Effects of exposure to predatory cues on territorial behaviour of male fathead minnows

We conducted a laboratory study to determine if male fathead minnows, Pimephales promelas, altered their territorial behaviour associated with reproduction in response to combinations of visual and chemical cues from northern pike, Esox lucius. We introduced the following stimuli to a territorial male: a brick (control), fathead minnow alarm pheromone, a pike fed brook stickleback, Culea inconstans, or a pike fed fathead minnow. The territorial behaviour of males did not change when the control was added. Male minnows experiencing threat from pike fed stickleback significantly reduced the frequency at which they performed three territorial behaviours, but, within 12 h, had returned to pre-exposure activity levels. Male minnows subjected to alarm pheromone alone and to pike fed fathead minnow significantly reduced their territorial behaviour, abandoned their nests, and did not return to pre-exposure levels of activity after 24 h. We suggest that because risk of predation triggers prolonged decreases in territorial defense, it may affect competition between nesting males and female mate choice. We conclude that fathead minnows can assess the severity of predatory threat and adjust their reproductive behaviour accordingly.     

Effects of northern pike on patterns of nest use and resproductive behavior of male fathead minnows in a boreal lake

We conducted a two-part study to assets predator avoidance byreproductive male fathead minnows (Pimephales promelos) subjectedto predation threat from northern pike (Esox lucius). First,we determined if patterns of nest use by egg-guarding male minnowsin a boreal lake were related to pike densities. We samplednorthern pike and identified four areas of "high pike-density"and three areas of "low pike-density." We censused natural nestsand placed nest boards in these areas. We found eggs on naturalnests more frequently in areas with low densities of pike thanin areas with high densities of pike. However, we could notfully explain the distribution of nests by predation risk. Second,we evaluated the behavioral response of egg-guarding males toa control stimulus (a piece of wood) or a live pike in a wirecage. We used time to return to the nest after a stimulus asa measure of risk taking. Males took different amounts of riskbased on predation threat; males in the predator treatment tooklonger to return to their nests than control males. Risk takingwas not related to the number or age of the eggs but to distanceto nearest egg-guarding neighbor; males with close neighborsreturned sooner than more isolated males. Males in the predatortreatment had lower total activity and egg rubbing than controlmales after they returned to their nests. We conclude that malefathead minnows altered their reproductive behavior in waysthat reduced predation risk, but the cost of predator avoidancemight include egg predation, lost mating opportunities, or usurpationof nests     

Overriding the oddity effect in mixed-species aggregations: group choice by armored and nonarmored prey

Because "odd" individuals often suffer disproportionately highrates of predation, solitary individuals should join groupswhose members are most similar to themselves in appearance.We examined group-choice decisions by individuals in armoredand nonarmored species and predicted that either (1) the oddityeffect would result in preference for conspecific groups forsolitary individuals of both species, or (2) individuals inthe armored species would prefer to associate with groups containingindividuals of the more vulnerable species. Armored brook sticklebacks(Culaea inconstans) and nonarmored fathead minnows (Pimephalespromelas) have the same predators and often occur together instreams. In mixed-species shoals, yellow perch (Perca flavescens)attacked minnows earlier and more often than sticklebacks. Wetested whether solitary minnows and sticklebacks preferred toassociate with conspecific or heterospecific shoals under conditionsof both low and high predation risk. When predation risk washigh, minnows preferred to associate with conspecifics overheterospecifics, as predicted by the oddity effect. In contrast,sticklebacks preferentially associated with groups of minnowsover groups of conspecifics when predation risk was high. Whenpredation risk was low, solitary individuals of both speciespreferentially associated with conspecific over heterospecificshoals. Stickleback shoal choices under low-risk conditionsmay have been influenced by interspecific competition for food.In feeding experiments, minnows were more efficient foragersthan sticklebacks, so it should benefit sticklebacks to avoidminnows unless predation risk is high. Therefore, for armoredprey, the benefits of associating with more vulnerable preyappear to override the costs of both the oddity effect and foodcompetition when predation risk is high.     

Variation in tolerance to hypoxia in a predator and prey species: an ecological advantage of being small?

Three physiological variables, haematocrit, haemoglobin concentration and ventilation frequency, were measured to test how fathead minnows Pimephales promelas and small and large yellow perch Perca flavescens responded to three different dissolved oxygen concentrations. All fish were monitored continuously for any indications of stress in response to these manipulations. Within and between species, smaller individuals were the most tolerant of hypoxic environments. A species effect, however, did contribute to this observation, with fathead minnows being more tolerant of hypoxic environments than similar-sized yellow perch. In aquatic ecosystems where smaller fishes are more tolerant to hypoxia than their larger predators, hypoxic environments may have the potential to act as a refuge from predators.     

Chemical alarm signals increase the survival time of fathead minnows (Pimephales promelas) during encounters with northern pike (Esox Lucius)

We tested the hypothesis that exposure to a conspecific alarmpheromone improves survival of fathead minnows (Pimephales promelas)during staged encounters with an unfamiliar predator (northernpike: Esox luaus). Minnows exposed to the alarm pheromone survived39. 5% longer than controls. This difference in survival timeappeared to result not from direct inhibition of the pike butrather from some aspect of the minnows' antipredator behavior.Minnows exhibited significant increases in both shoaling andshelter use after exposure to the alarm pheromone. For controlminnows, the degree of shoaling was positively correlated withsurvival time, suggesting that increased shoaling is an effectiveantipredator response. This study provides the first directexperimental evidence that chemical alarm signals in fishesimprove survival of receivers.     

The role of experience and chemical alarm signalling in predator recognition by fathead minnows, Pimephales promelas

Young-of-the-year, predator-naive fathead minnows, Pimephales promelas , from a pikesympatric population did not respond to chemical stimuli from northern pike, Esox Indus , while wild-caught fish of the same age and size did. These results suggest that chemical predator recognition is a result of previous experience and not genetic factors, Wild young-of-the-year minnows responded to pike odour with a response intensity that was similar to that of older fish, demonstrating that the ability to recognize predators is learned within the first year. The intensity of response of wild minnows which had been maintained in a predator free environment for 1 year was similar to that of recently caught minnows of the same age, suggesting that reinforcement was not required for predator recognition to be retained. Naive minnows that were exposed simultaneously to chemical stimuli from pike (a neutral stimulus) and minnow alarm substance exhibited a fright response upon subsequent exposure to the pike stimulus alone. Predator-naive minnows exposed simultaneously to chemical stimuli from pike and glass-distilled water did not exhibit a fright response to the pike stimulus alone. These results demonstrate that fathead minnows can acquire predator recognition through releaserinduced recognition learning, thus confirming a known mechanism through which alarm substance may benefit the receivers of an alarm signal.     

Differential learning rates of chemical versus visual cues of a northern pike by fathead minnows in a natural habitat

We stocked 39 juvenile pike, Esox lucius, into a previously pike free pond which contained a population of approximately 78 000 fathead minnows, Pimephales promelas. Fathead minnows sampled prior to pike stocking did not show a stereotypic fright response to either visual or chemical cues from pike. After stocking pike, we sampled minnows every two days for a period of two weeks. Minnows sampled six days after stocking still did not show a fright response to the sight of a pike, but those sampled eight days after stocking did exhibit a significant fright response, indicating that acquired predator recognition based on vision occurred between six and eight days. Minnows sampled two days after stocking did not show a fright response to chemical cues of a pike. Those sampled four days after did, however, exhibit a significant fright response, indicating that acquired predator recognition based on chemical cues occurred between two and four days. These data indicate that acquired predator recognition occurs very rapidly and that the rate of learning of predator identity differs for chemical versus visual cues.     

Effects of social structure on reproductive activity in male fathead minnows (Pimephales promelas)

The selection of alternative reproductive phenotypes is oftenthought to be the result of physiological state, with smallindividuals forced energetically to postpone the allocationof resources to reproduction. However, for male fathead minnows(Pimephales promelas), we show that seasonal reproductive activityis modulated by social status. In enclosure and pond experiments,small males advanced their reproductive condition, held nesting territories, and spawned earlier in the reproductive seasononly when large males were absent or removed from the population.Since differences in the timing of reproduction among smallmales were not size- or condition-dependent, the common explanationfor the selection of alternative reproductive phenotypes, basedon state-dependence, is insufficient. In the absence of large,socially dominant individuals, small males produced comparablenumbers of offspring as the treatment with large males, although the offspring of these uninhibited small males were smallerat the end of the growing season than the young of large males.Thus, interactions among conspecifics may account for muchof the phenotypic diversity observed within and among naturalfathead minnow populations, through their direct and indirecteffects on growth, recruitment and survival.     

Distribution,abundance, and mortality of small littoral-zone fishes in Sparkling Lake,Wisconsin

Synopsis During 1982 and 1983, seining, fyke netting, and SCUBA observation were used to determine the depth distribution, abundance, and mortality of small littoral-zone fishes (bluntnose minnows, shiners [primarily mimic shiners], yellow perch, logperch, johnny darters, Iowa darters, and mottled sculpins) in Sparkling Lake, a small moderately-productive lake in northern Wisconsin. During the summer cyprinids, darters, and mottled sculpins were most abundant in areas shallower than 1 m, while yellow perch were most abundant at depths of 2 to 4 m. Between August and October cyprinids and yellow perch moved to water 1 to 2 m deeper, while the depth distributions of darters and mottled sculpins generally remained unchanged. Large within- and between-year variations in abundances and mortality rates were evident for all species. In 1982, most of the mortality of cyprinids and darters occurred during a short period in late spring and early summer, with relatively little afterwards. In 1983, this pattern was reversed for cyprinids and Iowa darters, but not logperch and johnny darters. Between-year differences in abundance were greatest for young-of-the-year yellow perch; they were 400 times more numerous in 1983 than in 1982. Darters had their lowest mortality rates and highest reproductive success in 1983, while the opposite was true for cyprinids. This lack of synchrony between darters and cyprinids suggests that these two taxa responded differently to changes in environmental conditions in Sparkling Lake. Predation may have accounted for much of the variability in darter population parameters, but appeared to be a less important source of variation for cyprinid population parameters.     

Egg crypsis and clutch survival in three species of blackbirds (Icteridae)

For most open-nesting passerine birds, the primary function of egg pigmentation is crypsis. Interspecific variation for egg coloration is quite high, even among species that nest in the same locale. This variation might be maintained by selection to blend into species-specific nest cups, which vary in pattern because they are constructed from different materials. To test this hypothesis we photographed 30 clutches of Brewer's blackbirds ( Euphagus cyanocephalus ), 26 clutches of red-winged blackbirds ( Agelaius phoeniceus ), and 24 clutches of yellow-headed blackbirds ( Xanthocephalus xanthocephalus ), and quantified crypsis in terms of pattern (PD crypsis), brightness (MB crypsis) and brightness disparity (BD crypsis). The species were equally cryptic in terms of pattern crypsis, but differed significantly ( P < 0.01) in MB and BD crypsis. Intraspecific variation in pattern crypsis was surprisingly high. Mean clutch pattern disparity was highly correlated with mean background pattern disparity ( r = 0.999); this suggests selection for disparity matching, rather than for crypsis. We also monitored 25 clutches (7 Brewer's, 9 red-winged, and 9 yellow-headed blackbirds) for a 9-day period to determine if survival was related to crypsis. For Brewer's and yellow-headed blackbirds, successful nests were no more cryptic than failed nests. For red-winged blackbirds, there was a tendency ( P = 0.1) for less cryptic clutches to have higher survival; in addition, successful red-winged blackbird clutches had significantly ( P < 0.05) higher egg pattern disparity and background pattern disparity. Red-winged blackbird clutches are not concealed by overhanging vegetation, and high pattern disparity may enable them to blend into a large-scale heterogeneous background.     

A Test for Male Parental Care in a Fundulid, the Bluefin Killifish, Lucania goodei

Under field conditions, breeding male bluefin killifish, Lucania goodei, have been observed aggressively defending territories from other breeding males, non-breeding females, and minnows (mainly Notropis harperi). We performed an aquarium experiment to test whether male aggression serves to protect newly deposited eggs from predation. We allowed a male and a female to spawn in a yarn mop, removed the female, and exposed the eggs to one of four treatments (spawning male present, two minnows present, spawning mal+two minnows present, no adult fish present). Mops were censused daily for seven days. Egg predation rates were highest in the male+minnows and male only treatments. Egg predation rates in the male+minnows treatment did not differ from the predicted predation rate (sum of male only and minnows only treatments). Hence, there is no evidence for male parental care in L. goodei. In addition, we compared the egg predation rates (filial cannibalism) between males of 3 different color morphs and found no evidence for differential egg cannibalism.