We found a SMPE for two life history traits (survival until emergence and emergence success) that was similar across both temperatures supporting the first hypothesis, but not the second hypothesis. In addition, we detected a trend for SMPE for one physiological trait (protein content) but only at the high temperature matching our second hypothesis. Expression of other life history traits were in most cases affected by warming and predator cues, but for these traits we did not find SMPEs. The other physiological traits that we quantified showed fixed expressions across treatments, indicating that life history and physiological traits were to some extent decoupled in the study system.
Consistent with SMPE patterns, survival and emergence success were highest in larvae in the E + L group and lowest in those of the L + E group. These results support previous ones 9,18,29,79,80 and confirm that intraspecific competition, including cannibalism, benefits larger individuals. In addition, early-hatched larvae in mixed phenology groups may have benefited from an increased accessibility to food due to a reduced number of competitors in the containers, i.e. thinning effects 81. Contrary to the second and third predictions, neither temperature nor predator cues affected the strength of SMPEs in survival and emergence success. We suggest that the impact of these two factors were offset by antagonistic larval interactions. Antagonistic interactions can change larval behavior to avoid predation 32. Previous studies showed that life history traits in damselflies are altered by non-consumptive predator stress 82,53,83,33,64,84 and temperature 85,86,53. But in these studies, the focus was on predator stress on the egg stage or on individually reared larvae, thereby precluding cannibalism. Our current results add that SMPEs in key life history traits can affect population size, but that the strength of SMPEs is weakly altered by other environmental factors.
Despite the highest survival until successful emergence of E + L individuals, we did not detect SMPE in development rate. This result does not support previous results in other ectotherms, including the damselfly L. sponsa, which showed SMPE in development time as well as in other life history traits 9,12,87. In the case of L. sponsa, early larvae from mixed groups had the shortest development times 9. Therefore, we suggest that the differences could be caused by different life cycle characteristics. L. sponsa overwinters in the egg stage and is a strictly univoltine damselfly, while I. elegans overwinters in the larval stage and has a variable voltinism – with uni- and bivoltine life cycles in the study region 61,66. These life cycle characteristics influence larval behavior, life history and physiology 88. As species under high time constraints usually grow and develop faster 89,90, a strictly univoltine species, L. sponsa, due to its short larval period after wintering in the egg stage is under higher pressure for rapid growth compared to the univoltine I. elegans, which spends winter in the larval stage. However, a fraction of the I. elegans population may complete a second generation within the season (this as a result of cohort splitting, resulting in univoltine and bivoltine fractions), hence proceeds for direct larval development and emergence with no overwintering stage, and therefore is likely more time constrained (but still less than egg-overwintering L. sponsa), than the larval overwintering univoltine fraction 88. Hence, the bivoltine fraction is likely more prone for SMPE. In the experiment, all phenology groups reared at the high temperature finished their larval development and emerged within 100 days (Fig. 2B). This fits the time necessary for the bivoltine fraction to finish its second generation within a season, especially in high temperature conditions, as recorded in local populations of I. elegans 66,91. More studies, preferably in (semi)natural thermo-photoperiod conditions, allowing larval direct development until emergence (bivoltine) and larval overwintering (univoltine) are needed to clarify the effect of within population variation in voltinism on SMPE in damselflies.
Early hatched I. elegans from both mixed- and non-mixed phenology groups had shorter development times than late hatchlings. Shorter development times in early hatchlings were accompanied with elevated growth rates, and this led early hatchlings to reach similar mass at emergence as late hatchlings. Hence, there was apparently no trade-off between age and mass at emergence, which is often reported in ectotherms 92–96, but see 97. These plastic life history responses of early hatchlings may be adaptive. Early emerged individuals mature early in the season and have higher mating success than delayed ones 98. Usually there is also a positive association between adult mass and components of mating success 99,100. Additionally, we did not detect mortality costs of fast development rate, which is often reported 101–103. Early emergence is likely adaptive by allowing the completion of an extra generation within a year, i.e. bivoltinism, especially when temperature conditions are permissive 61,66,104. However, selection for early emergence is probably relaxed because of highly unsynchronized mating over the flight season in I. elegans 105. Contrary, in damselflies with synchronized, early season emergence and mating such as Coenagrion armatum106, Coenagrion hastulatum or Coenagrion puella 107 selection for early date emergence, fast development and early maturation is likely strong. Finally, maternal effects could have also played a role in shaping the faster development of early vs late offspring. It has been reported that when mothers age, their condition may drop and the offspring quality decrease 108–110. A higher quality of early hatchers could positively affect their development rate and decrease age at emergence with no trade-off between these two traits. The few studies that focused on maternal effects in damselflies reported weak or no impacts of the mother on her offspring quality 72,111,112. In the current study we could not determine if maternal effects had an impact on life history and physiology traits, but it is worth investigating in the future.
We found ecologically important temperature effects on life history traits which did not show SMPEs. As expected, the high temperature decreased development time, and the shorter development time resulted in a lower mass at emergence. This elevated temperature-driven trade-off was somewhat reduced by increased growth rates at the high temperature, yet, the increase of growth rate was not strong enough to fully compensate the shorter development time. A similar incomplete compensating mechanism under warming was shown in previous studies, including studies on damselflies 113 and is considered one major mechanism for the here observed temperature-size rule where animals get smaller at higher temperatures 114.
It has been demonstrated that non-consumptive predator effects can change prey life history traits 32,115,116, and could therefore potentially weaken or remove SMPE in prey, by, for example, reduced foraging rate in prey due to predator avoidance 117–119. Here, we show that predator cues affected damselfly life history, but without having an effect on SMPEs. Specifically, predator cues reduced larval growth rate, leading to a lower mass at emergence, but only in the high temperature treatment (predator cue × temperature interaction for growth and mass). This suggests that the expected temperature rise will likely increase non-consumptive predator stress in I. elegans, with potentially negative fitness consequences. Similar results were shown in previous studies on other ectotherms 45,46, including a damselfly 120. These results could be explained as follows: predator stress increases physiological stress in prey, causing more energy to be allocated to costly defence mechanism rather than growth rate 121. Taken together, current and previous results indicate that warming temperature may magnify the effects of predator-induced stress in prey, but that the increased predation stress may not affect SMPEs in prey.
The increased temperature lead to a weak SMPE in protein content (phenology × temperature interaction, p = 0.074), a fundamental component of various body structures, including muscles 122, whereby the early larvae in the mixed group had a higher protein content under warming. This matched our second prediction of SMPEs being stronger or more likely at the high temperature. SMPE may be more likely under warming because a higher metabolism allows faster and more pronounced reactions to interactions between organisms, as well as the latter being stronger in general. In cannibalistic species, increased interactions result in higher cannibalism rates 7,123. As conspecifics represent a rich source of proteins for cannibals 124, increased cannibalism may lead to a higher protein content which can have positive effects on body condition during the larval stage 125,126. As proteins make up an important part of the swim muscles in damselfly larvae, it may contribute to a better predator escape performance. Furthermore, this may generate positive carry-over effects across metamorphosis in the adult stage. For instance, proteins play an important role in ensuring proper wing elasticity, and as building blocks of flight muscles and the exoskeleton 127–129. Intriguingly, the increased protein content under warming was not traded off against a faster growth rate, as it happened in body mass. This suggests that the larvae invest more energy into proteins than into other traits shaping final body size. It would be interesting to study in detail into which tissues the early hatched individuals invested more in the context of SMPEs.
We did not detect SMPEs in immune function (PO activity) and energy storage (fat content). These traits had similar values across all experimental treatments, suggesting fixed responses. These results are surprising because previous studies showed that PO activity and fat storage increased under warming, and decrease under predator pressure but, again, when larvae were reared individually 50,130. That the physiological traits did not follow the SMPE hypothesis confirms previous results in L. sponsa 29. Yet, in the latter species trait values showed plastic responses when individuals were exposed to time stressed conditions: PO activity decreased and fat content increased 29. In the current experiment we did not impose time stress, but it would be interesting to study this stress on SMPE in I. elegans and link it with variable voltinism in this damselfly.
In summary, our results confirm that SMPEs caused by differences in hatching phenology are an important factor that by shaping survival and emergence success can promote early emergence of amphibious and cannibalistic organisms in a population. Other central findings of current study were that warming and non-consumptive effects imposed by a top predator did not affect SMPE for life history traits, yet warming did generate a weak SMPE for larval protein content that may adaptively carry over to the adult stage. In agreement with theory 20, and current results, we suggest that given the high tendency for larval cannibalism, SMPEs in I. elegans could lead to directional selection for early adult breeding.