Brian K. Sullivan

GEOGRAPHIC VARIATION IN SEXUAL SELECTION AMONG POPULATIONS OF AN IGUANID LIZARD, SAUROMALUS OBESUS (=ATER)

Abstract.— Geographic variation in selection pressures may result in population divergence and speciation, especially if sexual selection varies among populations. Yet spatial variation in targets and intensity of sexual selection is well studied in only a few species. Even more rare are simultaneous studies of multiple populations combining observations from natural settings with controlled behavioral experiments. We investigated how sexual selection varies among populations of the chuckwalla, Sauromalus obesus. Chuckwallas are sexually dimorphic in color, and males vary in coloration among populations. Using field observations and multiple regression techniques, we investigated how sexual selection acts on various male traits in three populations in which males differed in coloration. The influence of sexual selection on male coloration was then investigated in more detail using controlled experiments. Results from field observations indicate that phenotypic selection was acting on territory quality in all three populations. In two populations, selection was also acting either directly or indirectly on male coloration. Male color likely functions as an indicator of food resources to females because male color is based partly on carotenoid pigments. In controlled experiments, significantly more females from these two populations chose males with brighter colors over dull males, a result consistent with studies on carotenoid pigments in other taxa. In a third population, no evidence of sexual selection on male coloration was found in either the field study or controlled experiment. Lack of female preferences for male color in this population, in which chuckwalla densities are low and home ranges are large, may result from searching costs to females.

Sexual selection in Woodhouse's toad (Bufo woodhousei). II. Female choice

Abstract I studied female choice in a prolonged-breeding population of Bufo woodhousei australis in central Arizona during the spring of 1981 and 1982. Field observations revealed that variation in call rate was significantly greater between males than within males and that male call rate was correlated with mating success. Male size was not significantly correlated with mating success, or did I observe positive assortative mating with respect to body size. Females did not discriminate between calls of large and small males in choice experiments. However, males with high call rates were preferred, independent of size. For anurans, these results are the first to document clearly female choice based on male phenotype independent of male size. It is suggested that high call rate may be an indicator of male fitness in this female-choice system.

Female choice and selection on male calling behaviour in the grey treefrog Hyla versicolor

Variation in male calling behaviour and female choice was investigated in a population of the grey treefrog, Hyla versicolor, with a lek mating system in east-central Maine over 3 years. Operational sex ratios were male-biased and males obtained zero to four matings within a breeding season. In laboratory discrimination experiments in which all other acoustic parameters were held constant, females preferred long-duration calls over short, and calls broadcast at high rats. Males exhibited consistency in calling behaviour: repeatabilities (intra-class correlation coefficient) ranged from 0·14 to 0·65 for duration, pulse rate and dominant frequency of advertisement calls, and call rate. Selection differentials and standardized selection gradients did not reveal any consistent relationships between male calling behaviour and mating success. The only variable influencing male mating success was the number of nights of participation in chorus activity.

Sexual selection in the Gulf Coast toad,Bufo valliceps: female choice based on variable characters

Abstract Sexual selection on male size and calling behaviour was studied for 2 years in one population and for 3 years in a second population of the Gulf Coast toad. A large-male mating advantage was detected only at one site during 1 year. Measures of calling behaviour were available for 3 years at one of the sites: on a nightly basis in 2 years, males with higher call rates were significantly more likely to mate than males with lower call rates. However, there was no relationship between mating success and either call duration or calling effort on a nightly bias in any year. In discrimination experiments, females preferred higher call rates and calls of longer duration, but not calls of lower frequency. Biased male mating success with respect to call rate therefore appears to have been partially a consequence of female mate choice. Call duration is positively correlated with male size, while dominant frequency is negatively correlated with male size in this species. Because females did not discriminate between high- and low-frequency calls, and because there was no bias in mating success with respect to call duration, it appears unlikely that female choice resulted in the large-male mating advantage observed in 1 year. Despite a nightly bias in mating success with respect to call rate that appears to have resulted from female mate choice, there was no detectable sexual selection on average call rate in any year. Instead, the number of nights of chorusing activity was the only detectable direct target of selection in all 3 years at one of the sites. The repeatability of call rate within males was low, ranging from 0·08 to 0·33. Simulations suggest that within-male variation in a trait can reduce the strength of sexual selection on the trait. Lack of selection on call rate, despite female choice based on call rate, may thus have been partially a consequence of within-male variation in this trait.

Female mate attraction in ornate tree lizards, Urosaurus ornatus: a multivariate analysis

Recent reviews of mate choice have concluded that this phenomenon is rare or nonexistent in lizards. However, direct tests of lizard mate choice are few and generally limited in scope. We present results of a mate choice experiment with Urosaurus ornatus in which two male lizards differing in morphology, colour and behaviour were given the opportunity to attract a female lizard. Pairwise comparisons of selected and unselected males revealed that variation in individual traits failed to account for much of the variation in mate attraction success. However, a multivariate analysis revealed that mate attraction was related to multiple cues from a males phenotype: body mass and head size as well as body and tail coloration all contributed to mate attraction. We argue that the additive and interactive effects of several traits combined may be more important than single traits in mate choice. We suggest that mate choice studies should account for multivariate effects of male traits, and also allow mate attraction to occur over variable spatial and temporal scales, paralleling mate choice under natural conditions.

Variation in Advertisement and Release Calls, and Social Influences on Calling Behavior in the Gulf Coast Toad (Bufo valliceps)

We examined variation in calls and calling behavior in two populations of Bufo valliceps in south-central Texas. Both pulse rate of advertisement calls and call rate were positively related to body temperature, whereas call duration was negatively correlated with body temperature. Advertisement call frequency was not significantly correlated with toad temperature, but was negatively correlated with SVL. Of the temporal advertisement call variables, only call duration was significantly correlated (positively) with SVL. Release call frequency was negatively correlated with SVL. Field observations revealed significantly greater variation among than within males for call rate, call duration, and calling effort. However, despite the consistency of individual calling behavior, calling activity varied with social context. Calling effort was weakly, positively correlated with male density. Call rate and calling effort varied nonlinearly with nearest neighbor distance. Both variables were highest at intermediate distances (2.5-7.5 m), decreasing as nearest neighbors became very close ( 7.5 m).

Sexual selection and calling behavior in the American toad (Bufo americanus)

Mating system structure, variation in advertisement and release calls, and calling behavior were investigated during a three-year period in Bufo americanus in east-central Maine. Although the breeding season was relatively brief, chorus size was small, and males participated in chorus activity for an average of six or seven nights within a season. Pulse rate and duration of the advertisement call were significantly correlated with temperature, but neither frequency nor call rate was influenced by temperature over a 10 C range. Dominant frequency was the only advertisement call variable significantly (negatively) correlated with snout-vent length in one of two years for one of two populations studied. Similarly, dominant frequency was the only release call variable significantly (negatively) correlated with male size (snout-vent length and mass). Males exhibited consistency in both call rate and call duration as measured over a series of nights of chorus activity: repeatability (intraclass correlation coefficient) was 0.52 for call duration and 0.24 for call rate. Females did not discriminate between high and low frequency calls in discrimination experiments; however, females did prefer calls broadcast at relatively high call efforts (rate x duration).

Sexual selection in Woodhouse's toad (Bufo woodhousei) I. Chorus organization

Abstract I studied the reproductive behaviour of a population of prolonged-breeding Bufo woodhousei in central Arizona. Choruses formed in discrete and consistent areas of ponds, and males actively defended calling sites within these aggregations. Call rate and calling site residency, but not size, were important factors determining the outcome of male-male disputes within choruses. Females visited choruses to select males, and oviposition occurred away from the calling site of selected males. Natural and experimental observations revealed that females prefer males with a high call rate. Preliminary results indicated that large males did not achieve greater reproductive success, nor was there any suggestion of assortative mating in size of mated pairs. It is concluded that B. woodhousei has a lek mating system, possible because of its prolonged breeding period. These results are discussed in relation to other anuran mating systems and sexual selection in general.

Sexual selection in the Great Plains toad ( Bufo cognatus).

Breeding aggregations of Bufo cognatus were observed in southern Arizona and southwestern New Mexico during June, July and August, 1980-82. Most breeding aggregations lasted only two or three nights following summer rainstorms. Males substantially outnumbered females at choruses, and females actively selected mates from among displaying males. Some small males adopted satellite positions near calling males, and the percentage of time a male was observed calling was significantly correlated with snout-vent length. Male mating success was positively correlated with the percentage of time a male was observed calling, but male size and mating success were not correlated. There was no indication of positive assortative mating with respect to body size. It is concluded that Bufo cognatus has a lek mating system.

Significance of size, temperature and call attributes to sexual selection in Bufo woodhousei australis.

Advertisement calls of a population of Bufo woodhousei australis were recorded in the field to determine possible effects of size and temperature on call attributes. Analysis of dom- inant frequency, pulse rate and duration of advertisement calls revealed that only frequency was correlated (negatively) with size. Observations of the same individuals on successive nights showed there was substantial variation in call attributes independent of temperature. Only pulse rate was correlated (positively) with body temperature; frequency and call duration were independent of body temperature over a 5?C range. It is concluded that females might distinguish males of different sizes on the basis of frequency, but temperature has no effect on this possible size indicator. These results are discussed in relation to the work of Fairchild (1981) on Bufo woodhousei fowleri.