Stalhandske performed 82 trials in which male Pisaura mirabilis were given no gift, or small, medium, or large size gifts to offer females. For hypothesis 3 to be supported, males without gifts would be expected to approach females, and females to act more aggressive toward mates without a gift. Results showed just the opposite. Males without gifts attempted copulation, and the presence of gifts enhanced female aggression (they were violent only towards males with gifts) (Stalhandske 2001). However, females only mated readily with males procuring gifts (90% mated) and avoided coupling with non-gift givers (40% mated). Additionally, gift size was positively correlated with sperm transfer duration and confidence in paternity (Stalhandske 2001). Thus, sexual cannibalism is an ingrained part of the mating interaction. It occurs when mating occurs, and is not offset by a nutritional proxy. Instead of offering protection, nuptial gift giving represents male mating effort, a tactic for tempting the female, accessing the copulation site, and increasing copulation time (Stalhandske 2001).
Glossary
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Observation Within a Species Hypothesis Testing - A method of testing a hypothesis in which organisms of a single species are monitored, often in a field setting, to determine if differences between individuals have adaptive significance. This method demands extensive knowledge of the organisms’ habits and attributes.
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Confidence in Paternity - When females mate with more than one male in a breeding season (polyandry), the sperm donated by a single male is in danger of being supplanted by that of a later rival. A male’s confidence in fathering the offspring of the female decreases as her number of mates increases. Male spiders may try to increase their confidence in paternity by allowing themselves to be eaten, which may increase time of sperm transfer (Andrade 1996).
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Cost-benefit approach - An adaptively valuable trait must have a higher ratio of fitness benefits to fitness costs than alternate versions of that trait. This approach acknowledges that phenotypes are associated with varying costs and benefits to the individual as their genes interact with their environment (Humphries 2003).
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Experimentation Hypothesis Testing - A method of testing a hypothesis in which a researcher methodically varies an environmental factor (explanatory variable) and observes its effects on animal behavior (responding variable). This technique only applies to behaviors that can alter in response to manipulated external variables. Experimentation is used on behaviors that are immediately traceable; it is a short-term method that does not track the evolutionary response to change.
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Fecundity - An individual’s reproductive capacity. The number of viable, surviving offspring a female produces is a function of her fecundity.
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Foraging Strategy Hypothesis - The evolution of sexual cannibalism in some species is explained by its fitness benefits to females. Females of species having low to moderate sexual size dimorphism improve their fecundity and reproductive yield through eating the male soma (Barry et al. 2008).
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Intersexual conflict - Reproductive strategies of males and females usually differ due to incongruence in gamete size and number (females inherently invest more gamete resources in the common young). Each parent wants to produce the greatest number of surviving offspring while minimizing costly resource investment. This imposes asymmetric costs of reproduction on the sexes, resulting in conflict within mating interactions (Schneider 1998).
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Mate size dimorphism (MSD) - Difference in size between a pair of male and female individuals (Wilder and Rypstra 2008).
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Natural Selection - The process that occurs when members of a species vary in their traits due to genetic differences, and an individual’s unique traits cause them to have more surviving offspring than others in the population. Surviving offspring inherit parents’ favorable traits, which are then sustained or evolved in future generations.
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Nuptial Gift - Nutrient resources supplied to females by courting males prior to, during, or shortly after copulation (Stalhandske 2001). See
[link] for further information.
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Pedipalp - A pair of extremities located posterior to the fangs that serves as a copulatory organ in spiders. Each pedipalp inserts into one of the females paired genital openings and injects sperm into a female storage organ (Foellmer and Fairbairn 2003).
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Paternal Investment - Activities conducive to producing and rearing surviving offspring which are costly to the female or male parent and reduce their ability to produce offspring in the future. Include nutrient (food), energy, time, and risk-taking forms of investment (Schneider and Lubin1998).
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Proximate cause - An immediate cause, based on the operation of an individual’s physiological mechanisms. Proximate causes concern the biological structure of an animal that enables it to behave in a particular way.
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Secondary Sexual Characteristics - Traits that are unique to a particular sex in a species, but are not used directly in mating and reproduction. These characteristics are often the result of sexual selection for traits that improve the desirability or competitiveness of the mate.
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Sexual Cannibalism - The consumption of a male by a female in the context of mating. Females consume courting males before, during, or immediately after mating. Occurs prominently in arachnids, insects, and amphipods (Wilder and Rypstra 2008).
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Sexual Dimorphism - A difference between traits of males and females in a species.
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Sexual Selection - The process that occurs when members of a species vary in their ability to compete with others for mates or attract individuals of the opposite sex. When this variation is due to genetic differences, sexual selection drives genetic changes in the population. It is a form of natural selection.
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Sexual size dimorphism (SSD) - Mean difference in size between males and females for a species (Wilder and Rypstra 2008).
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Spandrel - When selection in one context has implications for another context. A trait is viewed as an incidental by-product of selection on suites of correlated traits. The concept of spandrel is invoked as an alternative to adaptionist explanations for the evolution of traits (Gould 1997). Originates from an architectural term meaning a space between two arches, or between an arch and a rectangular enclosure.
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Sperm Competition - When a polyandrous female has accepted sperm from multiple males, sperm competition determines the portion of her eggs fertilized by each male. The extent of the male’s paternity is positively related to the amount of sperm he transfers. A male can increase his chances in sperm competition through behaviors like mate guarding (seen in harems of elephant seals, for example), increasing copulation time, or doubly inserting palpal emboli. See
[link] for more information.
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Scramble Competition - Polygynous males (that is, males who fertilize several females’ eggs in a given reproductive season) compete for widely dispersed, more sedentary females by “scrambling” over long distances to find them first (Vollrath and Parker 1992). The successful male is determined more by his fortune to be in the right place at the right time, than by particular male characteristics.
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Ultimate cause - Evolved causes that are based on the past and current usefulness of the behavior in promoting lifetime fitness and reproductive success. Contrasts with
proximate causes which are the physiological mechanisms resulting in an act.
