July was the hottest month ever recorded in Washington, D.C., in Oklahoma City, Oklahoma, and in Wichita Falls, Texas, as measured by the National Weather Service. In fact, the NWS has issued an "excessive heat warning" for a huge swath of middle America extending from northwestern Illinois and central Iowa in the north to central Texas in the south. The Centers for Disease Control and Prevention warn each year that people can easily become ill or even die from extreme heat: from 1979 to 2003, 8,015 people died due to excessive heat exposure. In fact, more people in the US died from heat exposure during that 24-year period than from hurricanes, lightening, tornadoes, floods, and earthquakes combined.
The CDC's top recommendation for avoiding the negative effects of extreme heat is "stay in air-conditioned buildings as much as possible." Easy enough if you're a human whose job doesn't require being outdoors all day - but what if you're a non-human animal? Some animals live in extremely hot environments and must go about their days without refrigerated water, air conditioning, or electric fans. Some heat-dwelling species have to find other ways to deal with the excessive heat, sometimes involving cooperation with others. It's hard to imagine a situation in which increased heat would lead to increased cooperation - personally, I get increasingly irritable and annoyed as the heat increases!
The Kentish plover (Charadrius alexandrinus), however, is a small shorebird that lives and breeds in deserts where ground temperatures may exceed 140 degrees Fahrenheit (60 degrees Celcius). And they actually become more cooperative in the heat.
Biologist Monif Al-Rashidi and colleagues decided to use this extreme environment as a way to investigate the way that male and female Kentish plovers work together to protect their eggs from the oppressive heat of the Saudi Arabian desert. Biparental care, which is the care of offspring by both male and female parents, represents a classic example of the trade-off between cooperation and conflict in social behavior in the animal kingdom. If they cooperate, parents can work to improve the odds of the survival of their offspring. By withholding care, however, an individual can potentially survive longer and increase the odds of successful breeding later in life. Assuming that biparental care is even possible in a given species, mathematical models expect it to occur anytime the possibility of offspring survival is significantly greater than when cared for by a single parent. In particular, the harsh environment hypothesis predicts that parents should both contribute to the care of their young in environments susceptible to harsh weather conditions, where food is scarce, where there is intense competition for resources, if desiccation of eggs is a possibility, or in areas where the offspring are regularly preyed upon. It is very hard to experimentally investigate this hypothesis. For one thing, it's very challenging to modify the environment in any sort of ecologically valid way. More critically, though, environmental factors tend to operate together, making it a challenge to identify which among a set of environmental variables might be the factor driving changes in parenting behavior.
The Kentish plover provided Al-Rashidi with the opportunity to conduct a particularly clever experiment. These birds lay their eggs on the ground, which means that the eggs as well as both parents have direct exposure to the surrounding environment. Some nests are located under bushes, and are therefore naturally protected from direct sunlight, while others are out in the open. This provided an obvious way for Al-Rashidi to create two experimental groups - one in direct sunlight and a second in the shade. In general, males tend to sit on the nest during the cooler nighttime, while females tend to take the daytime shift. The problem is that the females risk overheating if they incubate the eggs all day. The harsh environment hypothesis, therefore, predicts that the warmest nests will not only show evidence of more biparental care but that the two parents will take turns more often throughout the day.
Over the course of two breeding seasons, the researchers worked on Farasan Island in the Red Sea, which is part of the Kingdom of Saudi Arabia. At each of the 32 nests included in the study, the researchers recorded the number of eggs as well as its geographic location. They were then classified according to the amount of shade they had: no covering, less than 25% covered, 25-50% covered, 51-75% covered, and greater than 75% covered. The researchers visited each nest multiple times to determine whether or not the amount of shade cover remained constant over time. Finally, they noted the distance to the closest water source. In addition, through the use of hidden cameras and small radio tags, they were able to record every twenty seconds whether a parent was incubating the eggs, and which parent it was. Temperature recordings verified that the exposed nests were 10-15 degrees warmer during the day than the covered nests. In addition, birds at the exposed nests showed more signs of heat stress, including panting and belly-soaking.
As expected, males and females both spent more time sitting on the exposed nests than the covered ones over the whole day (left, below), and as predicted by the harsh environment hypothesis, there were more change-overs - that is, they took turns more often - during the hottest part of the day at the exposed nests (right, below). In both graphs, the white circles represent exposed nests, while the black circles represent the covered nests. Taken together, this showed that there was indeed more biparental parenting as the exposed nests, compared with the covered nests.
Then, Al-Rashidi took things one step further by introducing an experimental manipulation. For a period of one day, the researchers changing the availability of covering at each nest. At each covered nest, they removed the plant, and each exposed nest received a new covering over it. The birds were not particularly disturbed by this: it took them between 1 and 107 minutes to return to the nests after the researchers left. In addition, data was not recorded until six hours had passed, to give the parents time to relax.
Again, the results were totally consistent with the predictions of the harsh environment hypothesis: parents at cover-added nests spent less time incubating their eggs, while parents are cover-removed nests increased the time they spent sitting on the eggs (left). At cover-added nests, there were fewer change-overs, and at the cover-removed nests, there were more change-overs, as compared with the previous day (right). Also as expected, these changes were most obvious during the hottest parts of the day.
Taking all these results together, this study suggests that the level of biparental care in Kentish plovers is not determined strictly by genetics - instead, it varies according to environmental constaints. In other words, all Kentish plovers appear to have the capability for high levels of parental cooperation, but the level of cooperation depends critically on the environment. In this particular environment, the main variable seems to be ground temperature. As the heat increases, the level of cooperation also increases. By taking turns more often, each parent can fly to the nearby Red Sea (the only source of water nearby), where they can take a swim and wet their feathers (a process known as "belly-soaking"), which allows them to cool themselves as well as their eggs. This experiment also revealed another important finding: each parent monitored their mate's behavior and altered their behavior in response to compensate. When one parent left the nest to cool off, the other parent almost immediately began their nest-sitting duties.
"But wait!" you're about to say. "Increased cooperation in the heat is all well and good, but why build a nest out in the open in the desert in the first place, rather than in the shade?" As with most things in behavior and biology, there is a trade-off: building a nest in the open might make it hotter, but it also allows for easier detection of predators, and therefore faster escape from predators, compared with nests built under plants. In that sense, building a nest in the cooler shade could actually represent a greater risk to the plovers than building a nest in the desert heat.
So you see, turning up the heat can lead to increased cooperation. If you're a Kentish plover living in the desert, at least. For me, I'll stick with the air conditioning.
Monif AlRashidi, András Kosztolányi, Mohammed Shobrak, Clemens Küpper, & Tamás Székely (2011). Parental cooperation in an extreme hot environment: natural behaviour and experimental evidence Animal Behaviour, 82, 235-243 : 10.1016/j.anbehav.2011.04.019