Geographic Variation In Sexual Size Dimorphism Research Articles

The effect of seed traits on geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Acanthoscelides macrophthalmus.

Explaining large‐scale patterns of variation in body size has been considered a central question in ecology and evolutionary biology because several life‐history traits are directly linked to body size. For ectothermic organisms, little is known about what processes influence geographic variation in body size. Changes in body size and sexual size dimorphism (SSD) have been associated with environmental variables, particularly for Bruchinae insects, which feed exclusively on seeds during the larval stage. However, the effect of important seed traits on body size variation has rarely been investigated, and whether SSD varies substantially among populations within bruchine species is poorly known. Using the seed‐feeding beetle Acanthoscelides macrophthalmus infesting its host plant Leucaena leucocephala, we investigated whether specific seed traits (hardness, size, water content, carbon/nitrogen ratio, and phenolic content) were determinant in generating geographic variation in body size and SSD of A. macrophthalmus. We also examined the relationships between body size and SSD with latitude and altitude. The body size of both sexes combined was not related to latitude, altitude, and any of the physical and chemical seed traits. However, the female body size tended to vary more in size than the males, generating significant variation in SSD in relation to latitude and altitude. The females were the larger sex at higher latitudes and at lower altitudes, precisely where seed water content was greater. Therefore, our results suggest that water content was the most important seed trait, most severely affecting the females, promoting geographic variation in SSD of A. macrophthalmus.

Geographic variation in sexual-size dimorphism of the American black bear (Ursus americanus) in areas of western North America

Patterns of geographic variation in cranial characters of the American black bear (Ursus americanus) were examined for 11 localities in the western US and Canada. Forty-four dimensions were recorded from skulls of 290 (149 males, 141 females) adult (≥4.5 years old) animals using digital calipers. Results showed that males are generally larger than females and the degree of sexual-size dimorphism for characters among localities is relatively consistent. Most averages (84.1%) of mean values across characters varied from 7.0% to 18.0%. The majority (90.9%) of character ranges (difference between highest and lowest value for a trait at each locality) of mean values for sexual dimorphism varied from 5.0% to 14.0%. Overall, there was no significant statistical difference in the degree of sexual dimorphism across localities in 43 of 44 characters. Measurements for males were significantly correlated with those of females for 38 of 44 characters. Results indicated that the level of sexual-size dimorphism remained relatively low and constant across localities. No statistically significant relationship existed between degree of sexual-size dimorphism and selected environmental variables. Although overall causes for levels of sexual differences in morphological features of black bears remain uncertain, bioenergetics appears to be a determining factor associated with sexual-size dimorphism in black bears.

Biphasic Geographic Variation in Sexual Size Dimorphism of Turtle (Mauremys leprosa) Populations Along an Environmental Gradient in Morocco

Abstract The varying influences of selective forces throughout a species' range can result in geographic variation in sexual size dimorphism (SSD). The Moroccan turtle, Mauremys leprosa, occupies an extremely wide variety of ecoregions and habitats, including coastal rivers, mountain streams, oases, and intermittent rivers in the northern fringe of the Sahara Desert. To assess geographic variation in SSD, we collected specimens along an environmental gradient in central Morocco, including Oued (River) Ksob on the Atlantic coast, Oued Zat in the High Atlas Mountain foothills, and Oued Drâa in the Sahara Desert. Only turtles with conspicuous secondary sexual characteristics were included in our analysis. We calculated a sexual dimorphism index (SDI) using the mean size of the larger sex divided by the mean size of the smaller sex and subtracted one from that ratio. The direction of SSD was biphasic: in one population males and females exhibited the same body size, whereas, in two other populations, females ...

LATITUDINAL VARIATION IN SEXUAL SIZE DIMORPHISM OF SEA-RUN MASU SALMON, ONCORHYNCHUS MASOU

Sexual size dimorphism (SSD), a difference in body size between the sexes, occurs in many animal species. Although the larger sex is often considered invariable within species, patterns of selection may result in interpopulation variation or even reversal of SSD. We evaluated correlations between latitude and female body size, male body size, and relative body size (male body size/female body size) in 22 populations (ranging from 37 degrees N to 49 degrees N) of sea-run masu salmon (Oncorhynchus masou) that spawn in rivers along the Sea of Japan coast. Male size and the relative body size increased with latitude, but female size did not correlate with latitude. In addition, increase in male size with latitude was sufficient to result in a reversal of SSD, the switch-point being around 45 degrees N. We suggest that the positive correlation between latitude and male size is due to increasing operational sex ratios or sexual selection on sea-run male body size that result from sex-biased patterns of anadromy. In conclusion, our study provides the first example of predictable geographic variation in SSD shaped by apparent patterns of sexual selection.

Geographic Variation in Growth and Sexual Size Dimorphism of Bog Turtles (Clemmys muhlenbergii)

We examined sexual size dimorphism (SSD) and growth rates in samples of bog turtles (Clemmys muhlenbergii) from North Carolina, Pennsylvania and New Jersey. Mean carapace length (CL) of males was significantly greater than mean CL of females in all three states. However, the degree of SSD varied significantly among states. Specimens from North Carolina had the greatest SSD, the largest mean adult CL, and the largest mean hatchling CL. Growth rates were rapid until about age 6 and a CL of 80 mm. Males grew faster than females thereafter. A comparison of the three parameters estimated from von Bertalanffy growth equations showed significant differences for asymptotes between sexes and states for the North Carolina and Pennsylvania samples, but not for the growth rate constant (parameter k). Geographic variation in SSD appears to be a result of differences in growth related to the timing of maturity and the approach to asymptotic body size caused by indeterminate growth, or both. We suggest that male-biased SSD in this species is ultimately a consequence of the advantage conferred to larger males in male-male interactions and during mating.