Color Changes in Old Aposematic Thorns Spines and Prickles

Among the various colorful plant/animal communication systems, adaptive color changes are known to take part in the two extensively studied gene dispersal systems: pollination and frugivory. Young and unrewarding animal-pollinated flowers and young and unripe fleshy fruits are usually green and cryptic. Flowers usually become colorful and conspicuous only towards anthesis, when they open and offer nectar and pollen as rewards to pollinators. Many flowers retain their conspicuous advertising colors until they wilt. However, many others change color after pollination (Weiss 1991, 1995 ; Weiss and Lamont 1997) . A change in flower color that occurs during an inflorescence may reduce the flower's advertising intensity, and thus its detectability by pollinators. On the other hand, retaining the coloration after pollination, or after such flowers turn unreceptive, may reduce pollinator visits to unpollinated flowers, thus diminishing the plant's reproductive success. By simultaneously reducing the reward after pollination and their attractiveness by changing their color, plants direct pollinators to unpollinated flowers within the same inflorescence or plant. Floral color change is a well-documented phenomenon in various taxa and life forms on all continents except Antarctica (Weiss 1991, 1995; Weiss and Lamont 1997; Bradshaw and Schemske 2003) . Fleshy fruits usually become colorful (yellow, pink, orange, red, brown, blue, purple and black) only toward ripening, when they become edible by lowering the content of protective, poisonous, and otherwise harmful secondary metabolites, and by increasing their sugar, protein and fat contents as well as their flavor and softness (Ridley 1930; van der Pijl 1982; Snow and Snow 1988; Willson and Whelan 1990; Schaefer and Schaefer 2007), a phenomenon that is also considered to be at least partly adaptive (Willson and Whelan 1990).

While the adaptive significance and the broad occurrence of color change in flowers (Weiss 1991, 1995) ; fruits (van der Pijl 1982; Willson and Whelan 1990) and leaves (Matile 2000; Archetti 2000; Hamilton and Brown 2001 ; Hoch et al. 2001; Lee et al. 2003; Schaefer and Wilkinson 2004; Lev-Yadun and Gould 2007) has been widely discussed, the phenomenon of color change in thorns, spines and prickles has only recently been described as being a widespread phenomenon and discussed as such (Lev-Yadun and Ne'eman 2006).

Patterns of color changes of senescent colorful aposematic thorns, spines and prickles were described in Lev-Yadun and Ne'eman (2006) ; Color changes make them less conspicuous, and they lose most or even all their aposematic character. The scale of this phenomenon on a taxon, flora, continent or global scale is still unknown. Lev-Yadun and Ne'eman (2006) emphasized that color changes in thorns, spines and prickles are not mandatory. Color changes and the aposematic character losses occur when the defended organs become less edible to large herbivores because of their increased size, mechanical rigidity or chemical defense, or when there is no need for defense. Reducing the cost of defense seems to be the reason for the ephemeral nature of the conspicuousness of plant thorns, spines and prickles (Lev-Yadun and Ne'eman 2006) ; The adaptive value may lie in reducing the investment in coloration, since a thin ephemeral coloration layer demands fewer resources. Keeping a thorn, spine, or prickle colorful for a long time is more costly, and the benefit of being aposematic is smaller in older, larger, or otherwise better protected organs. The tendency of plants to lower the cost of defense by thorns, spines and prickles is a well-known phenomenon. For instance, African acacias and other woody plants have longer thorns on the lower branches than on the higher ones (Cooper and Owen-Smith 1986; White 1988; Milewski et al. 1991; Brooks and Owen-Smith 1994; Young and Okello 1998; Gowda and Palo 2003) ; Certain trees (e.g., various citruses and palms) have large thorns or spines only when juvenile and none or fewer when mature (e.g., Kozlowski 1971; Cooper and Owen-Smith 1986; Cornett 1986; Clement and Manshardt 2000) ; Moreover, like several other types of induced defenses, thorns and spines are known to increase in size and number following herbivory (e.g., Milewski et al. 1991; Perevolotsky and Haimov 1991; Young et al. 2003). There is no theoretical difficulty in proposing that color changes in thorns, spines and prickles also reflect conservation of resources (Lev-Yadun and Ne'eman 2006). However, a simple alternative explanation exists: the thorns, spines, and prickles are colorful simply because the hard polymers composing them are colorful by nature. Lev-Yadun and Ne'eman (2006) dismissed this possibility because the thorns, spines and prickles that lose or change color remain hard and functional. The layer of coloration does not seem to have a significant, or even any, role in producing their sharpness. The broad taxonomic distribution of color changes in thorns, spines and prickles indicates that this character has evolved repeatedly and independently (convergent character) in both gymnosperms and angiosperms, probably in response to selection by visually oriented herbivores.

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