Biparental care of eggs or young is uncommon in the animal kingdom although it does occur among insects, fishes, amphibians, birds and mammals [1–4]. Biparental care, however, is a common behaviour in certain groups of animals: for instance 40% of cichlid fish genera and 32% of primate species are biparental . These species provide excellent opportunities to investigate how and why biparental care evolves, and to tease apart the roles of parental investment, sexual selection and conflicts in breeding system evolution [sensu-[6–9]].
Two major groups of hypotheses have been proposed to explain biparental care [reviewed by [2, 10, 4]]. On the one hand, both parents may be essential for successful rearing of the young; the parents may need to share incubation, brood defence or protection of the territory in order for the young to survive [11, 12]. Biparental care may be essential if parents breed in resource poor environments, or the physical environment is harsh and challenging . Experimental removal of one parent (usually, the male) supports the hypothesis that biparental care provides direct benefits by enhancing offspring survival, and/or by putting less strain on the female [2, 14–17]. On the other hand, parents may benefit in future from staying together and sharing care provisioning ; for instance by keeping their partner for future matings and therefore avoiding the costs related to finding/attracting a new mate. Staying with the mate and helping him/her might be particularly beneficial if there are few opportunities for finding a new mate .
Biparental care is particularly common among birds: approximately 50% of bird species have biparental incubation and/or brood care [11, 12, 19, 20]. Although in these species the parents cooperate to rear the young, there are also elements of conflict because the benefit of care, i.e. the offspring, is shared between biological parents whereas each parent pays the cost of care itself. Therefore each parent prefers the other to invest more resources in rearing the young [sexual conflict over care [21, 22, 7, 23]].
Incubation is essential for successful reproduction in nearly all bird species, because eggs require heat for embryonic development, and the incubating parent can defend the clutch from potential predators [24, 19]. However, incubation can be costly to the parents because it demands time and energy, and the incubating parents themselves become exposed to predators [25, 26]. By sharing incubation, the parents reduce the costs of time, energy and predation risk imposed upon them .
The optimal temperature for embryo development in most birds is between 36°C and 40.5°C, and if ambient temperature deviates from the optimum, parents regulate nest temperature by warming or cooling the eggs [27, 28]. Overheating and chilling (hyper- or hypothermia, respectively) reduce egg survival and may cause nest failure. Hyperthermia is more harmful than hypothermia, since hot temperatures induce embryonic mortality faster than cold ones; embryos may survive 0°C for a short time period, whereas no avian embryo survives above 44°C . Therefore parental care in hot environments, especially of ground-nesting birds where the eggs might be directly exposed to the heat of the sun, plays a vital role in preventing eggs from overheating [29, 30].
We investigated parental cooperation - defined here as mutually beneficial interactions between the parents to maximise their reproductive success  - in a small cosmopolitan ground-nesting shorebird, the Kentish plover, Charadrius alexandrinus (body mass approximately 42 g), which breeds in temperate and subtropical environments [32–34]. Nests are sparsely filled with material such as straw, pebbles, mollusc shells and algae which may act as insulation materials to help regulate egg temperature [32, 35]. Both parents participate in incubation: females usually incubate in the daytime whereas males incubate during night [36, 37], although after hatching of the eggs one parent (usually the female) may desert the brood. The Kentish plover is an ideal species for studying parental behaviour, since it has variable parental care both within and between populations. Monogamy, polygyny and polyandry may all occur along with male-only, female-only and biparental brood care within a single population [38–41]. All three types of brood care that occur in Kentish plovers were recorded in the Arabian Desert, although biparental care of young appears more common than in temperate zone populations such as Hungary and France . The transition from biparental incubation to biparental/uniparental brood care is an excellent paradigm to understand how and why animals shift from biparental care to uniparental one.
Here we investigate the division of parental effort during biparental incubation in Kentish plovers in the Arabian Peninsula where ground temperatures may exceed 60°C at midday during the breeding season. The objectives of our study were to answer two questions: i) Is the behaviour of the male or the female influenced by ambient temperature? ii) Does ambient temperature influence parental cooperation during incubation? We predicted that (i) male contribution to incubation should increase with ambient temperature to assist female incubation, and (ii) total incubation will increase with ambient temperature, so that parental coordination will be tight during the hottest part of the day.