mimic)
4. Animal mimicry
(plant
(animal
Inviting mimicry may be difficult to evolve in plants because they are sessile.
Vane-Wright’s ( 1976) best animal examples involve the aggregation of similarly
coloured prey species or individuals which disperse when approached by a
predator to confuse it through a myriad of shifting stimuli. Clearly, the sessile
nature of plants would render useless such a defence. An example of inviting
mimicry in plants, cited by Vane-Wright (1976) and Wickler (1968) is the case of
‘useful weeds’ such as corn and oats that developed seeds and seed-dispersal
mechanisms that mimicked those of cultivated wheat and became crops
themselves. This exception is somewhat uninstructive in the context here because
originally the weeds were not useful and fit into the class of aggressive mimicry,
and after becoming useful the new crops are cultivated separately from wheat.
The autonomy of plant appendages probably precludes much selective
advantage to warning mimicry in plants. In the classical Miillerian (synergic
warning) example, the cost of educating naive predators is much less to noxious
plants than to noxious animals.
Defensive mimicry required avoidance of the mimic by a deceived operator that
in the absence of deception would approach the mimic. I suspect that the sessile
aggregated conditions of most plants and appendages provides the operator with
several options that interdict deception. First, an operator may inspect a plant
mimic throughly before acting, whereas with a mobile animal mimic the operator
must react quickly. Second, the operator may selectively and cautiously sample a
portion ofa plant mimic or its model without the dire consequences associated with
sampling some animal mimics and their models, such as wasps and bees. Third,
after inspection or sampling or both, the operator may devastate a mimic whose
parts are aggregated. Fourth, after inspection, sampling and consuming a plant
mimic’s parts, the operator may remember the plant’s location and rely less on
morphological identification for subsequent return attacks.
The same arguments about the sessile and aggregated conditions of plants
applied to defensive mimicry also apply to aggressive mimicry, but the latter is
apparently more common in plants. A distinguishing feature is the mimic’s relative
benefit of deception and cost of detection in defensive versus aggressive mimicry.
In the former case both the benefit ofdeception and the cost ofdetection are great,
56
G . B. WILLIAMSON
and in the latter case the benefit of deception is great but the cost of detection is
little or none. This distinction is important since deception is never complete and
mistakes are part of mimicry systems. Furthermore, naive operators devoid of
experience with models or mimics are produced through operator reproduction
and continually impose the cost of detection. Naive operators will impose a high
cost to mimics in defensive mimicry but no cost to aggressive mimics.
SUMM.4RY
Mimicry reviews and classifications often cite nearly all the potential mimicry
systems known (Wickler, 1968; Vane-Wright, 1976; Wiens, 1978). In contrast, I
have limited plant mimicry to cases where I believe evidence has accumulated in
support of the mimicry hypothesis because my purpose is to delimit the conditions
ofknown plant mimicry. This approach concludes that plant mimicry, as presently
known, is usually aggressive mimicry. Wiens (1978) has noted that plant mimicry
is largely unstudied. I heartily concur and suggest furthermore that future
investigations in plant mimicry give special attention to the class of aggressive
mimicry. The preponderance of animal mimicry is warning or defensive, so our
knowledge of animals may have channelled prejudicially the search for plant
mimicry into these classes (Ford, 1971 ;Wiens, 1978). Aggressive mimicry occurs in
animals, but not commonly. It usually involves a resting predator or parasite
luring prey items to it (Vane-Wright, 1976) and this temporary resting is obviously
analogous to the sessile condition of plants.
There are many more cases of plant-animal mimicry than plant mimicry and
the former include a wider variety of mimicry classes (Vane-Wright, 1976).
Plant-animal systems suffer some, but not all, of the constraints of plant mimicry.
Plant-animal mimicry involving a plant mimic of an animal model suffers the
constraint of sessile, aggregated mimics with autonomous appendages. However,
the mobility of the animal model and its possible association with the plant mimic
may cause the operator’s experience to be a random or alternating sequence of
models and mimics and thereby circumvent the sessile, aggregated constraint.
Plant mimics of animals are quite common as effective pollination mechanisms :
flower mimics of female Hymenoptera to attract males, flower mimics of flies,
beetles and butterflies to attract the models, and flower mimics of animal products
such as carrion or blood to attract flies and beetles (Wickler, 1968; Wiens, 1978).
These flower mimics include the best known examples of chemically mediated
mimicry through imitation odours of urine, dung, carrion and sex attractants.
Where the floral mimic actually includes a reward, the mimicry is usually synergic
inviting; otherwise, it is antergic aggressive (Table 2). An unusual case of a plant
mimic of an animal model is the dummy eggs produced by some PussiJloru species
to discourage oviposition by Heliconius females who normally lay only on shoots
without eggs in order to prevent cannibalism (Gilbert, 1975). These egg dummies
are an example of antergic defensive mimicry (Table 2).
Plant-animal mimicry systems that involve an animal mimic of a plant may be
restricted less by the sessile, aggregated constraint and not at all by the
autonomous appendage constraint of plant mimicry systems because the mimic is
the animal. Mantids which are brightly coloured mimics of flowers and capture
pollinators which they attract (Wickler, 1968) are an example of antergic
aggressive mimicry (Table 2). Sucking herbivores (Hemiptera and Homoptcra)
PLANT M I M I C R Y
57
that resemble plant thorns or flowers and so arrange themselves upon the stems on
which they feed (Wickler, 1968) are representatives of antergic defensive mimicry.
Beyond these two classes, the millions of known resemblances of animals to plants
fall under Vane-Wright’s (1980) definition of crypsis.
This brief characterization of plant-animal mimicry is presented to suggest that
i t is intermediate both in occurrence and in variety between animal mimicry and
plant mimicry (Table 2).
Finally, while present knowledge of plant mimicry suggests it is limited to
aggressive mimicry and many undiscovered examples of plant mimicry may fall
into this class, other classes of plant mimicry should not be ignored. Investigations
of potential plant mimicry which is inviting, defensive or warning ought to involve
thorough documentation and experimental testing of the mimicry hypothesis.
Such research, whether proof of mimicry is positive or negative, would serve to
define more clearly the conditions for plant mimicry. For example, tests of
potential cases of inviting mimicry are needed to determine if floral convergence
associated with pollination syndromes involves mimicry. In such cases, to merely
cite evidence of floral similarity and pollinator sharing is inadequate. Macior
(1971) has argued that pollinator sharing where plants are rare may lead to floral
mimicry, and Schemske (1981) recently documented such a possibility in Costus
species, although experimental evidence is sorely needed.
ACKNOWLEDGEMENTS
Edwin M. Black commented critically on an earlier draft of the manuscript.
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