Paul V. Switzer

Site fidelity in predictable and unpredictable habitats

SummarySite fidelity, the tendency to return to a previously occupied location, has been observed in numerous species belonging to at least three phyla. In this paper I develop a general model using dynamic programming to investigate conditions under which fidelity to a previously occupied territory will be advantageous. The results predict that site fidelity should be inversely related to heterogeneity in territory quality and the animals lifespan and positively related to the cost of changing territories, age and probability of mortality in the habitat. The predictability of reproductive outcome (defined as the probability that next periods outcome will be the same as this periods outcome) also affects site fidelity. In predictable habitats, changing territories may be favoured after a bad previous outcome. In contrast, settlement should be independent of the previous outcome in unpredictable habitats. Individuals should also be site-faithful in unpredictable habitats, as long as the mean territory quality is equal among available territories. I also investigate the success of two potential decision rules (‘always stay’ and ‘win-stay: lose-switch’) relative to the optimal settlement strategy. The results show that these rules may perform as well as the optimal strategy under certain conditions. The always stay strategy does well in unpredictable habitats, when the mean quality within a territory is equal among territories. In contrast, the win-stay: lose-switch strategy performs best in predictable habitats.

Past reproductive success affects future habitat selection

Abstract Correlational studies have shown that an individuals past reproductive success often increases its breeding site fidelity (i.e., the tendency to return to a previously occupied location), suggesting that individuals use their reproductive experience to assess habitat quality. However, the causality of the relationship between reproductive success and site fidelity is still uncertain. In a field experiment, the effect of mating success on site fidelity was isolated from potential confounding variables in a territorial dragonfly, the eastern amberwing (Perithemis tenera). The experiment controlled for site quality, intrinsic characteristics of males, previous territorial experience at the site, arrival order, and territorial evictions. Males that were prevented from mating were much more likely to change sites the following day than control males that were allowed to mate. This result was not affected by age, the amount of time a male spent on the site, or mortality. These results imply that individuals use their own reproductive success to assess the quality of the habitat. The benefit to an individual of using its reproductive success to determine habitat quality is discussed relative to other sources of information.

Olfactory function in apoE knockout mice.

Apolipoprotein E (apoE), a lipid transporting protein, has been shown to play a vital role in nerve repair and remodeling. Since the olfactory system is in a continuous state of remodeling, the present study tested the hypothesis that apoE is required for normal functioning of the olfactory system. Olfactory behavior of wild-type (WT) and apoE-deficient (apoE KO) mice was assessed by using three standard olfactory tests: (1) the buried food pellet (BFP) test; (2) the odor choice (OC) test; and (3) the odor cued taste avoidance (OCTA) test. ApoE KO mice performed poorly in all the three tests as compared to WT mice, although they learned the tasks at a rate comparable to WT mice. ApoE KO mice had a significantly longer latency to find the buried pellet than WT mice. In the OC experiment, apoE KO mice did not differentiate water from an odorant solution. Furthermore, in the OCTA test the apoE KO mice were significantly less successful than WT mice at avoiding water containing an odorant and a bad tastant. These data demonstrate that apoE deficiency in apoE KO mice leads to a deficit in olfactory function, suggesting an important role for apoE in the olfactory system.

ECOLOGICAL GAMES IN SPACE AND TIME: THE DISTRIBUTION AND ABUNDANCE OF ANTARCTIC KRILL AND PENGUINS

The distribution and abundance of organisms are affected by behaviors, such as habitat selection, foraging, and reproduction. These behaviors are driven by interactions within and between species, environmental conditions, and the biology of the species in- volved. Although extensive theoretical work has explored predator-prey dynamics, these models have not considered the impact of behavioral plasticity and life-history trade-offs on predicted patterns. We apply a modeling method that allows the consideration of a spatial, dynamic ecological game between predators and prey using a life-history perspec- tive. As an illustrative example, we model the habitat selection of Antarctic krill and penguins during the time when penguins are land-based for reproduction. Although envi- ronmental conditions and the life-history constraints of each species have both direct and indirect effects on both species, the penguins foraging rule (whether food-maximizing or time-minimizing) has the greatest effect on the qualitative distribution pattern of both species. Size-dependent diel vertical migration of krill also strongly affects penguin foraging patterns. This model generates suggestions for future research and qualitative predictions that can be tested in the field. The application of this method to a specific problem also demonstrates its ability to increase our understanding of important ecological interactions

When should a territory resident attack

Models of territorial defence tend to omit two characteristics of many territorial systems: repeated intrusions by the same individual and the learning processes of residents and intruders. Here we present state-dependent, dynamic models of feeding territories, designed to investigate temporal patterns of resident aggression towards intruders that are capable of spatial learning. We compare two types of models: (1) a nomadic intruder model, in which intruders never visit the same territory twice, and (2) a single, repeat intruder model, in which an intruder may repeatedly intrude into a given territory but is less likely to do so after being attacked. These two models produce qualitatively and quantitatively different patterns of aggression by residents. For instance, residents with intruders that may repeatedly intrude have high initial attack rates, regardless of initial probability of intrusion, but their attack rates decline over time. In contrast, residents in the nomadic intruder model do not attack intruders if intrusion rates are moderately high, and their attack rates are constant and high for most of the period of territory tenure. In addition, residents of both nomadic and repeat intruder scenarios stopped attacking intruders for a short period before voluntarily abandoning their feeding territories. The results of our models suggest that repeated intrusions and learning processes have a dramatic effect on territorial defence.

Pursuit of Heterospecific Targets by Territorial Amberwing Dragonflies (Perithemis tenera Say): A Case of Mistaken Identity

Although they are defending mating territories, territory residents of a wide variety of insect species have been observed to pursue heterospecifics in addition to the conspecifics that intrude on their territories. One species that has such heterospecific pursuits is the Eastern amberwing dragonfly (Perithemis tenera Say) (Anisoptera: Libellulidae). In this study, we tested five alternative hypotheses for the function of heterospecific pursuits in amberwings: competition for resources, prevention of interference while mating, predator deterrence, foraging, and mistaken identity. Resident males pursued both male and female conspecifics, as well as a species of horse fly (Tabanus spp.) and butterfly (Ancyloxypha numitor). Other intruding odonates, including Epitheca princeps, Erythemis simplicicollis, Libellula luctuosa, Pachydiplax longipennis, and Plathemis lydia, were relatively ignored. Because the horse fly and butterfly were similar to amberwings in body size, color, and flight height, and because they are not predators or prey of amberwings, we concluded that the pursuit of these heterospecifics was due to mistaken identity. The characteristics of the horsefly and butterfly likely correspond to the cues that the male amberwings use to identify conspecifics, and the relative rarity of intrusions by these two species (as well as by female amberwings) probably made it more costly to discriminate and pursue only conspecifics than to make some mistaken pursuits.

Effects of environmental and social conditions on homosexual pairing in the Japanese beetle (Popillia japonica Newman)

Homosexual pairing between males occurs under natural conditions in a wide variety of taxa, including many insect species, but few studies have investigated how environmental and social conditions affect same-sex pairing in insects. We investigated factors affecting homosexual pairing in male Japanese beetles (Popillia japonica Newman) in the field and in the laboratory. Specifically, we investigated how time of day, sex ratio, beetle density, and temperature affected the likelihood of homosexual pairing. In the field, male–male pairs constituted 1–6% of the pairs we collected. Homosexual pairs were more common in the afternoon than in the morning and the evening. Sex ratio, density, and temperature were all related to the likelihood of finding a homosexual pair, but the relationships were not linear. In the laboratory, higher male densities and relatively male-biased sex ratios were associated with an increase in the frequency of homosexual pairs. Homosexual pairs were more frequent at relatively low and relatively high temperatures. Males that mounted other males tended to be smaller than the males that they mounted. In addition, compared to males that were not homosexually paired, there was some indication that the mounting males were smaller, and the mounted males larger, than the unpaired males. Our data suggest that homosexual pairs are a result of males mistaking other males for females, and we hypothesize that the environmental and social factors cause changes in homosexual pairing through their effects on the frequency of pair formation and pair duration.

A model at the level of the foraging trip for the indirect effects of krill (Euphausia superba) fisheries on krill predators

We present a model at the level of the foraging trip for the effects of a fishery on krill (Euphausia superba) predators, using the Adelie penguin (Pygoscelis adeliae) as a model organism. The model involves: (i) the description of the biomass and spatial distribution of krill; (ii) the effects of the fishery on the krill; (iii) the description of penguin breeding; and (iv) the indirect effects of the fishery on penguin reproduction and survival. The objective is to make relative comparisons of penguin reproductive success and adult survival in the absence or presence of a fishery. The biomass of krill appropriate for the predators (and the fishery) fluctuates from one year to the next according to an age-structured, stochastic recruitment model. Furthermore, there is a spatial-temporal structure, determined by diffusion and advection, to krill availability in relation to the location of the penguin breeding colony. Fishing is assumed to change the spatial and temporal distribution of available krill. After fledging, offspring survival depends in part upon the amount of krill delivered to them during the feeding periods. We use empirical data to estimate parental and offspring needs and a standard life history model to set the upper limits for survival. Parental survival after breeding depends upon the krill deficiency (relative to needs) that parents accumulate while feeding their young. A sensitivity analysis of the breeding model shows that the predictions are robust for parameters about which little is known, to the functional forms relating krill abundance to offspring and parent survival, and to the rules that parents use to allocate krill to their offspring. We evaluate expected reproductive success (offspring survival) and expected parental survival as functions of the amount of krill captured by the fishing fleet. Over the range of catch in our study, the reductions in reproductive success are essentially linear functions of krill catch with slope 1.5 and reductions in adult survival are also linear functions of krill catch, with but slopes less than 1. That is, reductions in reproductive success and parental survival are linear functions of krill catch, but not 1:1. The reductions in offspring and parent survival are mainly determined by how long the fishing season lasts and the capacity for harvest, rather than when fishing begins.

Male Preference for Large Females and Female Reproductive Condition in the Japanese Beetle, Popillia japonica Newman (Coleoptera: Scarabaeidae)

Abstract In the field, paired (in copula) female Japanese beetles (Popillia japonica Newman) tend to be larger than unpaired females. In this study, we investigated whether this size pattern could be explained by a male preference for large females, and whether larger females tended to have more and/or larger eggs than smaller females. In a laboratory study, both small and large males, when given a choice of a large and small female, tended to choose the large female. We dissected field-caught paired and unpaired females, measured their body size, and counted and measured their eggs. Larger females tended to have more and larger eggs than smaller females, indicating that males may benefit from choosing larger females due to the egg characteristics of these larger females. Paired females in the field were consistently larger than single females; paired females also had more and larger eggs, even when body width was statistically controlled. Thus, although body width of the female correlates with her fecundity, males may either use cues in addition to a females body width to determine her immediate fecundity or fecund females may be more available for or less resistant to male mating attempts.

Choice of Lookout Posts by Territorial Amberwing Dragonflies, Perithemis tenera (Anisoptera: Libellulidae)

Many territorial insects use specific perches, or lookout posts, from which they perceive and react to conspecific males and females. We investigated the lookout post choice of territorial male amberwing dragonflies (Perithemis tenera). An observational study indicated that males rarely perched directly at their oviposition site; rather, they perched farther out from shore than, and within 2 m from, their oviposition site. In an experimental study, we provided an array of perches at different distances from the shore and oviposition site to eliminate perch limitation as a factor in perch choice. The results of the experimental study confirmed the patterns evident in the observational study; males perched farther from shore than their oviposition site was located. Interestingly, in both the observational and the experimental study, when neighbors were close, a male perched away from his closest neighbor, which usually resulted in his oviposition site being closer to his neighbor than he was. Thus, male amberwings apparently alter their perch choice within their territories in response to the location of other males. These lookout post locations may provide the best opportunity for the territorial male to perceive passing females and intruding males, while minimizing conflict with their neighbors.