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Reverse Mounting in Grebes ’ The Condor 91:341-346 0 TheCooper Ornithological Society 1989 REVERSE MOUNTING IN GREBES ’ GARY L. NUECHTERLEIN Department of Zoolom, North Dakota State University,Fargo, ND 58105 ROBERT W. STORER Museum of Zoology and Department of Biology, Universityof Michigan, Ann Arbor, MI 48109 Abstract. Reversemounting (females mounting males) is commonin manygrebe species. Of 328 mountingsobserved and filmed in Silvery Grebes(Podiceps occipitalis), 27% were by the female.Similarly, for HoodedGrebes (P. gallardoi) observedduring the sameperiod, 15% of 95 mountingswere reversed. For both species,reverse mounting was significantly more common early in the season.Reverse mounting in Silvery Grebesalso was more frequentlyassociated with pre- and postmountingcourtship display activity at the nestthan werecopulations. We concludethat reversemounting is not aberrantbehavior in grebes, but constitutesa regularand integralpart oftheir courtshipbehavior. Since reverse mounting alsoappears to occur in a wide diversity of other monomorphic bird species,we suggest that using mounting behavior to determine sex in these speciesmay be unreliable. Key words: Hooded Grebe;Silvery Grebe;Podiceps; reversemounting; copulation. INTRODUCTION that there may be important behavioral differ- Although reversemounting hasbeen reported for ences between reverse mounting and “normal” many bird species,most authors appear to regard copulation sequences.In an analysis of filmed the behavior as unusual or aberrant. The evi- sequencesof the Least Grebe (Tachybaptus dom- dence from easily sexed, strongly dichromatic i&us), for example, Storer (1976) found signif- bird speciesgenerally lends support to this as- icant differences related to whether or not the sumption (Morris 1954, 1955; Ficken 1963; No- roles of male and female were reversed. lan 1978; Thompson and Lanyon 1979), but in Several nonmutually exclusive hypothesescan grebes(Podicipedidae) and several other mono- be proposed to explain why reverse mounting chromatic species the behavior is sufficiently occurs in grebes. Reverse mounting may (a) re- widespreadto warrant re-examination of this as- sult in fertilization, (b) result from mistaking a sumption. partner’s sex,(c) be an aberrant behavior, (d) play The prevalence of reverse mounting in grebes a role in courtship and pair formation, (e) indi- has become particularly evident in our research cate a reversal of dominance in the sexes(Storer on the comparative courtship behavior of the 1976), or (f) stimulate the ovaries of the female colonial grebes.Because grebe pairs often engage to grow (Storer 1976). in repeated mounting attempts (sometimes five The primary objectives of the fieldwork re- or six copulations in a 15- to 20-min period), ported in this paper were to examine the contexts reversal in the roles of the sexes during such and behavioral details of reverse mounting in bouts is particularly conspicuous(see Storer 1969, Silvery Grebes (Podiceps occipitalis) and Hooded 197 1, 1976). Within colonies, sufficient sample Grebes (P. gallardoi), asking specifically the fol- sizesof both copulations and reverse mountings lowing questions: (1) Is reverse mounting aber- can be obtained to compare statistically the con- rant behavior? (2) When in the pair-formation texts and structural details of the two forms of processdo reversemountings take place?(3) Does mounting behavior. sperm transfer take place during reverse mount- Most authors describing copulation in grebes ing? (4) Are there consistent differences in the have used the neutral terms “passive bird” and platform displays of reverse mountings vs. cop- “active bird,” to avoid implying sex-specificbe- ulation? havior, but this practice ignores the possibility METHODS As part of a 3-year comparative study, we con- I Received2 1 July 1988.Final acceptance3 January ducted fieldwork on Silvery and Hooded grebes 1989. on small (lessthan 2 km2) snow-melt lakes in the [3411 342 GARY L. NUECHTERLEIN AND ROBERT W. STORER TABLE 1. Frequencyof copulations(male on female: larger than female bills (t = 6.0, df = 28, P < M/F), reverse mountings (F/M), and same sex mount- 0.001 for bill length; t = 3.9, df = 28, P -c 0.001 ings (M/M and F/F) in Silvery and Hooded grebes. for bill depth). When both measures were used in combination, there was almost no overlap be- Possible mountings M/F F/M M/M F/F tween male and female bill measurements. Al- though too few museum specimens exist for a Silvery Grebe 239 88 1 similar statistical analysis of bill size for the rarer Hooded Grebe 81 14 0 Hooded Grebe, a similar bill dimorphism was evident. foothills of the Andes in southern Patagonia, Ar- Most mounting activities were observed from gentina (52”S, 72”E). Observations on reverse 4 to 10 m distance using either binoculars or a mounting and copulation were most intensive in 15-60 x spotting scope. We also obtained films the 198 1-1982 season. of the behavior from within a distance of 5 m In November 198 1, we observed a group of using a 16-mm Beaulieu camera and the floating several hundred Silvery Grebes and 35 Hooded blind. For each mounting, we noted the sequence Grebes engagingin courtship activities on an un- of platform displays that occurred and whether named staging lake, which we named Laguna the active and passivebirds were male or female. Nevada. Unlike waterfowl (Anseriformes),grebes Display terminology used in this paper is that of cannot mount in open water (McAllister and Sto- Storer (1969). rer 1963), and we observed Silvery Grebes using In late November, the courting flocks of grebes flat, partly submergedrocks on the far north end on Laguna Nevada dispersedto other small lakes of the lake as copulation platforms. To obtain in the area for breeding. In December 198 1, closer observation and films of this activity, we Nuechterlein set up platforms on Laguna Blan- constructed platforms directly in front of our chillo, where over 100 Silvery Grebes and 30 camp. These platforms consistedof three woven Hooded Grebes eventually nested. Most birds basketsplaced in each comer of a floating trian- on this lake arrived already paired, so these ob- gular frame made of 3-m bamboo poles. The servations and those at the nesting colony pro- entire frame was then anchored in 1 m of water vided us with a late-season sample that could be and protected from wave action by floating wave compared to that from Laguna Nevada. breaks. These platforms were readily adopted by the RESULTS grebesand over a 2-year period, we were able to We observed 328 mounting attempts in Silvery film or observe more than 400 copulations or Grebes and 95 in Hooded Grebes. All attempts reverse mountings. Later in the season, mount- involved a male and female except for one case ing at nest platforms within breeding colonies in Silvery Grebes where a male attempted to was also observed from a floating blind. In Pat- mount another male (Table 1). This case also agonia, both specieswere very tame and allowed was unusual in that the passive male neither the close observation distances necessaryto sex Reared nor Invited. the birds reliably. Reverse mounting was common in both Prior to mounting, all pairs in the sample were species,accounting for 27% of the mountings in sexed visually using bill size, which is greater in Silvery Grebes and 15% in Hooded Grebes (Ta- males for most or all grebe species(Storer, un- ble 1). This was particularly evident early in the publ. data). Only pairs that could be easily dis- seasonon the temporary platforms, where 40% tinguished before making the behavioral obser- of 185 mounting attempts by Silvery Grebes were vations were included in the sample. This sexing by females. Later, in the egg-laying period, re- technique was verified for Silvery Grebes by R. verse mounting was significantly less frequent W. Storer by measuring the bills of male and (9.8% of 142 attempts, Table 2). A similar tally female specimens using dial calipers. Measure- of reverse mountings vs. copulations in Hooded ments taken included depth of the closed bill at Grebes showed that pairs just beginning to build the level of the posterior edge of the nostril and their nesting platforms (first day) engagedin sig- bill length from the anterior edge of the nostril nificantly more reverse mountings than did pairs to the tip, both to the nearest 0.1 mm. For both with well-formed cupped nests during the same measures,bills of males were highly significantly 3-day period (33% vs. 5%, Table 2). REVERSE MOUNTNG IN GREBES 343 TABLE 2. Frequencyof copulation vs. reverse mounting by: (A) Silvery Grebes on platforms during the pair- formation period (26-27 November, Laguna Nevada) vs. at nests during egg laying (11-12 December, Laguna Blanchillo); and (B) Hooded Grebes during early and late stagesof nest building. Copulations A. Silvery Grebe Platforms 111 14 G* = 40.4, df = 1, P < 0.001 Nests 128 14 B. Hooded Grebe On new nesting platforms 22 11 G2 = 13.4, df = 1, P < 0.001 On cupped nests 59 3 A behavioral analysis of copulation and re- quences males were significantly more likely to verse-mounting sequencesin Silvery Grebes in- Rear and Wing-quiver (Table 3A) prior to In- dicates many significant differences(Table 3). In viting the female to mount. Postmounting dis- contrast to copulatory sequences, ejaculation play activity, particularly Head-flicking and never occurred in reverse mountings, although Habit-preening, also was more frequent when sample size for this behavior was somewhat lim- females mounted males (Table 3C). ited (n = 21) owing to the necessity for close observations of birds that were facing directly DISCUSSION away from the observer. Tail thrusting by the On the basis of our data from the Silvery, Hood- female, however, occurred in 29% of 72 reverse ed, and other grebes,we can reject several of the mounts, and the cloaca of the male often was proposed hypotheses.
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