Evidence for condition-dependent mate preference has begun to appear in the literature over the last two years, although it is still neither extensive nor conclusive. I will begin by describing examples of parasitic infection reducing female condition and strength of mate choice and then move onto other examples in direct benefit mating systems and an example where genetic variation in
condition affects female mate preference. I will conclude the section by describing a potentially interesting area of investigation where other examples of condition- dependent mate preference may be found.
Parasites often cause phenotypic changes in individuals, especially in behaviour, and some parasites have been shown to bring about a reduction in an individuals condition (Poulin 1993; Zuk 1998; Holmes and Zohar 1990). A reduction in condition is likely to occur because of the cost of immune response and the drain on physiological resources directly by the parasite. Lopez (1999) showed that unparasitised female guppies preferred to mate bright actively
displaying males when given a binary choice between a bright displaying male and a dull less actively displaying male. A second group of females were infected with a fluke parasite that attaches to the fins of guppies. These infected females chose randomly between the males and had significantly weaker preferences than uninfected females. The number of parasites on a female was negatively
correlated with female activity and strength of preference. Infected females also swam between males less frequently, indicating that parasite infection reduced female condition.
Poulin (1994) found a similar reduction in female mate preference in parasitised upland bullies, Gobiomorphus breviceps. Wild caught females were
given a choice between a large and a small male. After mate choice decisions were made females were dissected and the number of trematode parasite cysts in the body of the fish were counted. Poulin (1993) had previously shown that there is a negative correlation between the physical condition of a fish and parasite load. In surprising concurrence with Lopez (1999), more parasitised females were found to make fewer mate inspections and chose the smaller male more often than lightly parasitised females.
In contrast to these studies, Zuk et a l (1998) found that a parasitic
nematode infection that reduced condition in females did not effect female mate choice in red Jungle Fowl (Callus gallus). The authors commented that the low
cost of choice in this mating system was likely to allow even low condition females to express their mate preference.
Examples of changes in female preference with condition are far more obvious in systems with direct benefits to mate choice. In a number of species of cricket and grasshopper, females are the choosy sex when environmental
conditions are high, but males become the choosy sex when food becomes scarce (Gwynne 1985; Gwynne and Simmons 1990; Simmons 1994; Ritchie et a l 1998).
This is because males produce a spermatophore which has a spermatophylax attached to it. A male attaches the combined spermatophore to the females genitalia. The spermatophylax is a proteinous mass that the female eats. When finished the female often then removes the spermatophore (Gwynne 1982). Brown (1997) found that females fed on low quality food mated more quickly and were less choosy than females on high quality food. Simmons (1994) found a similar effect when female condition was reduced by a gut Protozoan parasite in bush crickets. Females no longer chose between males but mated at very high frequency to compensate for the nutritional loss to infection.
The most convincing evidence for condition-dependent mate preference was found by Bakker et al. (1999) in sticklebacks. Female laboratory bred
sticklebacks {Gasterosteus aculeatus) were allowed to choose between computer
animated courting males. Full-sib families of approximately equal size were raised under standardised conditions. The average family condition was calculated
as the mean body mass divided per unit body length of six randomly sampled males from each full-sib family. Even though rearing conditions were
standardised, the average family condition differed significantly between full-sib groups. The preference of two females from each full-sib family was assessed when given the choice between a male with red-throated breeding colouration and a male that was orange throated. Overall, females did not show a significant preference for either male, but female preference for the red throated male correlated positively with average family condition. Females from high average condition families preferred red throated females whilst females from low average condition families preferred orange throated males. This experiment differs from those above in that females in high and low condition still preferred a male phenotype, but differed in their preferred phenotype.
Where else might we expect to find condition-dependent mate choice? The cost of producing some of the sensory organs required to assess males may be high (Jennions and Petrie 1997). We can expect the development of these organs to be condition-dependent and may also be subject to temporary or permanent injury through infection.
For example, Jennions et al. (1994) found condition-dependent sensitivity
to male calls in the African painted reed frogs when carrying out a repeatability of mate choice experiment. Females were subjected to artificial calls from two equidistant speakers. Simulated male calls differed in frequency by 400Hz.
Almost all the females tested preferred the deeper male call. When the difference in male calls was reduced to 200Hz, females overall showed no preference. Individuals who preferred the deeper call were on average of greater body size (measured as snout to vent length). This suggests that the sensitivity of females to the male call be in some way related to body size. In this species, large males have deeper calls (Dyson and Passmore 1988) and so this may be seen as evidence of assortative mating for male size. Dyson et al. (1992) however have
shown that in the wild females do not mate assortatively for male size and so the importance of Jennions et al.'s experiment as an example of condition-dependent
mate choice is unproven.
Females from other species have been shown to have considerable
variation in their sensitivity to male sexual signals. In female katydids the size of a female thoracic spiracles determines auditory sensitivity to male calls (Bailey
1998). Gwynne and Bailey (1999) showed that female katydids have larger spiracles than males and that females with larger spiracles have a pairing
advantage. This trait is probably under direct sexual selection as females compete for nuptial gifts from males and such females will be able to locate males more quickly. No correlation of spiracle size with body size or mass was found so this is not an example of condition-dependent mate choice, but it does emphasise that variation in female sensitivity to male signals may produce assortative mating. If
the development of the sensory organs required for mate choice is costly, we might expect to find condition-dependent mate choice.
From the current literature it appears that female mate preferences can be dependent upon individual condition. We may expect condition-dependent mate preferences to evolve wherever there are significant costs to mate choice.