Reproductive Investment In Response Research Articles

COSTS OF AN IMMUNE CHALLENGE AND TERMINAL INVESTMENT IN A LONG-LIVED BIRD

An induced immune challenge can have two counteracting effects on an individual's reproductive investment. (1) The resource demand could increase to "fuel" the immunologic reaction, which in turn can lead to an adaptive decrease in investment in resource-costly activities, such as reproduction. One the other hand, (2) the individual could assume that the immune activity it experiences is indicative of a serious infection. The latter can lead to an adaptive increase in reproductive investment in response to the reduced prospects of survival and future reproduction, so called "terminal investment." To measure such life-history-related consequences of increased immune activity, one group of incubating female Common Eiders (Somateria mollissima) was injected with a nonpathogenic antigen (sheep red blood cells, SRBC) while controls were injected with sterile saline. The eider is a long-lived sea-duck. Females, who incubate the eggs and care for young without assistance from the male, engage in facultative anorexia during incubation leading to a large reduction in body mass. Eiders can abandon their young to other females at the cost of reduced young survival. The immune challenge resulted in a larger mass loss, a prolonged incubation period, and reduced return rate, demonstrating both short- and long-term costs of immune challenge. Additionally, in response to what might have been interpreted as reduced survival chances in immune-challenged females, these females more often tended their own brood after hatching, despite having suffered higher costs during incubation.

Perceived risk of ectoparasitism reduces primary reproductive investment in tree swallows Tachycineta bicolor

Many birds have been shown to reduce their reproductive investment in response to infestation of nest sites by ectoparasites. Nest‐dwelling parasite populations increase throughout the breeding season, and can reduce the condition and future survival of both breeding adults and their offspring. Thus, avian hosts should be capable of assessing early cues that predict future ectoparasitism risk, and should be able to facultatively adjust their primary reproductive investment in response to anticipated future costs of parasites. We tested this hypothesis in the tree swallow Tachycineta bicolor, a cavity nesting passerine, by presenting a visual cue of avian fleas on the outer surface of nest boxes. This treatment manipulated perceived ectoparasitism risk without exposing birds to parasites, thereby allowing us to examine facultative responses in the absence of early physiological effects of parasites on female reproductive investment. During one of the study years, birds preferentially occupied control boxes, however, across both years, birds nesting in treatment boxes produced significantly smaller clutches, resulting in smaller broods at hatching, relative to those in control boxes. This difference in clutch size could not be explained by differences in phenotypic quality of breeding birds, indicating that for cavity nesting birds such as tree swallows, the perception of future ectoparasitism risk may be sufficient to induce a facultative reduction in reproductive investment early in the breeding season, before nest‐dwelling parasite populations have grown very large.

Interpopulation Variation in Developmental Titers of Vitellogenin, but Not Storage Proteins, in Lubber Grasshoppers

We examined simultaneous plastic and latitudinal interpopulation variation in the time course of hemolymph protein titers during egg production in the lubber grasshopper. Our goal was to gain insight into possible evolutionary changes in the physiology underlying reproductive plasticity. We used lubbers from three locations in the United States (Florida [FL], Louisiana [LA], and Georgia [GA]), each offered three daily food rations. Previous genetic analysis indicated that grasshoppers from FL (the low-latitude population) and GA (the high-latitude population) were phylogenetically closer to each other than to LA grasshoppers (the intermediate-latitude population). The ages at maximum titers of vitellogenin (Vg(max)) and three storage proteins that were referred to as major hemolymph proteins (MHP(max)) were used as indices of the progress of oocyte development. Age at Vg(max) was affected significantly both by diet and by population. Perhaps most importantly, age at Vg(max) was less for GA grasshoppers than for FL and LA grasshoppers; this pattern differs from the phylogenetic relationships of the populations. Age at MHP(max) was significantly affected only by diet and not by population. Hence, the regulation of these proteins may differ across populations. Finally, we found no evidence that plasticity of reproductive investment in response to food availability differs across populations (as indicated by nonsignificant interactions of population and feeding environment).

SEVERE DEFOLIATION OF SCOTS PINE REDUCES REPRODUCTIVE INVESTMENT BY ECTOMYCORRHIZAL SYMBIONTS

Reduction in the photosynthetic capacity of plants is presumed to negatively affect their fungal symbionts. To test this hypothesis under natural conditions, we artificially removed 100% of previous year needles in two successive years on Scots pine trees (Pinus sylvestris L.) to simulate pine sawfly attack. Despite a decline in the shoot growth of defoliated trees, root biomass did not differ from control trees. The ergosterol (fungal biomass) and starch concentration of fine roots, however, slightly declined in defoliated trees. Percent ectomycorrhizal colonization of fine root tips remained high in both defoliated and control trees. The dominant tubercle morphotypes were slightly more abundant in the control than in defoliated trees. In contrast to the relatively weak effects on vegetative ectomycorrhizae, reproduction declined near the defoliated pines. Average sporocarp numbers and, consequently, the relative fungal investment to reproduction of the estimated total fungal biomass were more than three times higher near controls than defoliated trees in the first treatment year. Defoliation also reduced the diversity of ectomycorrhizal species producing sporocarps. Mutualistic fungal symbionts may thus alter their reproductive investment in response to restrictions on host resources. Because fungal biomass in the roots as well as colonization percentage remained unchanged, Scots pine evidently continues to invest in the maintenance of the symbiosis despite the reduced photosynthetic capacity due to defoliation.

Reproductive Responses to Photoperiod by a South Florida Population of the GrasshopperRomalea microptera(Orthoptera: Romaleidae)

Reproductive tactics of some insects are affected by photoperiod because daylength is a reliable cue indicating seasonal change in environmental suitability. We tested the hypothesis that late-season (short-day) photoperiod affects the length of the oviposition cycle of the eastern lubber grasshopper, which is from a subtropical area with a winter dry season. We predicted that oviposition would occur sooner in short-day (11.5:12.5 [L:D] h) than in long-day (13.75:10.25 [L:D] h) individuals, and that costs associated with earlier oviposition would be expressed as smaller eggs or fewer eggs per female under the short-day photoperiod. Two diets (low and high food quantity) were tested at each photoperiod. Although MANOVA yielded no significant main effect of photoperiod on the timing of oviposition, number of eggs, and mean egg dry mass, there was a significant multivariate interaction between photoperiod and food for these variables. In the low food treatment, the short-day animals produced fewer, smaller eggs than did the long-day animals, but this multivariate difference between short-day and long-day animals was absent in the high-food treatment. Photoperiod and its interaction with food did not affect timing of oviposition in either multivariate or univariate analyses. The absence of the predicted change in timing of reproduction in an autumn photoperiod suggests that seasonal constraints on reproduction do not exert a strong influence on reproductive timing of this population. Our alternative hypothesis is that the combination of short days and low food availability serve as a cue to lubber grasshoppers of an oncoming dry season with potentially limiting food, and that they respond to this cue by reducing the mass of reproductive output but not by accelerating reproductive timing. Thus, we propose that these subtropical grasshoppers adjust reproductive investment in response to seasonal changes in resource availability.