Interspecific Social Interactions and Breeding Success of the Frog Rana latastei: A Field Study

Interspecific reproductive interference can affect fitness‐related breeding performance, thus influencing fitness and distribution of populations. Laboratory studies demonstrated the social interference of Rana dalmatina males on R. latastei breeding females: the presence of heterospecific males reduced the percentage of viable embryos in R. latastei eggs. Here, we tested if the negative effects of R. dalmatina males on R. latastei reproductive success occur in field conditions. We compared the percentage of viable embryos of eggs laid in field conditions from populations where R. latastei breeds alone with the percentage of viable embryos of populations where R. latastei cohabits with R. dalmatina. We did not find any significant difference in percentage of viable embryos between R. latastei populations syntopic and allotopic with R. dalmatina, nor a relationship between the relative abundance of heterospecifics and reproductive success. In natural conditions, the presence of heterospecific males does not seem to interfere with the reproductive success of R. latastei. The experimental procedure may influence the interaction among individuals. Therefore, we suggest to validate on natural populations the results of experiments dealing with complex interactions.     

Reproductive interference between Rana dalmatina and Rana temporaria affects reproductive success in natural populations

Experimental evidence suggests that reproductive interference between heterospecifics can seriously affect individual fitness; support from field studies for such an effect has, however, remained scarce. We studied reproductive interference in 25 natural breeding ponds in an area where two ranid frogs, Rana dalmatina and Rana temporaria, co-occur. The breeding seasons of the two species usually overlap and males of both species are often found in amplexus with heterospecific females, even though matings between heterospecifics produce no viable offspring. We estimated species abundance ratios based on the number of clutches laid and evaluated fertilization success. In ponds with low spatial complexity and a species abundance ratio biased towards R. temporaria, the average fertilization success of R. dalmatina eggs decreased, while this relationship was not detectable in spatially more complex ponds. Fertilization success of R. temporaria did not decrease with increasing relative numbers of heterospecifics. This asymmetry in fitness effects of reproductive interference may be attributed to R. temporaria males being more competitive in scramble competition for females than R. dalmatina males. Our study is among the first to demonstrate that in natural breeding populations of vertebrates interference among heterospecifics has the potential to substantially lower reproductive success at the population level, which may in turn affect population dynamics.     

A skeletochronological study of growth,longevity, and age at sexual maturity in a population of<Emphasis Type="Italic">Rana latastei</Emphasis> (Amphibia,Anura)

Longevity and age at sexual maturity in an Italian population ofRana latastei were studied by skeletochronology performed on the phalanges. Frogs collected in 1998 and 1999 by drift fences and pitfall traps were marked by toe-clipping. After marking, individuals were released and the cut phalanges were processed for skeletochronological analysis. The maximum age so far recorded was 3 years in males and 4 years in females. The smallest male and female that were sexually mature on the basis of histological analysis of the gonads were 36 and 35 mm snout vent length (SVL), respectively. In both sexes, most individuals were estimated to breed shortly after emergence from their first overwintering. Among the European Brown Frogs,Rana latastei appears to be one of the shortest-lived and one of the first to reach sexual maturity.     

Experimental manipulation of tail length in female barn swallows (Hirundo rustica) affects their future reproductive success

Previous studies have shown no significant effect of experimentaltail length manipulation in female barn swallows (Hirundo rustica)at the beginning of a breeding season on reproductive successor behavior during that breeding season. In the present study,we investigate if tail length manipulation had any effect onreproductive performance the following year, the so-called long-termeffect, in contrast to the short-term effects already studied.We found that females with experimentally elongated externaltail feathers at the beginning of a breeding season producedless offspring during the breeding season the following yearthan did females with shortened or unmanipulated tails. Theseresults suggest that tail elongation caused flight deficienciesthat deteriorated the condition of females and eventually reducedreproductive success. The finding of long-term effects but nosignificant short-term effects for female tail elongation suggeststhat female barn swallows have the ability to adjust immediateparental investment. Detrimental effects of long tails in femalesin terms of decreased reproductive success might explain whyfemale tails are not as long as those of males. Finally, femalesmated to long-tailed (sexually attractive) males decreased theirreproductive success the following year more than did femalesmated to short-tailed males, possibly owing to differentialparental effort causing a deterioration of their condition.     

Behavioral and endocrine correlates of reproductive failure in social aggregations of captive wolverines (Gulo gulo)

Sociality in mammals is often viewed as a dichotomy, with sociality contrasted against solitariness. However, variation within these broad categories may have strong effects on individual fitness. For example, reproductive suppression of social subordinates is generally associated with group living, but suppression may also occur in solitary species if the behavioral and physiological processes involved can be modulated by the demographic environment. To investigate whether behavioral and physiological traits that normally are associated with group living might be latent even in a solitary species, we explored the level of sociality and investigated causes and mechanisms of reproductive failure in female wolverines Gulo gulo that experienced a highly aggregated social environment in captivity. Behaviorally, females showed low levels of aggression and intermediate levels of social interactions. Reproductive failure seemed to have been related to low social rank and to have occurred between ovulation and implantation in 13 out of 15 breeding attempts. However, three of eight females observed to mate produced offspring, indicating that no individual female fully managed to monopolize breeding. Reproductive failure was not related to elevated levels of glucocorticoid stress hormones. Rather, elevated glucocorticoid levels during the mating season were associated with successful reproduction. We suggest that social tendencies and physiological mechanisms mediating reproductive suppression may be viewed as reaction norms to the social environment. We further suggest that the social flexibility of solitary carnivores might be greater than is commonly observed, due to ecological constraints that may limit aggregation.     

Breeding behavior and mating system in relation to body size in <Emphasis Type="Italic">Rana chensinensis</Emphasis>, a temperate frog endemic to northern China

Knowledge of how mating success is related to body size may provide insight into the evolution of social systems. This study investigated the mating system and relevant social behavior of a temperate anuran (Rana chensinensis) at three localities in northern China. During chorusing, males aggregated and persisted in the communal spawning ponds with a density of 10–26 frogs per m² water area and operational sex ratio of 15–28 males to 1 female. The males frequently grabbed any conspecifics they encountered, releasing the grip if the individual was a male, but holding the grip when it was a female. A significant positive relationship between male and female body lengths of pairs in amplexus was detected from all the sites during the five breeding seasons, but the average correlation coefficients of determination of 18% indicated that the size-assortative mating constituted a minority of the species’ mating system. Pairing probability decreased with increased size differences between sexes, which could have prevented a few adult animals in a population from forming pairs. The strong aggregation of males could limit the opportunities for large males to exhibit contest advantages in mating and for both sexes to choose a large mate. Infrequent occurrence of scramble competition (averaging 8% of the recorded amplectant pairs) and the lack of observed takeovers, probably because of reclusion of amplectant pairs at the bottom of spawning ponds, suggested a weak role of amplexus displacement in generating non-random mating. Our results suggest that although non-random mating with respect to body size may be expected in explosive breeders, the social environment characterized by male aggregation may also impose weak selective pressure for the evolution of non-random mating.     

Male ornamentation, timing of breeding, and cost of polygyny in the collared flycatcher

Highly ornamented males are often thought to be better ableto provide females with resources, parental assistance, or goodgenes. Individual variation in such male abilities may overridethe costs of polygyny and therefore largely explain within-populationvariation in mating patterns. We investigated the influenceof variation in male ornamentation and the environment on thecosts of polygyny for female collared flycatchers (Ficedulaalbicollis), using data from a long-term study involving 2733breeding attempts over 19 years. We show that females sufferreduced reproductive success when mated polygynously but thatthe costs of polygyny depend on an interaction between maleornamentation and timing of breeding. Among early breeders,polygynously mated females experience higher reproductive successwhen mated to less ornamented males, but among late breeders,females mated polygynously to highly ornamented males were moresuccessful. We suggest that a high effort spent on obtainingextrapair matings early in the season renders highly ornamentedmales less able to assist two females in caring for the young.Thus, a male's ability to simultaneously gain from extrapairmatings and polygyny may be limited through direct effects onfemale reproductive success. Given such limitation, extrapairmatings may be expected to be less frequent in species withbiparental care and a high level of social polygyny.     

Weight loss,reproductive output,and the cost of reproduction in the common frog,Rana temporaria

Summary The consequences of reproduction for body weight, growth and survival were studied in a Swiss population of the explosive breeder, Rana temporaria. Males and females continuously loss weight in the range of 0.5% of total body weight per day from the breeding migration throughout May. Females also lost about 33% (1983) and 29% (1984) due to spawning. In addition to this significant year-to-year variation, there was also considerable individual variation in reproductive output. Skeletochronological techniques indicated that breeding male or female frogs experienced a growth reduction of several millimeters relative to non-breeding frogs of the same body size. There was no relationship between an individual female's reproductive output in consecutive years or with her subsequent growth or survival. It was concluded that weight loss is caused by a seasonally elevated metabolism in combination with a lack of feeding and represents a basic energetic cost of reproduction, resulting in lowered growth. Individual variation in relative reproductive output is mostly environmentally induced and is not an expression of different reproductive strategies. This may explain the lack of trade-offs that are predicted by the cost-of-reproduction-hypothesis.     

Mechanical constraint on size-assortative paring success in a temperate frog: An experimental approach

In anurans with axillary amplexus, males may be unable to handle females much different in body size from them due to physical limitation. Such mechanical constraint during the grasping processes is thought to be one of the proximate mechanisms leading to pairs to form size-assortively. Using a pairing experiment, the purpose of this study was to test this prediction for a temperate frog (Rana chensinensis) wherein some size-assortative matings occur in natural populations. We found a reduced probability of pairing success as the difference between sexes. When one female was much larger than one male that attempted to grasp her, she tended to dislodge aggressively him, suggesting a role of mechanical constraint in facilitating female choice against small-sized mates. By contrast, when the male was much larger than the female, he often failed to grasp her effectively or remain her in amplexus for longer, indicating the restriction of mechanical constraint to male pairing attempts and to female preference for large-sized mates.     

Stimulatory effects on the reproductive axis in female songbirds by conspecific and heterospecific male song

Courtship vocalizations of male songbirds can profoundly enhance the reproductive physiology and behavior of conspecific females. However, no study has fully investigated the selectivity of conspecific song effects on reproductive development in birds. We studied the effects of conspecific and heterospecific song on reproductive development in domesticated (canaries) and wild songbirds (song sparrows). As expected, conspecific song enhanced follicular development. Unexpectedly, however, birds exposed to heterospecific song also underwent enhanced follicular development (compared to birds exposed to no song); conspecific and heterospecific songs were equally effective in enhancing ovarian development. In canaries exposed to 18L:6D, conspecific song induced oviposition earlier and at a greater frequency than in heterospecific and no song groups, with the fewest eggs being laid in the no song group. These results indicate that conspecific and heterospecific male song can enhance reproductive activity in female songbirds. Whether or not activation of the reproductive axis in female songbirds by heterospecific song occurs in the wild remains unclear. It is also unclear as to whether the ability of the reproductive axis to respond to heterospecific song performs a specific function, or whether it is simply a consequence of greater selection pressure acting upon behavioral responses to song.