Nuptial gift

Nuptial gifts are food items or inedible tokens that are transferred to females by males during courtship or copulation. Inedible tokens may include items such as a fragment of leaf or twig, a seed tuft, or a silk balloon.^[1] In many species of animals, including birds, insects, and spiders, this takes the form of a food item that is transferred from a male to a female just prior to copulation.^[2] This is a behaviour known as courtship feeding.^[3][4] In some species of insects such as katydids, the nuptial gift is packaged with the sperm of the male. The package is an edible spermatophore. These extra nutrients in the sperm are assimilated by the female and are thought to enhance the fitness of the offspring produced, thus increasing the probability that a male passes on its genes. Nuptial gifts may therefore be classified into oral and seminal gifts.^[5] Nuptial gifts may also be tokens that do not have any direct value as food but may serve as an indicator of male fitness.^[6][7][8]

Occurrence

Nuptial gifts are common in insects and other invertebrates, such as butterflies, fruit flies and katydids. They are less common in spiders, though the spider species Pisaura mirabilis is known for nuptial gift giving.^[9] However, this may in part be due to an experimental bias due to the ease of rearing it for experiments.^[10] Other taxa may commonly exhibit gift-giving behavior. Thus, more research is required to evaluate the scope of nuptial gifts in arachnid taxa. In many species of insects, birds and mammals, males acquire and donate food to females either before, during, or after copulation (termed mate provisioning, courtship feeding, nuptial gift giving, or meat-for-sex). Males may relinquish body parts, produce glandular secretions, or share prey or other food to gain fitness benefits via natural and sexual selection.^[11]

Courtship feeding is particularly common among many bird species.^[12]

Vertebrates

Great grey shrike

The male great grey shrike, a raptor-like passerine bird, gives prey (rodents, birds, lizards, or large insects) to females immediately before copulation. Shrikes are well known for impaling prey on thorns and sharp sprigs. Great grey shrike females select a mate according to the size of prey impaled, with larders thus serving as an extended phenotype of a male. If the amount of food stored by the males can drive female mate choice, food provided by males before copulation may also influence the female's decision to copulate. This applies both to the male's own partner and to other females.^[13]

Invertebrates

Arachnids

Pisaura mirabilis on Plantago lanceolata

Pisaura mirabilis

In spiders, nuptial gifts in the form of prey are restricted to a few species from two families belonging to the superfamily Lycosoidea: Pisauridae and Trechaleidae. In both families, the male courts the female by offering a prey wrapped in silk and mating occurs while the female consumes the gift. In the species Pisaura mirabilis (Pisauridae) the gift functions as a mating effort that increases male mating success. The nuptial gift consists of a prey the male has caught and wrapped up in silk. The male offers this gift during courtship, and if the female accepts the invitation she grabs the wrapped prey. While the female is eating, the male inserts the mating organ and sperm is transferred.^[14]

A similar function was recently suggested for the trechaleid spider, Paratrechalea ornata. In both species, males can obtain mating without a gift, but male mating success increases dramatically when a gift is offered. In Pisaura mirabilis, the male pushes up the female during mating and performs alternate pedipalp insertions into the female sperm storage organs place ventrally on the abdomen. After each insertion the male returns to a face-to-face position with the female, grabbing the gift in the chelicerae. Females often control mating duration and they often attempt to run away with the gift upon terminating the copulation. ^[15]

Females are less likely to succeed in stealing a wrapped rather than an unwrapped gift due to the silk wrapping of the gift. The silk wrapping facilitates male handling and control over the gift, as it facilitates a stronger hold of the silk covered package versus an unwrapped insect. Male spiders have a unique opportunity for gift manipulation through the gift wrapping trait, for example by preventing female assessment of the gift content. By disguising the gift content, males may deceive females to copulate, while the female attempts to consume the gift. In Paratrechalea ornata, males were observed wrapping prey carrion and occasionally inedible items such as plant seeds. In Pisaura mirabilis, males have been reported to carry gifts containing empty arthropod exoskeletons or plant parts, gifts of no nutritional value.^[15] However, some studies have shown that male spiders rarely cheat in nature.^[10] Cogent reasons for this disparate behavior are being explored. It is possible there are confounding factors which do not account for ecological implications. Other research suggests that sensory bias is not the main contributing force behind gift selection in P. mirabilis. Interestingly, females did not preferentially select unwrapped gifts over wrapped gifts.^[16]

Paratrechalea ornata

In the spider species Paratrechalea ornata, a Neotropical spider belonging to the Trechaleidae spider family, nuptial gift behavior was recently seen. Paratrechalea ornata males may present either a wrapped or an unwrapped nuptial prey gift. Prey wrapping seems to be triggered by perception of cues on the female's silk and increases in frequency according to the male's age.^[15]

Decorated crickets

In decorated crickets, Gryllodes sigillatus (Orthoptera: Gryllidae), the nuptial food gift is a spermatophylax, a large, gelatinous, sperm-free mass that surrounds a smaller, sperm-containing ampulla. Together, the spermatophylax and the ampulla constitute the male’s spermatophore, which is transferred to the female during copulation and remains attached outside her body at the base of her ovipositor. After mating, the female detaches the spermatophylax from the ampulla and feeds on it while the ampulla remains attached and is emptied of sperm.^[17] Once the female has consumed or discarded the spermatophylax, she removes the sperm ampulla, terminating sperm transfer. Female cooperation is required for successful spermatophore transfer, and thus, males cannot impose copulations on females.^[18]

Bushcrickets

Bushcricket males offer a spermatophylax that contains ampulla. The nuptial gift is also protein rich, which the females ingest into their reproductive tract. The size of the nuptial gifts positively influences the females refractory period and the males reproduction success. The size of the gift depends on the ampulla and serves as a sperm protection. In bushcrickets Ephippiger ephippiger the females prefer older males, who have larger spermophores and better nutritional value during mating. The nutritional value is related to the female's metabolism, which stands as a benefit for females feeding on the semaphores. Sometimes males produce lower dosages of sperm with a lower nutritional value the fourth time they mate.^[19] It may be possible the females are using age and gift quality as a proxy for mates with good genes as their offspring are likely to have high relative fitness.

Ornate Moth

During mating in the Ornate Moth (Utetheisa ornatrix), males provide the female with a spermatophore containing nutrients, sperm and alkaloids that serve as chemical defense from predators. Such nuptial gift accounts for up to 10% of the male's body weight and represents the total parental investment the male provides.^[20] Females receive spermatophores from several males and direct a postcopulatory selection process in which they decide what sperm will fertilize their eggs.^[21]

Six-spot Burnet

Six-spot Burnet

Nuptial gifts are widespread in insects such as the six-spot burnet (Zygaena filipendulae), and comprise food items or glandular products offered as paternal investment in offspring and/or to promote mating. Female Zygaena may use this gift for her own defense and to protect her eggs.^[22]

Scorpionfly

Scorpionfly with prey

In the scorpionfly (Panorpa cognata), males offer a salivary secretion as a nuptial gift before copulation. Exchange of the salivary secretion takes place after prolonged courtship interactions. The nuptial gift is more likely to be accepted by the female if premating duration is long. Males in poor condition with a limited supply of saliva may deliberately delay initiating copulations to decrease the probability that their costly gift is rejected and, thus, wasted. Males in good condition with ample mating resources, on the other hand, may afford the risk of wasting a salivary mass and therefore take every opportunity to mate.^[23]

Evolution, costs and benefits

Male sperm offers many nutrients to increase a female's lifetime and egg production. These nutrients include acids or sodium. Male sperm also protects females and their eggs from predators. The females would also gain a net benefit from a male's sperm. Female recipients are supposed to produce bigger offspring than those females who did not receive nuptial gifts. The eggs of female spiders who receive nuptial gifts may hatch at a higher rate than those without nuptial gifts.^[16] Historically, nuptial gifts were seen as nutritional substances given to females from males during mating.^[24] The male benefits from a net fitness. Although the gifts are costly to find or produce, the gifts will increase attraction and copulation with other females. One of the more recently identified costs to males is reduced running speed due to gift-carrying.^[10] This cost may be exacerbated in areas of high predation. Nuptial gifts can benefit a male by increasing his paternity share when females are promiscuous. In some insects, nuptial gifts allow the male to copulate longer and transfer more sperm to the female.^[5] In fruit flies, katydids, and scorpion flies, nuptial gifts contain substances that reduce a female’s receptivity to additional matings. While nuptial gifts also may boost female fecundity, from a male’s perspective, such investment will only be beneficial if it increases the number of his own offspring.^[8] In bell crickets, nuptial gifts may be necessary to avoid injury or death by cannibalizing females.^[25] This additional benefit allows gift-giving males to surpass the fitness of other males. In this way, females are exploiting the inherent sexual dimorphism of their species.

In species where males provide females with a nuptial gift during mating, there is a particular scope for males to manipulate females to acquire matings and prolong copulation to enhance their fertilization success. Typically, females control the duration and volume of sperm transfer throughout the mating process. Research has suggested that the gifts presented by males temporarily obstruct the female's capacity to manage the copulation event.^[26] Female choice for males with nuptial gifts could lead to the evolution of male deception by the use of toke gifts. For instance, males can decrease the costs of mating by re-using gifts or by offering worthless gifts. Males of some dance flies may deceive females by offering inadequate or false gifts. Although males that offer inedible gifts run a higher risk of being rejected and may suffer from shorter mating periods compared to males offering edible gifts the chance of acquiring an extra mating should make deception an attractive strategy for males. Hence, males of the dance fly Rhamphomyia sulcata that use inedible token gifts to obtain mates are as successful as males offering small genuine gifts.^[15]

When nuptial gifts are given it increases copulations and searches to find females to mate with. Albo and Costa conducted an experiment with Paratrechala ornata spiders to determine the function of the nuptial gift. Two groups of males were exposed to virgin females, 23 males with no nuptial gift (prey) and 21 males with a nuptial gift (prey). Mating ovulation were recorded and the researchers found that males that gave a nuptial gift had better mating success, longer copulations, and longer palpal insertions than those who did not give an nuptial gift. Longer copulations were associated with earlier egg sac construction and oviposition. The researchers' findings suggest that nuptial gift giving represents male mating effort for P. ornata. Nuptial gifts would allow males to control copulation duration and to accelerate female oviposition, improving sperm supply and paternity, and minimizing possible costs of remating with polyandrous individuals.^[15] Additionally, Prokop found that female mate choice is dependent on nuptial gifts rather than female reproductive status as an unmated or mated individual.^[27] This effect demonstrates sexual selection's ability to make one sex more discriminatory than the other, since females may negatively impact their output of offspring by refusing mating events with males that do not offer gifts.

References