Jep Agrell

COUNTER-STRATEGIES TO INFANTICIDE IN MAMMALS : COSTS AND CONSEQUENCES

Infanticide, the killing of nonspecific young, has been documented in numerous species of mammals and is considered an adaptive behavioral strategy to enhance the reproductive success of the perpetrator. The potential benefits of committing infanticide for males are obtaining nutritional gain and mating partners, and for females are acquiring access to resources such as food and nest sites. Some costs are associated with committing infanticide such as additional energy expenditure, risk of injury, and exposure to predation. However, the major costs associated with infanticide are borne by the victim female and the sire male in loss of fitness. In response to this selection, males and females use a variety of counter-strategies to protect their young from infanticide. We summarize the published accounts and theory associated with infanticide and the occurrence of counter-strategies in a variety of mammalian groups in order to explain how infanticide may influence individual behavior as well as the social systems of mammals. We focus on the behavioral strategies used, primarily by females, to deter major losses in reproductive success. These strategies include aggression, female choice of dominant males, and promiscuity to confuse paternity as defense against males, and territoriality, association with kin, reproductive suppression, and reproductive synchrony as defenses against females. Male counter-strategies are less well known, but intrasexual territoriality may in part function as defense against infanticide. The costs associated with the different male and female counter-strategies are likely to vary, but may include increased energy expenditure, exposure to predators and injury for both sexes, as well as increased competition for resources, limited mate choice, and postponed reproduction for females. We propose that the occurrence of infanticide does not only have the potential to affect the behavior of individuals (e.g. aggression, spacing and mate choice), but may also have consequences for the shaping of mammalian mating systems.

Density-Related Home-Range Size and Overlap in Adult Field Voles (Microtus agrestis) in Southern Sweden

Home-range dynamics and the relation between population density, home-range size, and spacing pattern in a Microtus agrestis population were examined in a 2-year-long mark-recapture study. Home-range size varied with vole density. At high densities during the nonbreeding season, ranges were half the size of those measured at low densities. Home ranges during the breeding season were considerably larger than those established during the nonbreeding season with high densities, but did not differ in size from those in the nonbreeding season with low densities. Ranges of males generally were larger than those of females. Established voles generally stayed in the same restricted area throughout winter. At low densities, males showed a tendency to change home ranges. With onset of breeding in spring, overwintered individuals usually settled in areas adjacent to their winter ranges. Adult females became distributed according to a territorial pattern in spring, whereas ranges of males still overlapped extensively during the early part of the breeding season. During the nonbreeding season, vole ranges overlapped extensively, especially at high densities. There was, however, no evidence of group structure with aggregation behavior in any of the sexes.

CO2 and light effects on deciduous trees: growth, foliar chemistry, and insect performance

Abstract This study examined the effects of CO2 and light availability on sapling growth and foliar chemistry, and consequences for insect performance. Quaking aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), and sugar maple (Acer saccharum Marsh.) were grown in controlled environment greenhouses under ambient or elevated CO2 (38.7 and 69.6 Pa), and low or high light availability (375 and 855 μmol m−2 s−1). Because CO2 and light are both required for carbon assimilation, the levels of these two resources are expected to have strong interactive effects on tree growth and secondary metabolism. Results from this study support that prediction, indicating that the relative effect of rising atmospheric CO2 concentrations on the growth and secondary metabolism of deciduous trees may be dependent on light environment. Trees in ambient CO2-low light environments had substantial levels of phytochemicals despite low growth rates; the concept of basal secondary metabolism is proposed to explain allocation to secondary metabolites under growth-limiting conditions. Differences between CO2 and light effects on the responses of growth and secondary metabolite levels suggest that relative allocation is not dependent solely on the amount of carbon assimilated. The relative growth rates and indices of feeding efficiency for gypsy moth (Lymantria dispar L.) larvae fed foliage from the experimental treatments showed no significant interactive effects of light and CO2, although some main effects and many host species interactions were significant. Gypsy moth performance was negatively correlated with CO2- and light-induced increases in the phenolic glycoside content of aspen foliage. Insects were not strongly affected, however, by treatment differences in the nutritional and secondary chemical components of birch and maple.

Herbivore-Induced Responses in Alfalfa (Medicago sativa)

The herbivore-induced response of alfalfa (Medicago sativa) was examined through assays with Spodoptera littoralislarvae and analyses of important secondary substances. In food preference experiments, larvae preferred young undamaged alfalfa plants over plants that had been damaged by feeding larvae 5 and 7 days earlier, while no difference in feeding preferences could be detected 1, 9, and 14 days after damage. This suggests a peak in the herbivore induced resistance of alfalfa approximately one week after initial damage. The induced resistance in young plants was also shown to be systemic, while older flowering plants failed to show increased resistance after defoliation. Larvae gained weight slower and had lower pupal mass when fed damaged alfalfa than when fed undamaged alfalfa. Levels of total saponins were increased in foliage of damaged alfalfa, and detailed analyses of specific saponin components revealed doubled concentrations of 3GlcA,28AraRhaXyl medicagenate (medicagenic acid bidesmoside) and 3GlcAGalRha soyasapogenol B (soyasaponin I). Levels of the flavonoid apigenin (as free aglycone) also were increased in herbivore damaged plants. The herbivore-induced response of alfalfa was significantly weaker than that of cotton: S. littoralis larvae given a choice of undamaged cotton and undamaged alfalfa preferred to feed on cotton, whereas preferences shifted towards alfalfa when plants were damaged.

Terrestrial trophic dynamics in the Canadian Arctic

The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.

Combined effects of elevated CO2 and herbivore damage on alfalfa and cotton

We examined herbivore-induced responses of alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) under different CO2 conditions. Plants were grown under ambient (350 ppm) or elevated (700 ppm) CO2 levels, and were either damaged or undamaged by Spodoptera littoralis larvae. At harvest, growth of undamaged (control) plants was determined, and foliar chemical composition of both undamaged and damaged plants was analyzed. Cotton grew faster overall and showed a greater increase in growth in response to CO2 enrichment than did alfalfa. Elevated CO2 levels increased starch and decreased nitrogen levels in damaged alfalfa and undamaged cotton plants. Alfalfa saponin levels were significantly increased by elevated CO2 and damage. Regarding specific saponins, medicagenic acid bidesmoside (3GlcA,28AraRhaXyl medicagenate) concentrations were reduced by high CO2, whereas zanhic acid tridesmoside (3GlcGlcGlc,23Ara,28AraRhaXylApi Za) levels were unaffected by the treatments. Soyasaponin I (3GlcAGalRha soyasapogenol B) was only detected in minute amounts. Alfalfa flavonoid analyses showed that total flavonoid levels were similar between treatments, although free apigenin increased and apigenin glucoside (7-O-[2-O-feruloyl-β-D-glucuronopyranozyl (1→2)-O-β-D-glucuronopyranozyl]-4′-O-β-D-glucuronopyranozide apigenin) decreased in CO2-enriched plants. In cotton, herbivore damage increased levels of total terpenoid aldehydes, gossypol, hemigossypolone, the heliocides H1 and H4, but not H2 and H3, whereas CO2 enrichment had no effect. These results demonstrate that combined effects of CO2 and herbivore damage vary between plant species, which has implications for the competitive balance within plant communities.

Delayed density-dependence in a small-rodent population.

The role of delayed density-dependent processes in the dynamics of animal populations poses a problem for ecologists; although generally assumed important in populations that show cyclic or chaotic fluctuations, little experimental evidence for such processes exist. Through manipulation of vole densities within enclosed areas it was shown that reproduction, recruitment, and body growth rate in introduced populations were negatively affected by high previous density. In addition, female movement patterns shifted, and territoriality as well as home-range size was increased after high density. The observed changes in female spacing-behaviour suggested that negative effects of previous density were partly mediated by social interactions, and agreed with the finding that smaller (less competitive) females were the ones suffering most from increased competition. Contrary to expectations from recent work, predation could be excluded as the cause of delayed density-dependence in this study. Instead, chemical analyses of a dominating food plant suggested that herbivory at high vole-density had delayed negative effects on food quality.

PCB congeners in precipitation, wash out ratios and depositional fluxes within the Baltic Sea region, Europe

Concentrations of PCB congeners were determined in precipitation and the annual and seasonal depositional fluxes were calculated for 16 (mostly rural) stations around the Baltic Sea during 1990-1993. The concentrations of individual congeners in precipitation were found to be influenced by atmospheric concentrations of PCBs, ambient temperature, precipitation volume and physico-chemical properties of the compounds. Median levels of PCBs in precipitation differed one order of magnitude between stations. When analyzing all data together to obtain regional trends, concentrations of PCBs in precipitation decreased with increasing temperature. This relationship was the same for all stations but differed in the magnitude of the slope for individual congeners. Low chlorinated PCB congeners showed steeper slopes for the temperature relationship than did high chlorinated congeners, a result explained by high atmospheric concentrations of the low chlorinated congeners during low temperatures. Annual wash out ratios were between 31 and 72 x 103 and tended to be higher for the high chlorinated congeners. Wash out ratios decreased with increasing temperature for all congeners except PCB-33. At snow scavenging events, the wash out ratio of PCBs increased with a factor of 2. Latitudinal trends for PCB concentrations in precipitation and deposition were generally not statistically significant. The calculated yearly deposition of PCBs to the Baltic Sea was 390 and 5-18 kg for individual congeners, with PCB-138 having the highest flux. Deposition of PCB congeners varied seasonally, with a factor between 2 and 3, and was generally highest during fall. Relatively higher deposition of low chlorinated congeners compared to high chlorinated congeners was found during winter

Natal dispersal in relation to population density and sex ratio in the field vole, Microtus agrestis

SummaryIn a sample of 240 juvenile field voles 8% of the males and 22% of the females reached sexual maturity within their natal home range. Among individuals retrapped as adults, 58% of males and 23% of females had dispersed, i.e. had moved more than one home range diameter. The mean distance moved for males (58.5 m) exceeded that for females (28.6 m). Male movement distances were negatively associated with total density, and with density of adult females, but not with male density. Female movements were not related to population density. There were no relation between sex ratio and distance moved. The distribution of distances moved for both males and females fit a geometrical distribution, suggesting the importance of competitive processes.

On the evolutionary stability of female infanticide

Abstract Territoriality among female rodents may have evolved as an adaptation to intraspecific competition for resources or, alternatively, to defend pups against infanticide. In order to evaluate the latter, we analyse the conditions that allow an infanticidal strategy to invade a population of non-infanticidal females, and the circumstances under which infanticide may become an evolutionarily stable strategy (ESS). Our game theoretical analyses indicate that infanticide has to be associated with some direct (cannibalism) or indirect (reduced competition) resource benefits in order to invade a non-infanticidal population. We also expect that females will primarily kill litters of nearby neighbors, thereby removing the closest competitors while keeping costs at a low level. However, once established in a population, infanticide may be an ESS, even if females do not gain any resource benefits. This is theoretically possible if a female through infanticide can reduce the possibility that other, potentially infanticidal, females establish and/or stay close to her nest. While behavioral data indicate that these special circumstances sometimes occur, they may be too specific to apply generally to small rodents. Therefore, we expect that the evolutionary stability of infanticide often requires resource benefits, and that female infanticide in small rodents may, in fact, be a consequence rather than a cause of territoriality.