Photoperiodic effects on diapause-associated gene expression trajectories in European Leptinotarsa decemlineata populations.

Behavioural and physiological changes during diapause, an important strategy of insects for surviving harsh seasonal conditions, have been intensively studied. The genetic and molecular mechanisms underpinning diapause development are less well known. We took a candidate gene approach to study prediapause gene expression patterns in the Colorado potato beetle (Leptinotarsa decemlineata), an invasive insect that has rapidly spread northwards to high seasonality environments. Newly eclosed beetles originating from southern (Italy) and northern (Russia) Europe were reared under short- [12 h light (L):12 h dark (D)] and long-day (18L:6D) photoperiods for 10 days. This time period includes the sensitive period for the photoperiodic induction and initiation of diapause. Gene expression trajectories of 12 diapause-related genes (regulatory, metabolic and stress-resistance) were analysed from 0-, 5- and 10-day-old beetles. Gene expression differences increased with age, deviating significantly between populations and photoperiods in 10-day-old beetles. The gene expression profiles, particularly those related to energy metabolism and stress-resistance, indicate that beetles originating from Russia also prepare for diapause under the long-day photoperiod and show qualitative differences in the diapausing phenotype. Our study shows that population-dependent differences seen in behavioural and physiological traits connected with diapause in L. decemlineata are also evident in the expression trajectories of diapause-related genes.

Selection for cryptic coloration in a visually heterogeneous habitat.

We studied selection by predators for cryptic prey coloration in a visually heterogeneous habitat that consists of two microhabitats. It has been suggested that the probability of escaping detection in such habitats might be optimized by maximizing crypsis in one of the microhabitats. However, a recent model indicates that a coloration that compromises the requirements of different microhabitats might sometimes be the optimal solution. To experimentally study these hypotheses, we allowed great tits (Parus major L.) to search for artificial prey items in two different microhabitats (background boards): small patterned and large patterned. On each board there was one prey item that was either small-patterned, large-patterned or medium-patterned and thus compromised. Search time was used as the measure of crypsis and was on average longer on the large-patterned than on the small-patterned background. On the small-patterned background, the small-patterned prey was more cryptic than the compromised prey, which was in turn more cryptic than the large-patterned prey. On the large-patterned background, the small-patterned prey was least cryptic, but the compromised prey did not differ significantly from the large-patterned prey. The compromised coloration had lower predation risk than the matching colorations. This indicates that in some conditions a compromised coloration might be the best strategy for the prey and has important implications for the study of animal coloration.

Predator experience on cryptic prey affects the survival of conspicuous aposematic prey.

Initially, aposematism, which is an unprofitable trait, e.g. noxiousness conspicuously advertised to predators, appears to be a paradox since conspicuousness should increase predation by naive predators. However, reluctance of predators for eating novel prey (e.g. neophobia) might balance the initial predation caused by inexperienced predators. We tested the novelty effects on initial predation and avoidance learning in two separate conspicuousness levels of aposematic prey by using a 'novel world' method. Half of the wild great tits (Parus major) were trained to eat cryptic prey prior to the introduction of an aposematic prey, which potentially creates a bias against the aposematic morph. Both prey types were equally novel for control birds and they should not have shown any biased reluctance for eating an aposematic prey. Knowledge of cryptic prey reduced the expected initial mortality of the conspicuous morph to a random level whereas control birds initially ate the conspicuous morph according to the visibility risk. Birds learned to avoid conspicuous prey in both treatments but knowledge of cryptic prey did not increase the rate of avoidance learning. Predators' knowledge of cryptic prey did not reduce the predation of the less conspicuous aposematic prey and additionally predators did not learn to avoid the less conspicuous prey. These results indicate that predator psychology, which was shown as reluctance for attacking novel conspicuous prey, might have been important in the evolution of aposematism.

Negatively condition dependent predation cost of a positively condition dependent sexual signalling.

Predation is considered as an important factor constraining the expression of sexual signals. Nevertheless, direct quantitative evidence for predation provoking significant viability costs on individuals signalling at high rates is scarce. Moreover, it is unclear whether high rate signallers are able to balance presumably increased predation costs. We examined whether a condition dependent audible sexual signal, drumming, makes Hygrolycosa rubrofasciata male spiders more prone to predation by pied flycatchers (Ficedula hypoleuca), and whether sexual signalling rate is related to escaping ability once attacked. When birds were given a choice between two spider males manipulated to drum either one or three bouts per minute using playbacks, naïve birds attacked the males randomly regardless of the drumming rate. However, experienced birds chose significantly more often the males with high signalling rate. When spiders were allowed to escape, males with high sexual signalling rate tended to be better at escaping attacks than males with low sexual signalling rate. This study provides evidence that high signalling rate increases the risk of predation, but simultaneously males with high mobility, which correlates positively with signalling rate seemed to be better at compensating this cost.

Strong antiapostatic selection against novel rare aposematic prey.

The evolution of aposematism, a phenomenon where prey species conspicuously advertise their unprofitability to predators, is puzzling. How did conspicuousness evolve, if it simultaneously increased the likelihood of an inexperienced predator to detect the prey and presumably kill it? Antiapostatic selection, where rare prey is predated relatively more often, is considered as another major difficulty for aposematism to evolve. However, the risk of being conspicuous in low frequencies has not been experimentally tested. We designed an experiment to test how frequency (4%, 12%, 32%) of conspicuous aposematic prey and its dispersion type (solitary vs. aggregated) affect an initial predation risk of the prey and in avoidance learning of predators. Wild great tits (Parus major) were predators on artificial prey in a "novel world." The relative mortality of aposematic prey was antiapostatic, thus the frequency-dependent predation was most severe at low frequencies. In all frequencies, aggregated aposematic prey survived better than solitary prey. Surprisingly, learning was not determined by a fixed number of unpalatable prey eaten, but at low frequencies fewer aposematic individuals eaten generated predators' avoidance learning. However, per-capita risk for the prey remained highest at low frequencies. Our results underscore the problems of initial evolution of rare conspicuous morphs. Aggregated prey suffered less from predation, indicating selective advantage of aggregation over solitary living for a conspicuous individual.