Atrophic egg is anegg whose function is notreproduction butnutrition; in essence, the trophic egg serves as food for offspring hatched from viable eggs. In most species that produce them, a trophic egg is usually anunfertilised egg. The production of trophic eggs has been observed in a highly diverse range of species, includingfish,amphibians,spiders andinsects. The function is not limited to any particular level ofparental care, but occurs in somesub-social species of insects, the spiderA. ferox, and a few other species like the frogsLeptodactylus fallax andOophaga, and the catfishBagrus meridionalis.

Parents of some species deliver trophic eggs directly to their offspring, whereas some other species simply produce the trophic eggs after laying the viable eggs; they then leave the trophic eggs where the viable offspring are likely to find them.

Themackerel sharks present the most extreme example of proximity between reproductive eggs and trophic eggs; their viable offspring feed on trophic eggsin utero.

Despite the diversity of species and life strategies in which trophic eggs occur, all trophic egg functions are similarly derived from similar ancestral functions, which once amounted to the sacrifice of potential future offspring in order to provide food for the survival of rival (usually earlier) offspring. In more derived examples the trophic eggs are not viable, being neither fertilised, nor even fully formed in some cases, so they do not represent actually potential offspring, although they still represent parental investment corresponding to the amount of food it took to produce them.

Trophic eggs are not alwaysmorphologically distinct from normal reproductive eggs; however if there is no physical distinction there tends to be some kind of specialised behaviour in the way that trophic eggs are delivered by the parents.

In some beetles, trophic eggs are paler in colour and softer in texture than reproductive eggs, with a smoother surface on thechorion.^[1] It has also been found that trophic eggs in ants have a less pronounced reticulate pattern on the chorion.^[2]

The morphological differences may arise due to the fact that mothers invest less energy in the production of trophic eggs than viable eggs.

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The behaviour of trophic egg-laying species depends highly on their environment and can be modified viaadaptive plasticity in response to environmental variation. The ratio of trophic to viable eggs is determined by the availability of resources, although the absolute number of trophic eggs does not always change.^[4]The production of fewer viable eggs ensures that each hatched nymph will have a larger provision of trophic eggs; and therefore give each individual an enhanced chance of survival when external resources are limited. Females can adaptively adjust the egg ratio in response to environmental drivers prior to oviposition.

When resources are limited, the presence of trophic eggs greatly increases the maturation and survival rates of offspring. There are some species such as the subsocial burrower bugCanthophorus niveimarginatus (Heteroptera:Cydnidae) whose offspring cannot survive at all without the provision of trophic eggs. The nymphs starve to death because trophic eggs are the only thing they are able to feed on.^[5] However, when other suitable sources of food are plentiful, feeding on trophic eggs has little effect on brood success.^[6]

Sibling cannibalism, common in many spider species, is not affected by the proportion of trophic eggs, since viable eggs are oviposited and hatch synchronously, before trophic eggs are laid. In the spiderAmaurobius ferox, trophic eggs are laid the day after spiderlings emerge from their egg sac. The mother's reproductive behaviour is modified by the behaviour of her offspring, and their presence inhibits the second generation of eggs from maturing; instead they are released as infertile trophic eggs. Converting the second generation into food for the first ultimately boosts the mother's reproductive success.^[7]

There are no concrete explanations for the evolution of trophic eggs. The two main conflicting arguments are:

• They are an evolved maternal phenotype
• They are simply a failed generation of offspring, produced as a result of reproductive stochasticity.

If they have evolved (and are now distinct) from functionless by-products of failed reproduction, then trophic eggs should bemore easily available and providemore nutrients to the offspring than their evolutionary predecessors. There seems to be clear evidence of this adaptation in many species. This can be seen in mothers making an effort to distribute trophic eggs to their offspring; or eggs which are specialised for the nutritional needs of the offspring. However, in many species, the two types of egg are indistinguishable. Various hypotheses could potentially be tested to determine whether trophic eggs are indeed an evolved phenotype.^[3]

It has been suggested that trophic egg-laying evolved as a consequence oflimited egg size, since larger eggs with more nutrient supply would require the mother to have a larger body size. Thus, the production of more eggs, some of which are not intended to reach maturity. It is relatively simple for the mother to adjust the ratio of fertilised to non-fertilised eggs, in response to environmental conditions.

An alternative to trophic egg-laying is sibling cannibalism; however this requires the mother to regulate thesynchrony of hatching times. However, in this case eggs which are not eaten would continue to develop. If it is difficult for the mother to achieve this synchrony, trophic eggs are a sensible alternative in ensuring that the offspring that hatches will be fed sufficiently.

• Trophic egg-laying is found relatively commonly insub-social insects, one of the most commonly studied being the bugAdomerus triguttulus (Heteroptera:Cydnidae). Nymphs are provisioned with nettle seeds, and the ratio of trophic eggs to viable ones is higher when seeds are less well-developed or in lower quantities, indicating that they are filling the deficit of the alternate food source.^[4]
• Worker bees in manyeusocialstingless bee species,Paratrigona subnuda, have ovarian development and can lay trophic eggs within the brood combs that are later eaten by the queen bee and her progeny, including workers and future queen larvae.^[8]
• Many ant species produce trophic eggs, although in the case ofPachycondyla apicalis (Formicidae:Ponerinae) the trophic eggs are laid by workers and offered to the queen rather than to developing offspring. However this depends on transmission ofpheromones from the queen, since workers lacking contact with the queen may instead start to layreproductive eggs.^[9]
• Some spider species lay a batch of trophic eggs the day after the viable offspring have emerged. This precise timing is based on close interactions between the mother and her offspring, including rotating and drumming behaviour by the mother, which stimulates the spiderlings to climb onto her body at the exact time of release of the trophic eggs. Consumption of trophic eggs can more than double the body weight of the spiderlings, greatly increasing their chances of survival.^[10]
• Some species of frogs produce trophic eggs in the same location as their reproductive eggs. Species such asOophaga (formerly included inDendrobates) lay both types of eggs within water-filled tree holes, bromeliad reservoirs, and pitcher plants; where the trophic eggs provide nutrition for the emerging tadpoles.^[11] Another frog species,Leptodactylus fallax, shows extraordinarily high levels of parental care, with both parents remaining near the burrow, and females feeding each brood a total of 10,000–25,000 trophic eggs, their only source of nutrition.^[12]
• Intrauterine cannibalism is common in theviviparous shark orderLamniformes (commonly known as mackerel sharks). This strategy is effective in applying the mother's available resources to production of the optimal number of offspring viable in theecological niche of a large marinepredator. Such cannibalism may take the form of oophagy (eating sibling eggs) or adelphophagy (literally, eating one's brother, but in context meaning the eating of one's siblings). In either form intrauterine cannibalism certainly minimises waste of nutrient resources, such as surplus eggs in the mother's reproductive system, and it prevents the development of excessive numbers of small offspring, rather than a small number of vigorously competitive young sharks. It has been speculated to ensure the survival of only the fittest offspring.^[13] However, that speculation has no substance, simply because there is no systematic genetic difference, and hence no systematic difference in geneticfitness, between embryos produced at various times and places in the mother's reproductive system. An embryo that in genetic terms is poorly fit, but nearly mature, would have no difficulty in eating a fit egg or much younger sibling embryo, no matter how genetically fit they might be.

• Barlow, G W (2002). "Chapter 2 - Jaws Two".The cichlid fishes: nature's grand experiment in evolution. Basic Books, Nature.
• Cole, K S (2010). "Chapter 1 - Chondrichthyan Reproduction".Reproduction and Sexuality in Marine Fishes: Patterns and Processes. University of California Press.
• Polis, G (2008). "Chapter 5 - Intraspecific predation and "infant killing" among invertebrates". In Hausfater, G; Hardy, S (eds.).Infanticide: Comparative and Evolutionary Perspectives. Transaction Publishers.ISBN 9780202366838.
• Perry, J C (2004).The behavioural ecology of trophic egg-laying. Simon Fraser University.
• Capinera, J L (2008). "Cannibalism".Encyclopaedia of Entomology. Springer.