Aposematic Coloration in Thorny Spiny and Prickly Plants

There are three terms for sharp defensive plant appendages: thorns, when they are made of branches; spines, when they are made of leaves; and prickles, when they are made of cortical tissues (e.g., in roses). Thorns, spines and prickles provide mechanical protection against herbivory (Janzen and Martin 1982; Janzen 1986; Tomlinson 1990; Myers and Bazely 1991; Grubb 1992; Rebollo et al. 2002) because they can wound mouths, digestive systems (Janzen and Martin 1982; Cooper and Owen-Smith 1986; Janzen 1986), and other body parts of herbivores. Thus, theoretically, once herbivores learn to identify thorns, spines and prickles (and their bright colors or associated markings should help in their recognition), they can avoid the harmful plants advertising them. The fact that thousands of thorny, spiny and prickly species have colorful and sharp defensive structures or that they are otherwise conspicuous due to their white or colorful markings somehow escaped the notice of botanists and zoologists, although cacti and other spiny taxa are found in the majority of botanical gardens.

Since what is toxic to one animal might be harmless to another (Laycock 1978; Janzen 1979; Gleadow and Woodrow 2002), chemical-based aposematism may not operate for all herbivores. For sharp defensive organs, the situation is somewhat different. There are differences in the sensitivity of herbivores to sharp objects, but even specialized mammalian herbivores like woodrats and collared peccaries, which are well adapted to deal with and exploit very spiny Opuntia plants, tend to choose the less spiny ones (Brown et al. 1972; Theimer and Bateman 1992) . The need to touch and ingest sharp objects makes all large vertebrate herbivores sensitive to such plants. Thorns, spines and prickles may therefore be more universal than poisons in relation to aposematism.

The recent proposals that thorny, spiny and prickly plants may be visually aposematic (Lev-Yadun 2001, 2003a, b, 2006b; Midgley et al. 2001; Gould 2004; Midgley 2004; Lev-Yadun and Ne'eman 2004, 2006; Rubino and McCarthy 2004; Ruxton et al. 2004; Speed and Ruxton 2005; Halpern et al. 2007a, b; Lev-Yadun and Gould 2008; Lev-Yadun and Halpern 2008) were based on the fact that thorns, spines and prickles are usually colorful or are conspicuous because they are marked by various types of associated coloration in the tissues that form them, including white markings. Similarly, it has also recently been proposed that many spiny animals have colorful spines and so they are aposematic (Ruxton et al. 2004; Inbar and Lev-Yadun 2005; Speed and Ruxton 2005), a fact that was discussed only briefly in the classic monograph by Cott (1940).

After realizing that the thorns, spines and prickles of many wild plants in Israel are usually colorful or are associated with conspicuous white or colorful markings, I decided to examine whether this principle is true in four very spiny taxa (cacti, Agave, Aloe, Euphorbia. When the examination of many species of these taxa clearly indicated that the sharp defensive appendages are usually conspicuous, I proposed that these plants are visually aposematic (Lev-Yadun 2001).

Lev-Yadun (2001) showed that two types of thorn conspicuousness are typical of many plant species: (1) colorful thorns and spines, and (2) white and colorful spots and stripes associated with thorns and spines in leaves, stems, and fruits. Both types of aposematic coloration dominate the spine systems of taxa rich in spiny species: cacti and the genera Agave, Aloe, and Euphorbia. It has been recorded in over a thousand species originating in America and Africa. The colorful spine systems are commonly multicolored (spines are brown, yellow, red, white, gray, pink, black, and tan). For instance, in cacti (the spiniest taxon), in more than 50% of the species for which there are detailed data (e.g., Benson 1982), the spines are pigmented with 3-7 colors, and 88.6% of the 973 cacti species described in Preston-Mafham and Preston-Mafham (1994) have white markings associated with their spines (Lev-Yadun 2001) . It has been proposed that conspicuous spines are beneficial for plants, since herbivorous vertebrates remember the signal and thus tend to avoid sampling these conspicuous spiny plants subsequently. Furthermore, herbivores may pass over the aposematic individuals and eat their nonaposematic neighbors, thus reducing competition between aposematic and their neighboring plants (Lev-Yadun 2001). Rubino and McCarthy (2004) tested Lev-Yadun's (2001) aposematic hypothesis by examining the presence of aposematic coloration in thorny, spiny, and prickly vascular plants of southeastern Ohio, and because of their similar field results, reached the same conclusions.

This phenomenon of aposematism in thorny, spiny and prickly plants, which seems to be veiy common, has been described and discussed at three levels: (1) the floristic approach, where it is studied across large taxa (Lev-Yadun 2001) or floras or ecologies (Lev-Yadun and Ne'eman 2004; Rubino and McCarthy 2004); (2) the individual species level (Lev-Yadun 2003a; Lev-Yadun and Ne'eman 2006; Halpern et al. 2007a, b), and; (3) mimicry of the phenomenon (Lev-Yadun 2003a, b, 2006b; Lev-Yadun and Gould 2008). Although Midgley et al. (2001) and Midgley (2004) did not use the word aposematic, they described the typical conspicuous white thorns of many African Acacia trees as visually deterring large herbivores, supporting the aposematic hypothesis. Ruxton et al. (2004) and Speed and Ruxton (2005) elaborated on the principle that, unlike poisons, aposematic thorns advertise their own dangerous quality (self-advertisement).

Lev-Yadun ;2003a) showed that the rosette and cauline leaves of the highly thorny winter annual plant species of the Asteraceae in Israel (S. marianum) resemble green zebras. The widths of typical variegation bands were measured and found to be highly correlated with leaf length, length of the longest spine at leaf margins, and the number of spines along the leaf circumference. Thus, there was a significant correlation between the spininess and strength of variegation. Lev-Yadun (2003a) proposed that this was a special case of aposematic (warning) coloration. However, additional defensive and physiological roles of the variegation, such as mimicry of the tunnels of flies belonging to the Agromyzidae, reducing the number of insects landing on the leaves in general, just as zebra stripes defend against tsetse flies (Lev-Yadun 2003a and citations therein), were also proposed.

Was this article helpful?

0 0

Post a comment