Mark A. Elgar

PREDATOR VIGILANCE AND GROUP SIZE IN MAMMALS AND BIRDS: A CRITICAL REVIEW OF THE EMPIRICAL EVIDENCE

One commonly cited benefit to animals that forage in groups is an increase in the probability of detecting a predator, and a decrease in the time spent in predator detection. A mathematical model (Pulliam 1973) predicts a negative relationship between group size and vigilance rates. Over fifty studies of birds and mammals report a negative correlation between group size and vigilance behaviour and most conclude that the relationship at least partly explains why individuals forage in groups. This review evaluates the strength of these conclusions based on their evidence. Those variables that may confound the relationship between vigilance and group size are outlined, and their control is assessed for each study. The variables I consider to be important include the density and type of food; competition between individuals; the proximity to both a safe place and the observer; the presence of predators; the visibility within the habitat; the composition of the group; the ambient temperature and the time of day. Based on these assessments, most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behaviour and group size. Nevertheless, many studies reveal interesting features of the relationship between vigilance and group size that should provide fruitful avenues for future research.

The evolution of pheromone diversity

Pheromones are chemical signals whose composition varies enormously between species. Despite pheromones being a nearly ubiquitous form of communication, particularly among insects, our understanding of how this diversity has arisen, and the processes driving the evolution of pheromones, is less developed than that for visual and auditory signals. Studies of phylogeny, genetics and ecological processes are providing new insights into the patterns, mechanisms and drivers of pheromone evolution, and there is a wealth of information now available for analysis. Future research could profitably use these data by employing phylogenetic comparative techniques to identify ecological correlates of pheromone composition. Genetic analyses are also needed to gain a clearer picture of how changes in receivers are associated with changes in the signal.

9 – Sperm Competition and Sexual Selection in Spiders and Other Arachnids

The chapter focuses on Polyandry, which sets the stage for sperm competition, and may provide female spiders with both material and genetic benefits. The material benefits include a reduction in the costs of male harassment and, perhaps, an increase in fecundity through the provision of nuptial gifts, including the body of the male. The genetic benefits, which are modified by the patterns of sperm precedence, are less apparent in females among species with a predominantly first-male sperm priority. However, for male spiders, most of the opportunities for securing paternity appear to be related to the timing of mating, physically guarding females from rival males and blocking the genital region of the female with a plug, or increasing the duration of copulation. Moreover, the latter reduces female receptivity and increases fertilization success in competition with other males, although the mechanisms remain unknown. Therefore, the studies of sperm competition also benefit by examining precedence patterns involving more than two males. The clear sexual dimorphism and often elaborate courtship behavior of many cursorial spiders provide a rich seam of model systems to investigate the evolutionary significance of both overt and cryptic female choice.

Gender Differences in Publication Output: Towards an Unbiased Metric of Research Performance

We examined the publication records of a cohort of 168 life scientists in the field of ecology and evolutionary biology to assess gender differences in research performance. Clear discrepancies in publication rate between men and women appear very early in their careers and this has consequences for the subsequent citation of their work. We show that a recently proposed index designed to rank scientists fairly is in fact strongly biased against female researchers, and advocate a modified index to assess men and women on a more equitable basis.

House sparrows establish foraging flocks by giving chirrup calls if the resources are divisible

Abstract Numerous studies have demonstrated that individuals benefit by joining existing foraging flocks. However, few have discussed mechanisms by which individuals might actively establish such flocks. This study presents experimental evidence that free-living, solitary house sparrows ( Passer domesticus L.) that discover a divisible food source usually make ‘chirrup’ calls to recruit other sparrows before foraging. However, a sparrow that locates a non-divisible resource tends not to chirrup, nor does it wait to be joined before foraging. The time for a ‘pioneer’ sparrow to be joined by another sparrow is inversely proportional to the rate of chirrup calls made by the pioneer sparrow, and the chirrup call rate per individual sparrow is inversely proportional to the flock size. Playback experiments with recorded chirrup calls confirmed that these calls alone are sufficient to attract other sparrows.

Evolutionary compromise between a few large and many small eggs : comparative evidence in teleost fish

Covariation in reproductive variables across 26 families of teleost fish is examined to investigate an evolutionary trade-off between the size and number of offspring. Clutch size is positively correlated with fish length, but there is no significant correlation between egg volume and fish length. There is a significant, negative correlation between clutch size and egg volume after removing the effects of body size, suggesting an evolutionary trade-off. This pattern is found within both freshwater and marine fish. The product of clutch size and egg volume is not correlated with either clutch size or egg-volume after removing the effects of body size

The functional significance of silk decorations of orb-web spiders: a critical review of the empirical evidence

A number of taxonomically diverse species of araneoid spiders adorn their orb‐webs with conspicuous silk structures, called decorations or stabilimenta. The function of these decorations remains controversial and several explanations have been suggested. These include:(1)stabilising and strengthening the web;(2) hiding and concealing the spider from predators;(3) preventing web damage by larger animals, such as birds;(4) increasing foraging success; or (5) providing a sunshield. Additionally, they may have no specific function and are a consequence of stress or silk regulation. This review evaluates the strength of these explanations based on the evidence. The foraging function has received most supporting evidence, derived from both correlative field studies and experimental manipulations. This contrasts with the evidence provided for other functional explanations, which have not been tested as extensively. A phylogenetic analysis of the different decoration patterns suggests that the different types of decorations are as evolutionary labile as the decorations themselves: the analysis shows little homology and numerous convergences and independent gains. Therefore, it is possible that different types of decorations have different functions, and this can only be resolved by improved species phylogenies, and a combination of experimental and ultimately comparative analyses.

Female control of paternity in the sexually cannibalistic spider Argiope keyserlingi

Sexual conflict theory predicts an antagonistic coevolution, with each sex evolving adaptations and counter–adaptations to overcome a temporary dominance of the other sex over the control of paternity. Polyandry allows sexual selection to operate after mating has commenced, with male and female interests competing for control of fertilization. There are numerous examples of male control of paternity, but few studies have unambiguously revealed female control. Attributing variance in paternity to females is often difficult since male and female influences cannot be separated unambiguously. However, we show that polyandrous female orb–web spiders Argiope keyserlingi (Araneidae) control the paternity of their offspring by adjusting the timing of sexual cannibalism. Our experiments reveal that females copulating with relatively smaller males delay sexual cannibalism, thereby prolonging the duration of copulation, and that these males consequently fertilize relatively more eggs.

The influence of ants on host plant selection by Jalmenus evagoras, a myrmecophilous lycaenid butterfly

Summary(1) Females of the myrmecophilous lycaenid butterfly, Jalmenus evagoras are far more likely to lay eggs on plants that contain their attendant ants, Iridomyrmex sp. 25 than on plants without ants, although the clutch sizes of individual egg masses laid in either situation is the same. (2) Ovipositing females respond to the presence or absence of ants before they alight on a potential food plant. Once they have landed, they are equally likely to ley eggs whether or not they encounter ants. (3) Ovipositing females prefer to lay eggs on plants that contain ant tended homopterans than on plants that contain only a few foraging ants. The presence of ant tended homopterans can act as a strong stimulus to induce females to lay eggs on plant species that differ from their original host species. (4) Ant dependent oviposition behavior has been described or suggested in 46 species of lycaenid and one riodinid. In general, the more dependent a species is upon ants for either food or protection, the more likely it is to use ants as cues in oviposition. Prominent characteristics of lycaenids that have ant dependent oviposition are described and discussed. (5) Myrmecophilous lycaenids that may use ants as cues in oviposition feed on a significantly wider range of plants than non-myrmecophilous lycaenids. Possible reasons for this pattern and its ecological significance are discussed.

Flocking and predator surveillance in house sparrows: Test of an hypothesis

The argument that individuals benefit from foraging in flocks through spending less time in predator surveillance and hence more time foraging and feeding, has been formalized in terms of a mathermatical relationship (Pulliam 1973). We have tested the model using house sparrows (Passer domesticus), and our results support its prediction of an inverse relationship between the time spent in predator surveillance and the flock size. The data did not fit the model exactly, and we attribute this to an unrealistic assumption.