Scott K. Sakaluk, Professor of Zoology

RESEARCH INTERESTS

i) Evolution of Post-copulatory Female Choice

Sexual differences in behavior, morphology and ornamentation are ubiquitous throughout the animal kingdom and are thought to arise through the action of sexual selection. Such traits evolve because of the advantages they confer on individuals in competing for mates or because they render the individuals that bear them more attractive to members of the opposite sex. However, sexual selection does not cease once copulation has been completed. In many species of insects, females are well positioned to determine the fate of their mates’ gametes through various means, including control of copulation duration, premature removal of spermatophores, and internal manipulation of ejaculates. Because it is in the interests of males to have all of their sperm utilized in fertilizations, female control of sperm transfer and usage is often at odds with the reproductive interests of their sexual partners. Theoretically at least, such control represents a potent selective force: a wide array of male secondary traits in the insects have been attributed to selection on males to circumvent post-copulatory female preferences.

Crickets offer an ideal model with which to examine the evolution of post-copulatory female choice: the ejaculate of a male typically remains attached outside the female’s genital opening after mating in the form of an externally attached spermatophore, and females are thus well positioned to determine the fate of their mates’ gametes through the judicious removal of spermatophores. In decorated crickets, the spermatophore includes a large gelatinous mass, the spermatophylax, which the female detaches and feeds on after mating. While the female consumes this nuptial food gift, sperm are evacuated into her reproductive tract from the remaining portion of the spermatophore, the sperm ampulla. Smaller spermatophylaxes require less time to consume, and males providing such gifts are penalized in the form of premature ampulla removal and reduced sperm transfer. In studies supported by the National Science Foundation, I tested the hypothesis that post-copulatory manipulation of ejaculates by female decorated crickets constitutes a potent form of female choice, imposing significant sexual selection on males. By experimentally altering sperm transfer of rival males and using genetic markers to assess the paternity of offspring, I was able confirm that female ejaculate manipulation, manifest in the removal of the externally attached spermatophore, directly affects male fitness. This work helped establish that nuptial food gifts function to entice females into relinquishing at least some of their control of the insemination process, and thereby aid males’ own selfish reproductive interests.

ii) Cryptic Sexual Conflict In Gift-Giving Insects: Chasing The “Chase-Away”

Holland and Rice (1998) recently proposed a new hypothesis to account for the evolution of elaborate male sexual displays, which incorporates a conflict between the sexes over mating rate. According to their model, display traits initially arise in males because they exploit preexisting sensory biases in females and consequently induce females to mate in a sub-optimal manner. This in turns selects for female resistance or decreased attraction to the trait, which in turn leads to greater selection on males to exaggerate the display trait to overcome this resistance. The resultant cycle of antagonistic coevolution forms the basis of the “chase-away” model of sexual selection.

Nuptial food gifts, an integral feature of the mating systems of a wide variety of insects, may be a frequent conduit through which males attempt to influence the mating behavior of females against females’ own reproductive interests. Such gifts may arise as a form of sensory trap that exploits the normal gustatory responses of females, favoring the selective retention of sperm of gift-giving males. In previous work, I tested this hypothesis by offering foreign food gifts, synthesized by males of one cricket species, to females of three non-gift-giving species. Females provisioned with novel food gifts were “fooled” into accepting more sperm than they otherwise would in the absence of a gift. These results provide support to the incipient stage of Holland and Rice’s (1998) chase-away process as it applies to the evolution of food gifts, namely, that display traits first arise in males because they exploit preexisting sensory biases in females.

More recent work in my laboratory suggests that food gifts may contains substances that at one time inhibited the sexual receptivity of females, but that females have evolved resistance to these substances. The conflict over female receptivity only became apparent when food gifts of a gift-giving species, Gryllodes sigillatus, were fed to females of a non-gift-giving species, Acheta domesticus. Female A. domesticus allowed to consume food gifts of male Gryllodes took significantly longer to remate than when given no such opportunity. In contrast, the consumption of food gifts appears to have no comparable effect on the propensity to remate in female Gryllodes. I suggest that the reason why females of the non-gift-giving species show a reduction in sexual receptivity is that, having had no evolutionary experience with food gifts, they have been under no selection to evolve any kind of immunity to the receptivity-inhibiting substances contained in food gifts. If, as I suggest, the evolution of food gifts is explicable within the context of the chase-away model of sexual selection, it requires that: 1) males benefit by inducing a delay in remating by their mates and 2) that females suffer a reduction in fitness from any such delay. Current research in my lab is directed at testing these two critical predictions.

Recent Publications:

Sakaluk, S.K., Avery, R.L. and C.B. Weddle. 2006. Cryptic sexual conflict in gift-giving insects: chasing the chase-away. American Naturalist 167: 94-104.  PDF reprint

Sakaluk, S.K., Campbell, M.T.H., Clark, A.P., Johnson, J.C. and P.A. Keorpes. 2004. Hemolymph loss during nuptial feeding constrains male mating success in sagebrush crickets. Behavioral Ecology 15: 845-849. PDF reprint

Sakaluk, S.K., Schaus, J.M., Eggert, A.-K., Snedden, W.A. and P.L. Brady. 2002. Polyandry and fitness of offspring reared under varying nutritional stress in decorated crickets. Evolution 56: 1999-2007.  PDF reprint

Sakaluk, S.K. 2000. Sensory exploitation as an evolutionary origin to nuptial food gifts in insects. Proceedings of the Royal Society of London B, Biological Sciences 267: 339-343.   PDF reprint

Sakaluk, S.K. and T.M. Ivy. 1999. Virgin-male mating advantage in sagebrush crickets: differential male competitiveness or non-independent mate choice? Behaviour 136: 1335-1346.   PDF reprint