Sexual cannibalism
Encyclopedia
Sexual cannibalism is a special case of cannibalism
Cannibalism (zoology)
In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species...

 in which a female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

 organism kills and consumes a male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 of the same species before, during, or after copulation. On rare occasions, these roles are reversed.

Prevalence

Sexual cannibalism has been observed in arachnid
Arachnid
Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...

s, insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, and amphipods, with some evidence for its occurrence in gastropods, copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, and cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s. Though it is rare overall, sexual cannibalism is common in most families of spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s and scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

s, and can affect population size
Population size
In population genetics and population ecology, population size is the number of individual organisms in a population.The effective population size is defined as "the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under...

 and sex ratio
Sex ratio
Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....

. In most species in which it occurs, the female usually cannibalises the male, due to the large size of the female.

The significance of sexual cannibalism has been played down by some. Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

 argued that sexual cannibalism was too rare to be significant. However, males can be a significant food source for females, ultimately leading to increased fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

.

Evolution and maintenance

Adaptive foraging hypothesis – Sexual cannibalism offers adult females direct, material benefits as part of an adaptive foraging strategy. An economic model showed that sexual cannibalism can be explained as the result of adaptive tradeoffs between the nutrient payoffs of cannibalising a male and the fertility benefits of mating with a male.

There are multiple predictions that can be derived from the adaptive foraging hypothesis. For a starving female, a courting male's value as food is higher than his value as a potential mate. As a result, hungry females are more likely to cannibalise than gorged females. Next, the female's previous mating status should affect the likelihood of pre-copulatory – but not post-copulatory – sexual cannibalism. Virgin females should be less likely to attempt to cannibalise males before sperm transfer than mated females. Next, the male's body mass determines his value as a meal, and thereby influences the chance of sexual cannibalism. Lastly, if pre-copulatory sexual cannibalism is an adaptive foraging method at the cost of mating, precopulatory attacks should be more successful than post-copulatory attacks, in a way that an attack is followed by a meal; otherwise, the most gainful approach for a female would be to copulate to secure sperm and then kill the male afterwards for food.

Aggressive spillover hypothesis – Sexual cannibalism may be the result of a "spillover" of aggression from juvenile life stages. High levels of aggression may be selectively advantageous in juveniles
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

, giving them the ability to hunt and consume more prey and thereby grow more rapidly. If a high level of aggression is still present in adulthood, it can interfere with an individual's ability to mate and lead to sexual cannibalism. Positive behavioural correlations for aggression levels can develop across ontogenetic stages (juvenile-adult), as well as across behavioral contexts (foraging-mating). Natural selection then acts on the collection of correlated aggressive behaviours, instead of enhancing aggressive levels in each individual context. The term "behavioural syndrome" describes such collections of correlated behaviours. Thus, this hypothesis represents an example of how a behavioural syndrome may produce behaviours that appear suboptimal when observed in an isolated context. It also predicts that females with an aggressive feeding history should grow larger and heavier than less aggressive females and are more likely to be cannibalistic. Some confirmation of the aggressive spillover hypothesis comes from a study of fishing spiders (Dolomedes triton
Dolomedes triton
The six-spotted fishing spider, Dolomedes triton, is an arachnid from the nursery web spider family Pisauridae. This species is from the genus Dolomedes, the fishing spiders. This species of fishing spider is named after the mythological Greek god Triton who is the messenger of the big sea and the...

).

Mate choice – Females may cannibalise unwanted males as a method of mate rejection or use cannibalism to regulate the timing of copulation. Direct mate choice
Mate choice
Mate choice, or intersexual selection, is an evolutionary process in which selection of a mate depends on attractiveness of its traits. It is one of two components of sexual selection...

 via sexual cannibalism is a result of a specific discrimination by females, often by size, since size is strongly correlated with a variety of fitness characteristics. This is inferred from a female's diminished aggression towards certain males and increased aggression towards others. For example, studies have shown that adult female wolf spiders, Schizocosa uetzi, prefer to mate with mates of a familiar phenotype. Males of an unfamiliar phenotype were more likely to be cannibalised. Researchers investigating direct mate choice suggest that: (a) females should react differently to some conspecifics (discriminate between them), (b) the discrimination should occur under natural conditions, (c) discrimination by the female between dissimilar males should cause differential reproductive success for males, (d) the female bias should be related to specific male morphological or behaviour traits, and (e) the variation among males in traits that are used by females to discriminate should be heritable
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...

.

Mistaken identity – The simplest explanation of sexual cannibalism is that it is a case of mistaken identity, in which a male is not acknowledged as a potential mate in time. However, in most spiders, elaborate courtship
Courtship
Courtship is the period in a couple's relationship which precedes their engagement and marriage, or establishment of an agreed relationship of a more enduring kind. In courtship, a couple get to know each other and decide if there will be an engagement or other such agreement...

 is thought to prevent this.

Polyandry advantage hypothesis – Polyandry is a mating system
Mating system
A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to higher animals, it specifies which males mate with which females, under which circumstances; recognised animal mating systems include monogamy,...

 in which one female mates with more than one male. The polyandry advantage hypothesis predicts that females will attempt to prevent male monopolisation of their eggs through sexual cannibalism.

Environmental and evolutionary circumstances – Spider species vary extensively in the degree of sexual size dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

, which is a result of variation among species in the strengths of fecundity selection acting to increase female size and viability selection, along with other selective pressures acting to maintain or decrease male size. As a result, this degree of sexual size dimorphism has consequences for sexual cannibalism, since it will affect male vulnerability to female attacks.

Male self-sacrifice

In some cases, sexual cannibalism may characterise an extreme form of male monogamy
Monogamy
Monogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...

, in which the male will sacrifice itself to the female. Males may gain reproductive success from being cannibalised by either providing nutrients to the female (indirectly to the offspring), or through enhancing the probability that their sperm will be used to fertilise the female's eggs. Although sexual cannibalism is fairly common in spiders, male self-sacrifice has only been reported in six genera of araneoid spiders
Araneoidea
The Araneoidea are a superfamily of araneomorph spiders. They contain families of eight-eyed spiders:* Anapidae* Araneidae* Cyatholipidae* Linyphiidae* Mysmenidae* Nephilidae* Nesticidae* Pimoidae* Sinopimoidae* Symphytognathidae* Synaphridae...

. In the most studied example, the Australian redback widow spider (Latrodectus hasselti, Theridiidae), males position their abdomen in front of the female's chelicerae
Chelicerae
The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...

 in a somersault behaviour and are consumed during copulation; sexual cannibalism occurs in about 65% of matings. Relating to the adaptive self-sacrifice theory, cannibalised redback males double their paternity compared to rival males. In addition, male orb-weaving spiders of the species Argiope aemula and Argiope aurantia, which are sexually cannibalistic, die immediately after mating, even if they are not attacked. Since they appear to expect no future mating success, they may have nothing to lose by being cannibalised, making the benefits of sexual cannibalism outweigh the costs.

Reversed sexual cannibalism

Reversed sexual cannibalism may transpire primarily in species in which males are larger than females. King cobras do feed on other snakes apart from rodents and other natural prey of snakes. Male king cobras are larger in size than their female counterparts and may consume the female or mate with them. Male amphipods of the species Gammarus pulex
Gammarus pulex
Gammarus pulex is a species of amphipod crustacean found in fresh water across Eurasia. It is a greyish animal, growing to long.-Description:...

and G. duebeni celticus may eat smaller females, especially at the moult
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

, when they are most vulnerable. Likewise, female paddle crabs, Ovalipes catharus
Ovalipes catharus
Ovalipes catharus is a species of crab of the family Portunidae. It is found around the coasts of New Zealand, the Chatham Islands, and in south-eastern parts of Australia. Individuals from shallow waters, deep, have a carapace width of only , while those from are wide....

, are also defenseless at the moult when their shell is soft, and may be cannibalised during or after mating. In Gammarus
Gammarus
Gammarus is an amphipod crustacean genus in the family Gammaridae. It contains more than 200 described species, making it one of the most speciose genera of crustaceans...

species, sexual cannibalism would appear to be naturally opportunistic
Opportunism
-General definition:Opportunism is the conscious policy and practice of taking selfish advantage of circumstances, with little regard for principles. Opportunist actions are expedient actions guided primarily by self-interested motives. The term can be applied to individuals, groups,...

 and not have any mate choice consequences.

Evidence has also been found of a male eating a female in the mantid Tarachode afzeli. Males of the water spider Argyronetia aquatica are larger than females, prefer to mate with larger females, and may cannibalise females that are smaller than themselves. Females prefer to mate with larger males, but are more cautious of them; they flee from larger males more often than from males that are closer to their own size. The vulnerability to sexual cannibalism is a key in A. aquatica and would propose that an indirect mechanism, instead of direct mate choice, yields the size-dependent reversed sexual cannibalism that is observed. While the presence of the male-biased size dimorphism and the relative size of the female determining her vulnerability, smaller females are likely to be cannibalised. In all cases of reversed sexual cannibalism, there appears to be little hope that the female will produce viable offspring for the male, or the need for food exceeds the fitness gain that can be achieved through mating. Since most males will tend towards promiscuity
Promiscuity
In humans, promiscuity refers to less discriminating casual sex with many sexual partners. The term carries a moral or religious judgement and is viewed in the context of the mainstream social ideal for sexual activity to take place within exclusive committed relationships...

 in order to maximise their reproductive output, it is difficult to visualise mate choice through sexual cannibalism for males, unless the sex ratio is strongly female-biased, offering the males the opportunity to be choosy or the sex roles are reversed.

See also

  • Interlocus sexual conflict
    Interlocus sexual conflict
    Interlocus sexual conflict is a type of sexual conflict that occurs through the interaction of a set of antagonistic alleles at one or more different loci in males and females, resulting in the deviation of either or both sexes from the fitness optima for the traits.Interlocus sexual conflict...

  • Evolutionary arms race
    Evolutionary arms race
    In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...

  • Sexual conflict
    Sexual conflict
    Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...

  • Traumatic insemination
    Traumatic insemination
    Traumatic insemination, also known as hypodermic insemination, is the mating practice in some species of invertebrates in which the male pierces the female's abdomen with his penis and injects his sperm through the wound into her abdominal cavity . The sperm diffuse through the female's hemolymph,...

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