Sexual selection and 'species recognition' revisited: serial processing and order-of-operations in mate choice

Abstract

Following the modern synthesis, mating signals were thought of principally as species recognition traits, a view later challenged by a burgeoning interest in sexual selection-specifically mate choice. In the 1990s, these different signal functions were proposed to represent a single process driven by the shape of female preference functions across both intra- and interspecific signal space. However, the properties of reliable 'recognition' signals (stereotyped; low intraspecific variation) and informative 'quality' signals (condition dependent; high intraspecific variation) seem at odds, perhaps favouring different signal components for different functions. Surprisingly, the idea that different components of mating signals are evaluated in series, first to recognize generally compatible mates and then to select for quality, has never been explicitly tested. Here I evaluate patterns of (i) intraspecific signal variation, (ii) female preference function shape and (iii) phylogenetic signal for male cricket call components known to be processed in series. The results show that signal components processed first tend to have low variation, closed preference functions and low phylogenetic signal, whereas signal components processed later show the opposite, suggesting that mating signal evaluation follows an 'order-of-operations'. Applicability of this finding to diverse groups of organisms and sensory modalities is discussed.

Keywords: Gryllus; crickets; phylogenetic signal; preference functions.

Figure 1.

Conceptual models of mate discrimination; (a) shows process in pale rectangles leading to either a pool of recognized potential mates (blue ellipses) or preferred mates (orange ellipses); (b) shows potential fitness as a function of increasing investment in mate discrimination. Total fitness increases with each added level of mate discrimination, but the marginal fitness gain from signal recognition (i.e. selection of a generally compatible mate) far exceeds the marginal fitness gain from sexual selection among recognized potential mates. (Online version in colour.)

Figure 2.

Mean ± s.e. within-species variation in cricket call components, measured as coefficient of variation for 39 Gryllus species or genetic lineages. Call traits with grey bars are shorter time scale features of pulses: dominant frequency (FREQ), pulse rate (PRATE) and pulse duty cycle (PDC), which are evaluated first by females; those with black bars are longer time scale features of groups of pulses: pulses per chirp (PPC), chirp rate (CRATE) and chirp duty cycle (CDC) which are evaluated second. Traits with the same letter above the bars are not significantly different from each other.

Figure 3.

Phylocorrelograms indicating phylogenetic autocorrelation as a function of phylogenetic distance for each of the song traits. The solid black line is Moran's I autocorrelation index, with dashed lines indicating 95% confidence envelope. Significantly positive regions of autocorrelation are indicated in red. (a) Frequency, (b) pulse rate, (c) pulse duty cycle, (d) pulses per chirp, (e) chirp rate and (f) chirp duty cycle. (Online version in colour.)