Despite the potential anti-predator benefits that could be associated with adopting TI behaviours (discussed above), there are also likely to be related costs. Some of these costs will impact evolutionary trade-offs in species, influencing variation in TI expression and its prevalence. Alternative last-ditch anti-predator behaviours, such as struggling to break free and fleeing or aggressively attacking the would-be predator, will also probably have benefits and costs that will influence their expression. Hence, we might expect the prevalence and strength of TI to reflect conditions where the cost-benefit trade-off favours TI over alternative behavioural tactics. As an example of this, Gyssels and Stoks (2005) found that TI in larvae of the damselfly Ischnura elegans was more likely to be utilised in staged laboratory encounters with predators if an alternate anti-predator defence was negated by removal of lamellae; otherwise, lamellae can be sacrificed through autotomy, as an escape strategy, when grasped by a predator.
For the adzuki bean beetle (Callosobruchus chinensis), Ohno and Miyatake (2007) argue that flying away and entering a state of TI are two alternative tactics that are used in response to predators. In their study, lines of beetles were artificially bred for long or short duration of TI. Ohno and Miyatake measured flying ability in terms of how strongly an experimentally-dropped beetle influenced its downward trajectory (i.e. how far it landed from directly below its release point) and found that those bred for long-duration TI had weaker flying ability, while the lines bred for greater flight ability showed correlated lower TI duration. The exact mechanism underlying this genetic correlation was not determined, but the authors speculated that there may be competition for resources between investment in flight muscles and in reproduction; highly-reproductive individuals may thus have poor flying ability and separately (for reasons that are unclear to us) this may relate to a finding that larger individuals show longer duration of TI. Alternatively, Ohno and Miyatake suggest that individuals that show a low propensity for TI may be more active and, thus, more likely to find a means of escape from their rearing facility. As a final suggestion, and their most plausible speculation to our minds, they suggest that TI might be indirectly selected, despite a direct cost, because it is genetically correlated with other traits that confer higher fitness. This is reminiscent of a study by Nakayama and Miyatake (2009), who found that longevity, emergence rate and egg size were all correlated with the extent of TI in the adzuki bean beetle.
Genetic factors relating to the mechanism of TI have also been shown to influence when TI will be deployed as an anti-predatory strategy over fleeing. Miyatake et al. (2008a) found that locomotion and tonic immobility are pleiotropically correlated with a genetic factor related to a biogenic amine in the red flour beetle. Walking distance was significantly lower in strains artificially selected for longer (L-strains) than shorter TI duration (S-strains), and cross-breeding experiments suggested that tonic immobility and locomotor activity have the same genetic basis (Miyatake et al. 2008a). The authors suggest that the alternative behaviours of fleeing or tonic immobility are associated with the pleiotropic effects of a neuroactive substance in this species.
In some cases, aspects of the environment will also impact the adaptive significance of TI relative to other anti-predator strategies. For example, Miyatake et al. (2008b) found that TI occurred more strongly in two species of seed beetle (Callosobruchus macalatus and C. chinesis) when individuals were kept at lower temperatures. Miyatake et al. interpreted this as a consequence of higher temperatures allowing beetles to use fleeing as an alternative anti-predator behaviour more readily. Similarly, Saxena (1957) found that woodlice (Armadillidium vulgare) showed stronger TI at lower temperatures, again presumably a condition where the alternative anti-predator tactic of fleeing was less available. Saxena also showed, though, that woodlice showed stronger TI when lighting was reduced; in this condition, vulnerability to visual predators when stationary was reduced and so remaining immobile was probably a more viable anti-predator strategy.
Further, there is evidence that TI is used flexibly according to how ecological circumstances influence its costs and benefits relative to alternative anti-predator behaviours (most obviously fleeing to a refuge). Arduino and Gould (1984) demonstrated this well in their experiments on domestic chicks. Chicks were exposed to some stimulus—either representing a poor, fair, or good opportunity to escape—for 15 s, followed by 15 s of being manually restrained while still being exposed to the stimulus. This induced TI, the chick was then released and the duration of TI was measured. Arduino and Gould found, in numerous paired comparisons, that TI lasted longer in situations where it was thought that the chick would consider the chance of escape to be lower (e.g. when the stimulus was a hawk model facing the chick), compared to cases when this chance might be considered to be higher (e.g. when the hawk model was facing away from the chick). An earlier study by Gallup et al. (1971) also found that, in young chickens, individuals immobilised by manual restraint in the presence of a hawk model exhibited longer durations of TI than chickens immobilised without the presence of the model. Santos et al. (2010) suggest that Liolaemus occipitalis lizards also evaluate predation risk in adopting anti-predator strategies as, in their study of TI experimentally induced by manual handling, TI lasted longer if the human ‘threat’ remained in close vicinity; if a predator remains close, it is less likely that fleeing would be a useful tactic.
As well as likelihood of escape, proximity to a place of safety can also influence whether animals choose to adopt a strategy of TI or attempt to flee from a predator. Ewell et al. (1981) demonstrated in a laboratory that rabbits (Oryctolagus cuniculus) decreased the duration of TI as proximity to a human decreased or as proximity to their home cage increased. Likewise, Hennig et al. (1976) found that TI was of shorter duration in Carolina anole lizards (Anolis carolinensis) that had been immobilised near large bushes than those immobilised in the absence of nearby protective cover. Further, O’Brien and Dunlap (1975) reported that blue crabs (Callinectes sapidus) immobilised on a sandy substrate (in which they could seek cover by burying themselves) exhibited shorter durations of TI than individuals similarly immobilised on a hard surface (on which burying themselves for protection was not possible).
As an important aspect of an individual’s environment, the local predatory community has also been found to serve as a key influence on the evolution and deployment of particular adaptive anti-predatory behaviours such as TI. The predatory community present can influence the anti-predator strategy deployed by prey, with the prey typically carrying out the behaviour most likely to protect it in a predator-prey interaction. Gyssels and Stoks (2005) found that TI in larvae of the damselfly Ischnura elegans was more frequently induced experimentally in those from a pond in which fish predators were present compared to those from a pond where this important predatory group was absent. As well as demonstrating a genetic correlation in the laboratory between flight ability and duration of TI discussed previously, Ohno and Miyatake (2007) also found such a negative correlation across 21 wild populations of the adzuki bean beetle. They speculate that one reason for this may be that the two alternate tactics have differential effectiveness against different predators and that the predator communities differ between populations. Additionally, or alternatively, they speculate that one tactic (perhaps flying away) may be both more effective and more costly, and populations that face lower predation pressure are selected to make more use of the cheaper but less effective tactic (TI).
As well as cost-benefit trade-offs surrounding alternative anti-predator behaviours, TI traits may impact the evolution of other important fitness traits. In some species, duration and frequency of TI have been found to negatively correlate with predation experienced, but also negatively correlate with mating success. Nakayama and Miyatake (2010a) bred lines of the red flour beetle T. casteraneum for either longer duration and higher frequency TI (labelled the L-strain) or shorter and less frequent (labelled the S-strain). They then demonstrated that males of the L-strain suffered less from predation in an experimental arena with an Adanson’s house jumping spider, but had reduced mating success in a predator-free environment. This latter effect was interpreted by the authors as individuals of the L-strain being, overall, less active and thus encountering females less frequently; a genetic trade-off clearly exists between an individual’s ability to avoid attack and find mates.
Nakayama and Miyatake (2010b) similarly bred L- and S-lines of the adzuki bean beetle and found that the L-strain had reduced activity that translated into reduced mating success in males but not females. This is probably because male mating success is more dependent on frequency of encounters and, thus, on activity levels. Activity levels might also explain the finding (Nakayama and Miyatake 2009) that L-strain individuals had higher longevity, rates of emergence, egg size and faster development. These authors suggest that reduced activity may conserve energy, increasing longevity, and allowing diversion of energy to larger eggs that take less time to develop. So, while longer and more frequent TI may reduce the risk of predation at the cost of reduced mobility and mating opportunities, the resulting energy conservation may ultimately increase individual fitness and offspring fitness.
In investigating the red flour beetle T. castaneum, Kiyotake et al. (2014) inferred some different potential costs to TI behaviours. They demonstrated that L-type beetles—selected to show higher frequencies and longer durations of TI—were significantly more sensitive to environmental stressors, such as mechanical vibration and high or low temperatures, than S-type beetles – selected to exhibit lower frequencies and shorter durations of TI. This could have evolutionary consequences for the species, with the prevalence of TI behaviour potentially being linked with exposure to stressors in wild populations. Some of the authors of this study had been involved in a previous work (Miyatake et al. 2004) demonstrating that the frequency of predation by Adanson’s house jumping spider was significantly lower amongst L-type beetles than S-type beetles. However, they also found that the wild population showed a majority of S-type individuals. This suggests that longer and more frequent TI behaviours may bring an advantage in reducing predation but that, in the wild population, being more robust to environmental stressors and changes may be an even greater advantage in terms of overall fitness, such that shorter and less frequent TI behaviours are prevalent.
As another good example of a fitness trade-off relating to TI, Kuriwada et al. (2011) found that mated sweet potato weevil females showed longer duration TI than virgin individuals. The authors interpreted this in terms of a shifting trade-off between avoiding predation and being active to search for mates. They also found that older females decreased investment in TI, which they again interpreted as reduced relative costs to predation in older individuals. Males, however, showed no change with age. Kuriwada et al. argued that there should be a strong effect of increasing age (and, thus, mortality risk from factors other than predation) in females that must not only mate, but also collect food for investment in eggs and find oviposition sites; versus males, who need only find mates. Previously, Kuriwada et al. (2009a) had found an effect of copulation in both males and females. The authors suggest that the difference between studies was that the earlier study tested TI 5 h after the treatment (copulation or control) was imposed, increasing to 30 h in the second study. The authors speculate that the long interval during which no females could be encountered may have caused perception of mating opportunity to decline in males of both treatments, making a treatment effect difficult to pick up. Miyatake (2001a) had previously found that duration of TI in the sweet potato weevil decreased with increased duration of starvation, and that this effect was stronger in males. This last result can be explained by males being more susceptible to mortality through starvation than females. Miyatake (2001b) also observed a sex-difference in TI in the sweet potato weevil. Both sexes showed a reduction in TI at night, which Miyatake suggests might be because most of their predators are visual and so predation risk is generally lower at night. The effect was particularly strong in males, and the author interpreted this as a consequence of copulations occurring at night and males having more to gain from remaining active to obtain multiple matings than females.
In their study of adzuki bean beetles, Hozumi and Miyatake (2005) found that smaller beetles showed shorter duration TI, something the authors interpreted in terms of the shorter natural lifespan of smaller individuals. As touched upon previously, Nakayama and Miyatake (2009) bred this species for either long or short duration of TI. Fecundity was the same in the two lines, but those with longer-duration TI produced bigger eggs, that developed more quickly, were more likely to hatch and produced longer-lived offspring. This was interpreted as TI requiring less energetic investment than fleeing, and the energy saved being diverted into reproduction. However, Nakayama and Miyatake note that their stocks experienced an abundance of food and potential mates. In other ecological circumstances, the energetic savings of TI may have to be traded off against the lost time that could be devoted to finding food and mating.
Fascinatingly, particular aspects of an individual’s environment and personal experiences have also been shown to influence TI variation and propensity. An interesting vertebrate study by Odén et al. (2005) shows that social environment can affect expression of TI. Odén et al. reported that female laying hens showed shorter duration TI in response to standardised human handling if they had been kept in groups with males. The authors interpreted this as males acting as guards and lowering fear levels in the females.
It may also be that experience of previous environmental conditions can influence TI, as well as the currently experienced condition. Tojo (1991) found that common cutworm (Spodoptera litera) raised in crowded conditions in their 4th and 5th instar showed reduced periods of TI in their 6th instar compared to those raised in isolation. This might be seen as an adaptive response to a trade-off between avoiding predation and competing for food resources; crowding may favour individuals that succeed in accessing the limited per capita resources available ahead of those that show extended anti-predator responses but limit their mobility. Tojo also reported variation between natural populations in duration of TI and demonstrated, in cross-breeding experiments, that this had a genetic component. In general, variation and plasticity in TI could be driven by evolution since aspects of this behaviour are heritable; recently, for example, Mignon-Grasteau et al. (2017) have shown that some TI traits (i.e., duration and number of inductions) show moderate heritability in broiler chickens (Gallus gallus domesticus).
In a study investigating the effects of human contact and intra-specific social learning on TI in guinea pigs (Cavia porcellu), de Lima Rocha et al. (2017) found that, firstly, experience of handling and habituation did not prevent TI responses in subjects. However, habituation increased the latency of TI, while handling or habituation decreased the duration of the TI behaviour. Additionally, the cohabitation of unhabituated and habituated animals was found to reduce TI duration in the unhabituated. The authors concluded that experience of human interactions can reduce experimenter fear in guinea pigs and that unhabituated guinea pigs may learn not to fear experimenters by cohabitating with habituated guinea pigs. Experience and social environment can clearly be important in determining the occurrence of and variation in TI behaviours.
Further, the complexity of how TI, as an anti-predatory behaviour, interacts with other parts of an individual’s responses to the environment is demonstrated by the study of Kuriwada et al. (2009a) on the effect of copulations in the sweet potato weevil. In both sexes, encounters with individuals of the opposite sex where copulation was experimentally prohibited did not affect duration of any subsequent TI. However, an encounter leading to successful insemination led to reduced TI length in females but not males. Regardless of whether copulation lead to insemination, females reduced TI after a single copulation, and males after multiple copulations. Kuriwada et al. interpret the behaviour of males as follows. Sex recognition ability is poor in this species and so copulation rather than simply encounters may be needed to suggest to males that there is high current availability of females. Therefore, activity to find females should be prioritised over avoiding predation for a male, since it may take them longer to find and successfully mate with females. For females, copulations may signal high male activity and thus high risk of sexual harassment. Having allowed successful sperm transfer once, a female’s interest in the attentions of further males decreases and, thus, such females will be willing to accept higher predation risk in order to remain active to avoid unwanted male attentions. This interpretation seems plausible, but further work could rule out alternative explanations (e.g. involving inseminated females increasing the priority that they give to feeding relative to avoiding predation). No matter what, this work stands as an example of how TI can be used flexibly dependending on the state of the organism and of how it can be integrated into a complex of other behavioural and life-history traits.
Interestingly, a different study by Kuriwada et al. (2009b) found that mass-rearing over 71 generations in the sweet potato weevil (Cylas formicarius) caused no change in the duration of TI. This consistency in TI traits is surprising, since there was no predation pressure during these 71 generations, but the authors speculate that the behaviour was perhaps maintained—despite the lack of predation pressure—simply because TI is not very costly. TI here did not occur at the expense of resources required for other behaviours as the mass-reared weevils were provided with abundant food and had a high chance of successfully encountering mating partners. Additionally, we would add that TI may have been triggered very infrequently in any individuals during these 71 generations, and that there may be little expressed cost in retaining the capacity to perform TI in situations where that capacity is seldom triggered. However, the constancy of TI duration found by Kuriwada et al. (2009b) might not actually be very informative considering how different the captive situation it created was from natural conditions. Kuriwada et al. (2009b) also posit that TI behaviour may have been indirectly selected via positive genetic correlation with other traits that confer higher fitness, as appeared to be the case in Nakayama and Miyatake (2009). We find it likely that TI was not simply maintained due to being an inexpensive trait, but rather due to the influence it appears to have on other fitness factors such as breeding potential, egg size, growth rate, and fecundity.
We have discussed here the many influences TI traits and expression can have on a prey individual’s anti-predator strategies, life history, and overall fitness. The predator’s side of TI is more fully explored in the next section.