Richard K. O'Hara

Pathogenic fungus contributes to amphibian losses in the pacific northwest

Abstract Worldwide declines in amphibian populations have been the subject of numerous recent reports, and numerous hypotheses have been constructed to address the causes. There is no evidence for a single cause for the declines. We identify and describe the spread of a pathogenic fungus that appears to be largely responsible for egg mortality in one population of western toad Bufo boreas. This is the first study documenting contemporary mortality in an amphibian population with identification of the attributed pathogenic species. The fungus we identify is circumglobally distributed and we suggest that this fungal infection could also be a major contributor to other amphibian populations declines.

Kin recognition in Rana cascadae tadpoles: maternal and paternal effects

Abstract Cascades frog ( Rana cascadae ) tadpoles preferentially associate with full siblings over half sibling and half siblings over non-siblings when reared with siblings or as isolates. These tadpoles can use cues of maternal or paternal origin in distinguishing siblings from non-siblings, but maternal cues are preferred over paternal cues. This suggests that a hierarchy of cue importance may exist. Our results are consistent with both a phenotypic matching and a genetic recognition system of kin recognition. Thus, both learned and innate components may play a role in R. cascadae sibling recognition. Kin recognition may facilitate preferential treatment of kin, such as cooperation in food finding or in warning against predators, and therefore those individuals behaving altruistically in kin groups can increase their inclusive fitness.

An investigation of sibling recognition in Rana cascadae tadpoles

Abstract Sibling recognition was studied experimentally in tadpoles of the Cascades frog, Rana cascadae . Sibships were divided into two groups at an early developmental stage. One group was reared with siblings and the second with a mixture of siblings and non-siblings. Whe given a choice of associating with a sibling or non-sibling group, individuals reared with siblings preferred siblings. One mixed-reared group spent more time associating with unfamiliar siblings than unfamiliar nonsiblings. Tadpoles of another mixed-reared group preferred to associate with pure siblings over a mixture of siblings and non-siblings with which they were reared. When the stimulus animals were taken exclusively from the mixed-rearing tank, two mixed-reared groups showed no sibling preferences. Our results suggest that early association with non-siblings does not affect sibling recognition in this species.

Mating pattern variability among western toad (Bufo boreas) populations

SummaryBreeding ecology and mating patterns of the western toad, Bufo boreas, were examined in three large explosively breeding populations in the Oregon Cascade Mountains. Two mating patterns occurred variably within and among the three populations. First, a large male mating advantage was observed in two of the three populations when data from all the days of data collection were combined. When each day of breeding was analyzed separately, there was a large male mating advantage on 3 of 5 days at one population, and mating was random on all days at the two other populations. The second mating pattern, positive assortative mating by size, was observed at two of the three populations. This pattern was found on separate days of breeding as well as when data from all days were combined at one population, and on only one day of breeding at the second population.In a survey of anuran amphibian mating patterns, intraspecific variation was found in 13 of 15 species, including the present study of the western toad. Intrapopulation variation in mating patterns among breeding years has been observed in 5 of 8 anurans, whereas within-site, within-year mating pattern variation has only been reported for the western toad. These results strongly suggest that anuran mating patterns are frequently neither species-specific nor population-specific attributes. Variable mating patterns were most commonly observed in explosively-breeding anurans. Explosive breeders may be susceptible to variable mating patterns because they may be more sensitive to fluctuations in environmental conditions, demographic parameters, and the intensities of intrasexual competition and mate choice.

Kin preference behavior in Bufo boreas tadpoles

SummaryThe ontogeny of sibling recognition behavior was studied in the laboratory in tadpoles of the western toad (Bufo boreas boreas) to test the hypothesis that Bufo tadpoles associate with siblings and to compare this behavior with two species of anurans previously studied. Tadpoles reared exclusively with sibs demonstrated a preference to associate with sibs over non-sibs both early and late in development but tadpoles reared with sibs and non-sibs (mixed rearing groups) exhibited no preference. Larvae that developed a preference for sibs after being reared with them for 75 days lost this preference following exposure to a mixed group for 2 to 6 days. Additionally, larvae reared in a mixed group did not develop a preference for the familiar mixed group nor was a preference exhibited by individual larvae that were reared exclusively with 5 non-sibs. These results suggest that larvae learn by prior association to discriminate sibs from non-sibs and that preferences are rapidly modifiable following exposure to non-sibs. However, the latter two experiments suggest that social preferences are not totally labile and are not based entirely on familiarity. Field observations of larval activity, aggregation behavior, and dispersal patterns in B. boreas indicate that tadpoles mix with non-sibs from early larval stages throughout development. Because even short term exposure to nonsibs resulted in a loss of sib preference in our experiments, we suggest that a ‘kin selection’ interpretation of B. boreas aggregation behavior is not parsimonious. Results differ with those of other studies of sib recognition in larval anurans.

Kin preference behaviour is present after metamorphosis in Rana cascadae frogs

Abstract Kin recognition was investigated in newly metamorphosed Rana cascadae frogs. Previous work has shown that larvae of this species prefer to associated with siblings over non-siblings. Juvenile frogs from three clutches were reared with siblings and tested for sibling preference as larvae and at 4–12 days and 39–47 days after metamorphosis. Tadpoles and froglets of the three clutches displayed a significant preference to associate with siblings.

Kin recognition cues in Rana cascadae tadpoles

The importance of chemical, visual, and auditory cues in kin recognition behavior in Rana cascadae tadpoles was tested in laboratory choice experiments. Tadpoles were given a choice of associating with siblings or nonsiblings by using visual and chemical stimuli together (standard tests), visual stimuli alone, or chemical stimuli alone. We also monitored sound production of tadpoles. We conclude that tadpoles can distinguish between siblings and nonsiblings by using waterborne chemical cues alone that are probably sensed by olfaction or taste. Visual stimuli alone are not sufficient for such discrimination and sound production as a means of communication in tadpoles is probably unimportant.

Rana cascadae tadpoles aggregate with siblings: an experimental field study

Previous laboratory studies have shown that Rana cascadae larvae preferentially associated with siblings over non-siblings in choice tests. This study, conducted during three consecutive summers, tests the hypothesis that R. cascadae larvae aggregate nonrandomly with respect to sibship in natural ponds. Pairs of sibships were reared in separate tanks or together in the same tank in the laboratory. Each sibship within a pair was then stained with neutral red or methylene blue dye and released together in a natural pond. Over a period of several days, aggregations of tadpoles within test ponds were repeatedly captured, censused for sibship composition, and released. In control tests, two groups of tadpoles from the same sibship were dyed different colors and released together. In total, 25 different tests were conducted using tadpoles from 31 sibships and 456 aggregations were sampled. The distribution of color compositions of aggregations in control tests did not differ from an expected random distribution. Color compositions of aggregations in experimental tests differed from controls and from an expected random distribution. Aggregations in these tests tended to be dominated by one of the two colors (sibships). We conclude that R. cascadae tadpoles recognize and prefer to aggregate with siblings in natural field conditions. Circumstances of early rearing (i.e., whether tadpoles were reared with siblings or in mixed sibling/non-sibling groups) had no influence on preferences to associate with siblings, but there was an inverse correlation between group size and sibling association.

Aggregation behaviour in Rana cascadae tadpoles: association preferences among wild aggregations and responses to non-kin

Abstract In laboratory experiments, wild-caught Rana cascadae tadpoles preferred to associate with members of their natural aggregation over members from a second aggregation collected from the same pond. Laboratory-reared tadpoles also preferred to associate nearest to a sibling stimulus group over a stimulus group containing no tadpoles. However, when tadpoles were given a choice of associating between a stimulus group containing conspecific non-siblings and an empty compartment, there was random association. This suggests that tadpoles are gregarious with kin, but are not as likely to form aggregations composed solely of non-kin.

Kin recognition in Rana cascadae tadpoles: effects of rearing with nonsiblings and varying the strength of the stimulus cues

Experiments with two clutches of Rana cascadae tadpoles were undertaken to determine if (1) rearing tadpoles exclusively with nonkin affects sibling recognition behavior and (2) if tadpoles are capable of distinguishing between groups of varying sibling/nonsibling composition. Individual tadpoles reared from embryonic stages with 12 nonsiblings behaved as tadpoles reared with siblings by exhibiting a preference for siblings over nonsiblings in choice tests. This suggests that experience with siblings is not a necessary prerequisite for the development of sibling recognition in this species. Tadpoles reared with siblings preferred to associate with a group composed of 50% siblings and 50% nonsiblings over a group of 100% nonsiblings, but exhibited no preference for a group composed of 25% siblings and 75% nonsiblings versus one composed of nonsiblings only. Thus tadpoles are capable of discriminating between mixed groups of siblings and nonsiblings, but probably require some minimum proportion of siblings to do so.