NOVEL ASPECTS OF OVIPOSITION SITE PREPARATION BY FOOTHILL YELLOW-LEGGED FROGS (RANA BOYLII)

Abstract

male foothill yellow-legged frogs, Rana boylii, scrape the rocky substrate with their hind feet prior to oviposition. They observed that this behavior loosened algae and sediment and postulated that it could enhance the ability of egg masses to adhere to the substrate. Based on 2 episodes at a site in northwestern California, their observations provide a 1st glimpse of this behavior. Here, we add serial observations of this behavior by 2 additional female R. boylii from western Oregon. These observations increase our knowledge of substrate preparation behavior and suggest that females of this species evaluate oviposition site quality. Observations were made in June 2004 during an ongoing study of the northernmost extant population of R. boylii (Rombough 2002), the last known population remaining in the Willamette hydrographic basin (Borisenko and Hayes 1999). This site is located on a sharp (approximately 120') bend of the South Santiam River (Linn County, Oregon; 44040'97N, 122056'54W [datum: WGS84]; elevation 192 m). The substrate is primarily hard volcanic tuffs, which form a relatively flat bedrock surface approximately 400 m long and up to 150 m wide; this surface is scoured bare of nearly all loose rock <0.5 m in diameter. Seams of soft conglomerates in this bedrock have eroded, leaving numerous troughs and scour pools, the largest of which are lined by clumps of sedge and often contain smaller cobble and gravel. Oviposition by R. boylii takes place exclusively in a few off-channel pools and troughs with reduced flow velocities, decreasing the likelihood of egg scour (Rombough 2002; see also Kupferberg 1996a; Lind and others 1996). All observations reported here occurred immediately before and during oviposition. Observations took place in shallow (0.22 to 0.27 m) water near the edge of an 8-m-diameter scour pool where frogs have bred during each of the past 5 years (from 2000 to 2004). Located along the west margin of the active channel, the west side of the pool had a soil and riprap bank overhung by bigleaf maple (Acer macrophyllum). At least 40 m of bedrock flat separated the east side of the pool from the perennial river channel. During the breeding season, male R. boylii generally approach the site from the east, whereas females usually enter from the west, from the direction of the forested bank. On this