Life-history Adjustments Research Articles

Experiencing El Niño conditions during early life reduces recruiting probabilities but not adult survival.

In wild long-lived animals, analysis of impacts of stressful natal conditions on adult performance has rarely embraced the entire age span, and the possibility that costs are expressed late in life has seldom been examined. Using 26 years of data from 8541 fledglings and 1310 adults of the blue-footed booby (Sula nebouxii), a marine bird that can live up to 23 years, we tested whether experiencing the warm waters and food scarcity associated with El Niño in the natal year reduces recruitment or survival over the adult lifetime. Warm water in the natal year reduced the probability of recruiting; each additional degree (°C) of water temperature meant a reduction of roughly 50% in fledglings' probability of returning to the natal colony as breeders. Warm water in the current year impacted adult survival, with greater effect at the oldest ages than during early adulthood. However, warm water in the natal year did not affect survival at any age over the adult lifespan. A previous study showed that early recruitment and widely spaced breeding allow boobies that experience warm waters in the natal year to achieve normal fledgling production over the first 10 years; our results now show that this reproductive effort incurs no survival penalty, not even late in life. This pattern is additional evidence of buffering against stressful natal conditions via life-history adjustments.

Effects of social information on life history and mating tactics of males in the orb-web spider Argiope bruennichi.

Informed mating decisions are often based on social cues providing information about prospective mating opportunities. Social information early in life can trigger developmental modifications and influence later mating decisions. A high adaptive value of such adjustments is particularly obvious in systems where potential mating rates are extremely limited and have to be carried out in a short time window. Males of the sexually cannibalistic spider Argiope bruennichi can achieve maximally two copulations which they can use for one (monogyny) or two females (bigyny). The choice between these male mating tactics should rely on female availability that males might assess through volatile sex pheromones emitted by virgin females. We predict that in response to those female cues, males of A. bruennichi should mature earlier and at a smaller body size and favor a bigynous mating tactic in comparison with controls. We sampled spiders from two areas close to the Southern and Northern species range to account for differences in mate quality and seasonality. In a fully factorial design, half of the subadult males from both areas obtained silk cues of females, while the other half remained without female exposure. Adult males were subjected to no‐choice mating tests and could either monopolize the female or leave her (bigyny). We found that Southern males matured later and at a larger size than Northern males. Regardless of their origin, males also shortened the subadult stage in response to female cues, which, however, had no effects on male body mass. Contrary to our prediction, the frequencies of mating tactics were unaffected by the treatment. We conclude that while social cues during late development elicit adaptive life history adjustments, they are less important for the adjustment of mating decisions. We suggest that male tactics mostly rely on local information at the time of mate search.

Life history adjustments to intestinal inflammation in a gut nematode.

Many parasitic nematodes establish chronic infections. This implies a finely tuned interaction with the host immune response in order to avoid infection clearance. Although a number of immune interference mechanisms have been described in nematodes, how parasites adapt to the immune environment provided by their hosts remains largely unexplored. Here, we used the gastrointestinal nematode Heligmosomoides polygyrus to investigate the plasticity of life history traits and immunomodulatory mechanisms in response to intestinal inflammation. We adopted an experimental model of induced colitis and exposed worms to intestinal inflammation at two different developmental stages (larvae and adults). We found that H. polygyrus responded to intestinal inflammation by up-regulating the expression of a candidate gene involved in the interference with the host immune response. Worms infecting mice with colitis also had better infectivity (earlier adult emergence in the intestinal lumen and higher survival) compared with worms infecting control hosts, suggesting that H. polygyrus adjusted its life history schedule in response to intestinal inflammation.

Independent life history evolution between generations of bivoltine species: a case study of cyclical parthenogenesis.

Successive generations of bi- and multivoltine species encounter differing biotic and abiotic environments intra-annually. The question of whether selection can independently adjust the relationship between body size and components of reproductive effort within successive generations in response to generation-specific environmental variation is applicable to a diversity of taxa. Herein, we develop a conceptual framework that illustrates increasingly independent life history adjustments between successive generations of taxa exhibiting complex life cycles. We apply this framework to the reproductive biology of the gall-forming insect, Belonocnema treatae (Hymenoptera: Cynipidae). This bivoltine species expresses cyclical parthenogenesis in which alternating sexual and asexual generations develop in different seasons and different environments. We tested the hypotheses that ecological divergence between the alternate generations is accompanied by generational differences in body size, egg size, and egg number and by changes in the relationships between body size and these components of reproductive effort. Increased potential reproductive effort of sexual generation B. treatae is attained by increased body size and egg number (with no trade-off between egg number and egg size) and by a significant increase in the slope of the relationship between body size and potential fecundity. These generation-specific relationships, interpreted in the context of the model framework, suggest that within each generation selection has independently molded the relationships relating body size to potential fecundity and potential reproductive effort in B. treatae. The conceptual framework is broadly applicable to comparisons involving the alternating generations of bi- and multivoltine species.

Smelling the future: subtle life-history adjustments in response to environmental conditions and perceived transmission opportunities in a trematode.

A number of parasites with complex life cycles can abbreviate their life cycles to increase the likelihood of reproducing. For example, some trematodes can facultatively skip the definitive host and produce viable eggs while still inside their intermediate host. The resulting shorter life cycle is clearly advantageous when transmission probabilities to the definitive hosts are low. Coitocaecum parvum can mature precociously (progenesis), and produce eggs by selfing inside its amphipod second intermediate host. Environmental factors such as definitive host density and water temperature influence the life-history strategy adopted by C. parvum in their crustacean host. However, it is also possible that information about transmission opportunities gathered earlier in the life cycle (i.e. by cercariae-producing sporocysts in the first intermediate host) could have priming effects on the adoption of one or the other life strategy. Here we document the effects of environmental parameters (host chemical cues and temperature) on cercarial production within snail hosts and parasite life-history strategy in the amphipod host. We found that environmental cues perceived early in life have limited priming effects on life-history strategies later in life and probably account for only a small part of the variation among conspecific parasites. External cues gathered at the metacercarial stage seem to largely override potential effects of the environmental conditions experienced by early stages of the parasite.

Cancer and life-history traits: lessons from host-parasite interactions.

Despite important differences between infectious diseases and cancers, tumour development (neoplasia) can nonetheless be closely compared to infectious disease because of the similarity of their effects on the body. On this basis, we predict that many of the life-history (LH) responses observed in the context of host-parasite interactions should also be relevant in the context of cancer. Parasites are thought to affect LH traits of their hosts because of strong selective pressures like direct and indirect mortality effects favouring, for example, early maturation and reproduction. Cancer can similarly also affect LH traits by imposing direct costs and/or indirectly by triggering plastic adjustments and evolutionary responses. Here, we discuss how and why a LH focus is a potentially productive but under-exploited research direction for cancer research, by focusing our attention on similarities between infectious disease and cancer with respect to their effects on LH traits and their evolution. We raise the possibility that LH adjustments can occur in response to cancer via maternal/paternal effects and that these changes can be heritable to (adaptively) modify the LH traits of their offspring. We conclude that LH adjustments can potentially influence the transgenerational persistence of inherited oncogenic mutations in populations.

Threat or treat: the role of fish exudates in the growth and life history of Daphnia

In kairomone studies, it is often implicitly assumed that the only effect of predator exudates on their prey is to trigger anti‐predator defenses. However, chemicals originating from fish activity may also fertilize the environment and enhance the growth of zooplankton prey by increasing bacterial food availability. It is necessary to separate these two effects in order to examine the adaptive significance of zooplankton anti‐predation defenses and the ability of the prey to benefit from fish‐related food.Here, we have employed differential filtration of media to permit assessment of these two effects on growth rate and life history adjustments in two Daphnia species that differ in body size and the ability to collect small particles (greater in small‐bodied D. cucullata than in larger D. hyalina). Filters of three mesh sizes were employed: 2 μm (most bacteria pass, but detritus is retained), 0.45 μm (standard) and 0.20 μm (most bacteria and detritus retained).The concentration of kairomones in the fish medium was assumed to be unaffected by filtration. However, the abundance of bacteria in the fish and no‐fish media was dependent on the filter mesh size. This influenced the food level and its impact on Daphnia growth rate and life history. The effect of fish‐related bacterial food was greater under low than under high algal food conditions (0.1 and 1.0 mg C/L) and more significant in D. cucullata.Use of media with a diminished bacterial abundance revealed the effect of kairomones alone as (1) reduced growth rate, size at reproduction, number of pre‐adult instars, age at maturation, level of the integrated egg and body lipids, egg volume, and (2) increased number of eggs per clutch vs. body length. These effects were equally strong in both species, despite differences in body size, and were greater at the higher algal food concentration. The findings of this study indicate the need to re‐analyze and possibly revise the results of previous kairomone studies, especially those using media filtered through mesh sizes coarser than 0.2 μm that were performed under limiting algal food conditions or with species particularly able to benefit from bacterial food.

Transgenerational effects of food availability on age at maturity and reproductive output in an asexual collembolan species

Transgenerational effects of environmental conditions can have several important ecological and evolutionary implications. We conducted a fully factorial experiment manipulating food availability across three generations in the collembolan Folsomia candida, a springtail species that inhabits soil and leaf litter environments which vary in resource availability. Maternal and grandmaternal food availability influenced age at maturity and reproductive output. These effects appear to be cumulative rather than adaptive transgenerational life-history adjustments. Such cumulative effects can profoundly influence eco-evolutionary dynamics in both stable and fluctuating environments.

AN EXPERIMENTAL MANIPULATION OF LIFE-HISTORY TRAJECTORIES AND RESISTANCE TO OXIDATIVE STRESS

Optimal investment into life-history traits depends on the environmental conditions that organisms are likely to experience during their life. Evolutionary theory tells us that optimal investment in reproduction versus maintenance is likely to shape the pattern of age-associated decline in performance, also known as aging. The currency that is traded against different vital functions is, however, still debated. Here, we took advantage of a phenotypic manipulation of individual quality in early life to explore (1) long-term consequences on life-history trajectories, and (2) the possible physiological mechanism underlying the life-history adjustments. We manipulated phenotypic quality of a cohort of captive zebra finches (Taeniopygia guttata) by assigning breeding pairs to either an enlarged or a reduced brood. Nestlings raised in enlarged broods were in poorer condition than nestlings raised in reduced broods. Interestingly, the effect of environmental conditions experienced during early life extended to the age at first reproduction. Birds from enlarged broods delayed reproduction. Birds that delayed reproduction produced less offspring but lived longer, although neither fecundity nor longevity were directly affected by the experimental brood size. Using the framework of the life-table response experiment modeling, we also explored the effect of early environmental condition on population growth rate and aging. Birds raised in reduced broods tended to have a higher population growth rate, and a steeper decrease of reproductive value with age than birds reared in enlarged broods. Metabolic resources necessary to fight off the damaging effect of reactive oxygen species (ROS) could be the mechanism underlying the observed results, as (1) birds that engaged in a higher number of breeding events had a weaker red blood cell resistance to oxidative stress, (2) red blood cell resistance to oxidative stress predicted short-term mortality (but not longevity), and (3) was related with a parabolic function to age. Overall, these results highlight that early condition can have long-term effects on life-history trajectories by affecting key life-history traits such as age at first reproduction, and suggest that the trade-off between reproduction and self-maintenance might be mediated by the cumulative deleterious effect of ROS.

Explaining variable costs of the immune response: selection for specific versus non‐specific immunity and facultative life history change

The cost of the immune response is variable and may provide a sufficient selective pressure to produce adaptations that minimise those costs under high infection prevalence. Here, using invertebrates as a model, I suggest two possible mechanisms that maintain variation in responses that covary with costs. First I propose that infection prevalence should balance costs through the selection of optimal patterns of “specific” and “non‐specific” immune pathways concomitantly expressed in the immune response. Second, I propose that life history adjustments (e.g. earlier reproduction in response to infection) could have been selected to minimise the cost of successful immune responses and consequently may result in the maintenance of costly immunity.

Rodent population cycles: life history adjustments to age-specific dispersal strategies and intrinsic time lags.

Multi-annual population cycles can be generated by life history responses to density dependent changes in adult and pre-reproductive survival. The proximate mechanism linking population dynamics and demography of cycling rodents appears to be high pre-reproductive dispersal at peak density, or during periods of population increase. This model is similar to the Chitty hypothesis which can best be viewed as a special case of demographic control on population size. Normally, this control should be selfreinforcing and lead to damped oscillations toward a stable population density. Intrinsic time lags induced by variation in the length of the breeding season modify the dependence of demography on population size, and enable the cycles to persist.