Copulatory plugs do not assure high first male fertilisation success: sperm displacement in a lizard

Sperm competition selects for opposing male defensive and offensive reproductive traits, and its outcome may be determined by the effectiveness to which one trait has evolved to out-compete the other. We tested the effectiveness of a first male plug physical interference with a second male insemination (defence) vs the effectiveness of plug and associated sperm displacement by a second male (offence) on the outcome of sperm competition in Iberian rock lizards. We conducted a double mating experiment where we compared the proportion of eggs per clutch fertilised by the same second males (against the same first males) when they copulated with females 30 min (first male plug adhered firmly inside the female cloaca) and 4 h (first male plug loosely adhered or shed from the female cloaca) after first males. We found that second males fertilised the majority of the eggs per clutch in the 30-min treatment, whereas fertilisations were equally shared between the two males in the 4-h treatment. These results show that plugs have little defensive effectiveness, and thus, do not assure high first male fertilisation success. Instead, sperm displacement appears to be associated with plug displacement. That is, because sperm embedded in first male plugs, and displaced from competition for fertilisations by second males, is expected to increase in number with decreasing time allowed for female sperm transport, second males thus enjoy higher fertilisation success. This study shows that offensive plug displacement out-competes plug defensive role in Iberian rock lizards. Moreover, it reveals sperm displacement as a novel sperm competition mechanism in reptiles.     

中国穿山甲的食性与觅食行为初步观察

通过野外观察及粪便和胃内容物分析,对中国穿山甲的食性进行了调查和研究.结果表明,中国穿山甲的觅食行为可分为6个类别,即行走、搜寻、行走/搜寻、挖掘、取食、暂停;取食地点主要发生在杂草、乔木、灌木的基部,落叶层下,倒木/枯枝、死树桩附近及白蚁巢内;以蚁类为食,且表现出明显的选择和偏爱,被拒食的蚂蚁和白蚁种数分别占83.87%(26/31)和53.85%(7/13),最喜爱的蚁类是台湾乳白蚁(Coptotermesformosanus)、黄翅大白蚁(Macrotermesbarneyi)、双齿多刺蚁(Polyrhachisdives);食谱由11种蚁类构成,占大雾岭保护区蚁类组成25%,其中蚂蚁5属5种,白蚁4属6种,夏季食谱主要由蚂蚁构成,冬季食谱主要由白蚁构成,台湾乳白蚁、黄翅大白蚁很可能是穿山甲食谱中的关键食物.表6参25     

Paying for information: partial loads in central place foragers

Information about food sources can be crucial to the success of a foraging animal. We predict that this will influence foraging decisions by group-living foragers, which may sacrifice short-term foraging efficiency to collect information more frequently. This result emerges from a model of a central-place forager that can potentially receive information on newly available superior food sources at the central place. Such foragers are expected to return early from food sources, even with just partial loads, if information about the presence of sufficiently valuable food sources is likely to become available. Returning with an incomplete load implies that the forager is at that point not achieving the maximum possible food delivery rate. However, such partial loading can be more than compensated for by an earlier exploitation of a superior food source. Our model does not assume cooperative foraging and could thus be used to investigate this effect for any social central-place forager. We illustrate the approach using numerical calculations for honeybees and leafcutter ants, which do forage cooperatively. For these examples, however, our results indicate that reducing load confers minimal benefits in terms of receiving information. Moreover, the hypothesis that foragers reduce load to give information more quickly (rather than to receive it) fits empirical data from social insects better. Thus, we can conclude that in these two cases of social-insect foraging, efficient distribution of information by successful foragers may be more important than efficient collection of information by unsuccessful ones.     

Antennal responses of the western pine beetle, <Emphasis Type="Italic">Dendroctonus brevicomis</Emphasis> (Coleoptera: Curculionidae), to stem volatiles of its primary host, <Emphasis Type="Italic">Pinus ponderosa</Emphasis>, and nine sympatric nonhost angiosperms and conifers

Summary. Stem volatile extracts from ten trees that are sympatric with the western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) were assayed by gas chromatographic-electroantennographic detection analysis (GC-EAD). The extracts were from the primary host, ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinaceae); two nonhost angiosperms, California black oak, Quercus kelloggii Newb. (Fagaceae), and quaking aspen, Populus tremuloides Michx. (Salicaceae); and seven nonhost conifers, white fir, Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. (Pinaceae), incense cedar, Calocedrus decurrens (Torr.) Florin (Cupressaceae), Sierra lodgepole pine, P. contorta murrayana Grev. & Balf. (Pinaceae), Jeffrey pine, P. jeffreyi Grev. & Balf. (Pinaceae), sugar pine, P. lambertiana Dougl. (Pinaceae), Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco (Pinaceae), and mountain hemlock, Tsuga mertensiana (Bong.) Carr. (Pinaceae). Sixty-four compounds were identified from the ten trees, 42 of which elicited antennal responses in D. brevicomis, usually in both sexes. In addition, several synthetic compounds, including a number of the antennally-active compounds from the extracted trees and some bark beetle pheromone components, elicited antennal responses in a manner similar to that observed with the extracts. Of the antennally-active compounds known to be present in trees sympatric with D. brevicomis, only geraniol was unique to its host. Four antennally-active compounds were found in the host and in other conifers; five compounds were found only in nonhost conifers; eight compounds were found in either or both of the nonhost angiosperms; eight compounds were found in either or both of the angiosperms and in nonhost conifers, but not in the host; and 19 were found in both the host and in angiosperms and/or nonhost conifers. Several bark beetle pheromone components were found in the stem volatile extracts. Conophthorin was identified from both nonhost angiosperms; exo-brevicomin was identified in A. concolor; verbenone was identified from a number of nonhost conifers; and chalcogran was identified from P. tremuloides. The number of nonhost volatile chemicals that D. brevicomis encounters and is capable of detecting, and the diversity of sources from which they emanate, highlight the complexity of the olfactory environment in which D. brevicomis forages. This provides a basis for further work related to chemically-mediated aspects of foraging in this insect and perhaps other coniferophagous bark beetles, and highlights the need to consider foraging context in the design and implementation of semiochemical-based management tactics for tree protection.     

Social foraging and dominance relationships: the effects of socially mediated interference

In socially foraging animals, it is widely acknowledged that the position of an individual within the dominance hierarchy of the group has a large effect upon its foraging behaviour and energetic intake, where the intake of subordinates can be reduced through socially mediated interference. In this paper, we explore the effects of interference upon group dynamics and individual behaviour, using a spatially explicit individual-based model. Each individual follows a simple behavioural rule based upon its energetic reserves and the actions of its neighbours (where the rule is derived from game theory models). We show that dominant individuals should have larger energetic reserves than their subordinates, and the size of this difference increases when either food is scarce, the intensity of interference suffered by the subordinates increases, or the distance over which dominant individuals affect subordinates increases. Unlike previous models, the results presented in this paper about differences in reserves are not based upon prior assumptions of the effects of social hierarchy and energetic reserves upon predation risk, and emerge through nothing more than a reduction in energetic intake by the subordinates when dominants are present. Furthermore, we show that increasing interference intensity, food availability or the distance over which dominants have an effect also causes the difference in movement between ranks to increase (where subordinates move more than dominants), and the distance over which dominants have an effect changes the size of the groups that the different ranks are found in. These results are discussed in relation to previous studies of intra- and interspecific dominance hierarchies.     

Global optimization from suboptimal parts: foraging sensu lato by leaf-cutting ants

Central-place foraging theory has been unable to explain the load selection behavior of leaf-cutting ants (Atta spp., Attini: Formicidae). We suggest that this is due to incomplete consideration of the sequence of behaviors involved in resource acquisition by these ants. Unlike most central-place foragers, leaf-cutting ants do not return to their nests with food. Instead, the leaf fragments they gather must be processed within the nest to convert them to substrate for fungal gardens. We have shown previously that leaf fragment size affects the rate of distribution and processing of leaf tissue inside laboratory nests of Atta colombica. Including these tasks in the calculation of foraging rate may help explain load selection and other features of central-place foraging by Atta colonies. Here we develop a mathematical model of the complete sequence of external and internal tasks that lead to addition of substrate to fungal gardens. Using realistic parameter values, the leaf fragment sizes predicted to maximize a colony's rate of foraging in this broad sense correspond well with the mean fragment sizes actually collected by Atta colonies in the field. The optimal fragment size for global performance in the model is below the size that would maximize the delivery rate by above-ground foragers. The globally optimal size also fails to maximize the rate of either fragment distribution or fragment processing within the nest. Our results show how maximum collective performance of an ensemble of linked tasks may require behavior that would appear suboptimal in a piecemeal analysis of tasks.     

Strike-induced chemosensory searching by a teiid lizard,the golden tegu (Tupinambis nigropunctatus)

Summary Strike-induced chemosensory searching (SICS) is experimentally demonstrated in a teiid lizard,Tupinambis nigropunctatus. SICS consists of a concurrent post-strike elevation in tongue-flick rate (PETF) and searching movements after voluntary release or escape of bitten prey or removal of prey from the predator's mouth. The results are consistent with previous data showing that PETF and/or SICS occur in all families of scleroglossan lizards and snakes and all families of actively foraging lizards yet studied. The relatively short duration of SICS (2 min) in a lizard having lingual and vomeronasal structure highly specialized for chemosensory sampling and analysis suggests that phylogenetic and ecological factors may be more important than morphology in determining the duration. The greatest known durations occur only in the presumably monophyletic clade containing varanoid lizards and snakes, all of which have highly developed chemical sampling and chemoreceptor apparatus, but in addition feed on prey that has a high probability of being relocated by prolonged scent-trailing. Because only active foragers move through the habitat while tongue-flicking and exhibit lingually mediated prey chemical discrimination, only active foragers may be expected to use SICS. SICS would appear to be useless to an ambush forager and might disrupt its defensive crypticity, rendering it more detectable to predators and prey. Therefore, it may be predicted that SICS is adaptively adjusted to foraging mode.     

Colony-level selection effects on individual and colony foraging task performance in honeybees, Apis mellifera L.

In honeybees, as in other highly eusocial species, tasks are performed by individual workers, but selection for worker task phenotypes occurs at the colony level. We investigated the effect of colony-level selection for pollen storage levels on the foraging behavior of individual honeybee foragers to determine (1) the relationship between genotype and phenotypic expression of foraging traits at the individual level and (2) how genetically based variation in worker task phenotype is integrated into colony task organization. We placed workers from lines selected at the colony level for high or low pollen stores together with hybrid workers into a common hive environment with controlled access to resources. Workers from the selected lines showed reciprocal variation in pollen and nectar collection. High-pollen-line foragers collected pollen preferentially, and low- pollen-line workers collected nectar, indicating that the two tasks covary genetically. Hybrid workers were not intermediate in phenotype, but instead showed directional dominance for nectar collection. We monitored the responses of workers from the selected strains to changes in internal (colony) and external (resource) stimulus levels for pollen foraging to measure the interaction between genotypic variation in foraging behavior and stimulus environment. Under low-stimulus conditions, the foraging group was over-represented by high-pollen-line workers. However, the evenness in distribution of the focal genetic groups increased as foraging stimuli increased. These data are consistent with a model where task choice is a consequence of genetically based response thresholds, and where genotypic diversity allows colony flexibility by providing a range of stimulus thresholds. Received: 3 May 1999 / Received in revised form: 22 December 1999 / Accepted: 23 January 2000     

The theoretical value of encounters with parasitized hosts for parasitoids

A female parasitoid searching for hosts in a patch experiences a diminishing encounter rate with unparasitized and thus suitable hosts. To use the available time most efficiently, it constantly has to decide whether to stay in the patch and continue to search for hosts or to search for and travel to another patch in the habitat. Several informational cues can be used to optimize the searching success. Theoretically, encounters with unparasitized hosts should lead to a prolonged search in a given patch if hosts are distributed contagiously. The results of empirical studies strongly support this hypothesis. However, it has, to date, not been investigated theoretically whether encounters with already parasitized hosts (which usually entail time costs) provide a parasitoid with valuable information for the optimization of its search in depletable patches, although the empirical studies concerning this question so far have produced ambiguous results. Building on recent advances in Bayesian foraging strategies, we approached this problem by modeling a priori searching strategies (which differ in the amount of information considered) and then testing them in computer simulations. By comparing the strategies, we were able to determine whether and how encounters with already parasitized hosts can yield information that can be used to enhance a parasitoid’s searching success.