Our results show differences between black snakes from toad-exposed versus toad-naive populations both in their physiological resistance to toad toxin and in their willingness to eat toads. Importantly, both of these differences are in an adaptive direction; that is, we see an increased resistance to toxin and lowered preference for consuming toads in toad-exposed populations. These changes could be evidence of either plasticity (a change acquired within an individual snake's lifetime) or strong selection imposed by toads (with or without a response to that selection). To discriminate between these possibilities, we attempted to elicit acquired responses in toad-naive captive snakes. However, we found no evidence that black snakes can learn to avoid a toxic prey item, nor that they can acquire physiological resistance to toad toxin. Our inability to elicit acquired responses in either of these two traits suggests that the differences observed between toad-exposed and toad-naive populations are due to selection rather than phenotypic plasticity.
This interpretation of strong selection also depends, to some extent, on the degree to which our acquired resistance and learning experiments mimic reality (i.e. have we given the snakes a realistic opportunity to express plasticity?). In designing these experiments we operated under the premise that most black snakes will be unlikely to survive even a single encounter with a large toad (Phillips et al. 2003
; W. Seabrook & M. Fitzgerald 1996, unpublished work; R. Shine 2003, unpublished work). Any black snake that eats a large toad is likely to die. The window of sub-lethal toxin effect is relatively narrow (Phillips et al. 2003
), such that few toads will be large enough to cause illness but small enough to be non-lethal. Because it is unlikely, therefore, that an individual snake will have several chances to learn avoidance, our learning experiment was based on a single noxious encounter. The fact that black snakes did not learn to avoid the toxic prey is surprising in that Burghardt et al. (1973)
and Terrick et al. (1995)
elicited learnt aversion in garter snakes (Thamnophis
) after a single toxic encounter. Garter snakes are often sympatric with dangerously toxic newts and may have evolved this response to toxic prey (Brodie & Brodie 1999
). In contrast, black snakes are not known to be sympatric with any naturally occurring, dangerously toxic prey and thus may have been under little or no selection to learn avoidance. It remains possible, however, that some cue specific to toads could increase a snake's tendency to learn avoidance (much as aposematic coloration appears to enhance learned avoidance in gartersnakes: Terrick et al. 1995
). If this is the case, learning could occur but our experiment would not elicit it. Given that Australian snakes have no evolutionary history with toads or their toxins, and that toads are not aposematically coloured, it seems unlikely that a toad-specific cue would increase learning ability. Hence, given the apparent inability of naive black snakes to learn avoidance despite a near-lethal encounter, the observation of strong differences in prey preference between toad-naive and toad-exposed populations implies the impact of selection.
While most snakes probably experience only a single chance with a large toad, it is possible that they will consume several small toads with minimal ill effect and acquire an increased level of resistance through an immune or other physiological response. The acquired resistance experiment thus exposed snakes to four sub-lethal doses. One month after these dosings, toxin-exposed snakes were no better equipped to deal with toad toxin. Again, this result suggests that the differences between exposed and naive populations are the result of selection.
Given that we have selection, has there been an evolutionary response? Our data on toxin resistance suggest a steady increase in toxin resistance with time since exposure (), a pattern consistent with a response to selection. The data on prey preference, however, contained no variation within exposed populations (no toad-exposed snakes ate a toad) and so we are unable to infer a gradual shift in this trait. So, for prey preference at least, the possibility remains that exposed populations could be undergoing selection every generation, without response. Such a situation would require either zero heritability for prey preference or strong genetic constraints on prey preference because of genetic correlations with other traits (Merila et al. 2001
; Blows & Hoffmann 2005
). We consider these conditions unlikely for two reasons. First, prey preference has a highly heritable basis in garter snakes (Arnold 1981
) and so it seems likely that this trait will also have high heritability in black snakes. Certainly, our naive snakes exhibited variability in their tendency to eat toads (only half of the naive snakes consumed a toad; ), suggesting variance at this trait. Second, toads represent a very strong and completely novel selective force on black snake populations. The most general explanation for a true lack of heritability in large populations, even in multivariate trait space, is that selection has already used up most of the available variance (Fisher 1930
; Gustafsson 1986
; Merila & Sheldon 2000
; Blows & Hoffmann 2005
). The arrival of toads, however, probably represents a radical shift in selection pressure, such that equilibrial selection pressures are likely to be swamped by the new effect. Selection will be operating in a wholly new direction and any deviation from equilibrial selection is more likely to have some heritable variance with which to work.
It is important to note, however, that the shift in prey preference indicates either a congenital disposition to avoid toads or an evolved ability to learn from a single noxious encounter. The heritability of learning ability has never been measured in snakes and we detected little variation in this trait in naive populations of black snakes (although our sample sizes were small). Further work exploring the basis of the change in prey preference would be enlightening.
In light of the prey preference results (no snake from toad-exposed areas consumed a toad), it superficially seems paradoxical that we also detected evidence of selection on toxin resistance. This difference may be the result of historically strong selection when toads first arrived and toad-avoidance had yet to become fixed (or nearly fixed) in the population. Alternatively, if the prey preference result reflects an evolved ability to learn avoidance of toxic prey, there may be ongoing selection on toxin resistance. A third possibility is that spatial and temporal variation in relative prey abundances and/or levels of snake preadaptation might also lead to concurrent evolution in resistance and prey preference (Brodie & Brodie 1999
; Gomulkiewicz et al. 2000
). Clearly, we cannot discriminate between these possibilities here and future work will be necessary.
In fact, systems such as this—where a novel species strongly interacts with a native—promise to be particularly useful for examining the formation of coevolutionary interactions and geographical mosaics (Gomulkiewicz et al. 2000
; Brodie et al. 2002
). For example, our recent work documents a reduction in the relative head size of black snakes (and hence their ability to eat large prey items) as a consequence of exposure to toads (Phillips & Shine 2004
). Thus it appears that black snakes show adaptive change at multiple traits in response to the presence of toads. Given that the generation time of black snakes is approximately 3 years (Shine 1978
) and that toads have only been present in Australia for 67 years prior to our study, these adaptive changes have occurred remarkably quickly (in fewer than 23 generations). Furthermore, toads have now been shown to exhibit directional change in traits that mediate their toxicity to snakes. Both toad body size and relative toxicity appear to be decreasing as a consequence of time since colonization (Phillips & Shine 2005
). Therefore, while snakes appear to becoming better equipped to deal with toads, toads are independently becoming less dangerous to snakes. In total, these results emphasize that it is critical to consider the potential for adaptation when predicting the long-term impact of environmental change (Stockwell & Ashley 2004
). Our results also highlight the value of invasive species systems for understanding the genesis of coevolution (e.g. Brockhurst et al. 2003
; Forde et al. 2004
The current study demonstrates an adaptive response by a native species to an impact of conservation concern from an invasive. Invasive species are already on a growing list of environmental changes to which adaptive response has been demonstrated (Stockwell et al. 2003
). The challenge remains to determine which classes of environmental change encourage adaptation rather than extinction, and which species are likely to mount adaptive responses. Answering these questions will give us a truly long-term perspective with which to prioritise conservation efforts.