Egg phenotype matching by cuckoos in relation to discrimination by hosts and climatic conditions

Photoprotective egg pigmentation reduces negative carryover effects of ultraviolet radiation on stink bug nymph survival

Solar ultraviolet radiation (UV) can have a wide range of negative effects on animal fitness that take place not only during, but also after exposure (carryover effects). UV-induced carryover effects and potential adaptations to avoid or mitigate them are understudied in terrestrial animals, including arthropods and their potentially most vulnerable life stages. The spined soldier bug, Podisus maculiventris, increases the emergence of its eggs that are exposed to UV radiation by coating them in sunscreen-like pigmentation, but consequences of these conditions of embryonic development for nymphs and adults are unknown. We measured stink bug nymph survival, adult size and sex ratio following exposure of differently pigmented eggs across a range of UV intensities. Nymph survival to adulthood decreased with higher intensity of embryonic UV exposure and this carryover effect decreased with higher level of egg pigmentation, similar to previously observed effects on embryonic survival. Nymph development time, adult size and sex ratio were not affected by embryonic exposure to UV radiation nor by photoprotective egg pigmentation. This study is the first to demonstrate the potential for lethal carryover effects of UV radiation in terrestrial insects, highlighting the need for more studies of how this pervasive environmental stressor can affect fitness across life stages.

Fruit colour conceals endocarp dimorphism from avian seed dispersers in a tropical beach plant, Scaevola taccada (Goodeniaceae), found in Okinawa

Theory predicts that honest signalling strategies will not always be evolutionarily stable in interspecific communication, yet to demonstrate such a transition of signalling modality between honesty and dishonesty in the wild would be difficult. An endocarp dimorphism has been found in Scaevola taccada fruits: a morph with a cork substrate that facilitates ocean current seed dispersal and a morph without the cork. Both types of fruit are covered with sugar-containing flesh, and are similar in size and colour to one another (at least from a human perspective). The apparent lack of external differences between morphotypes could potentially degrade mutualistic relations between the plant and seed-dispersing birds because the presence of a cork could lower the fruit's nutritional value. Thus, unless seed dispersers can discriminate between the different types of fruit, this system may provide an example of a transition between honest and dishonest signalling. We examined S. taccada fruit and leaf colours from an avian visual perspective. Even though the fruits and leaves were different in colour from one another to birds, there was no perceivable difference in the colours between fruit morphotypes. Therefore, fruit colour is not an honest indicator of reward to seed dispersers. Further, we propose an adoption of a statistical method in avian visual modelling studies that avoids the common statistical errors, such as violation of the congruence principle.

Embryonic learning of vocal passwords in superb fairy-wrens reveals intruder cuckoo nestlings

How do parents recognize their offspring when the cost of making a recognition error is high? Avian brood parasite-host systems have been used to address this question because of the high cost of parasitism to host fitness. We discovered that superb fairy-wren (Malurus cyaneus) females call to their eggs, and upon hatching, nestlings produce begging calls with key elements from their mother's "incubation call." Cross-fostering experiments showed highest similarity between foster mother and nestling calls, intermediate similarity with genetic mothers, and least similarity with parasitic Horsfield's bronze-cuckoo (Chalcites basalis) nestlings. Playback experiments showed that adults respond to the begging calls of offspring hatched in their own nest and respond less to calls of other wren or cuckoo nestlings. We conclude that wrens use a parent-specific password learned embryonically to shape call similarity with their own young and thereby detect foreign cuckoo nestlings.