Love at first smell: Can birds choose mates by their odors?

Mate choice is often the most important decision in the lives of humans and animals. Scientists at the Konrad Lorenz Institute of Ethology at the Vetmeduni Vienna have found the first evidence that birds may choose their mate through odor. The researchers compared the preen gland chemicals of black-legged kittiwakes with genes that play a role in immunity. Kittiwakes that smell similarly to each other also have similar genes for immunity. Since the birds prefer to mate with unrelated mates, the scientists have now found the likely mechanism by which they recognize relatedness. The scientists published their findings in Nature's Scientific Reports.

It has long been understood that reproducing with close relatives may have profoundly negative effects on offspring. It is therefore not surprising that biologists have discovered in some species that breeding individuals have evolved ways to detect their genetic similarity with those of prospective partners. Over 20 years ago it was discovered that female mice were able to choose unrelated over related males as mates. Females achieved this by comparing the smell of the urine of each male and comparing it with their own odors. Amazingly, the urine odors reflected the genetic composition of each mouse. More specifically, the odors were correlated with a special group of genes called the "major histocompatiabilty complex," or MHC, which helps individuals resist diseases. Thus, by pairing with MHC-dissimilar mates, breeders produce offspring with a more diverse collection of disease-resistant genes.

This discovery in mice was followed by similar findings in other mammals. More recently it has been shown that birds in several species also avoid breeding with MHC-similar mates. This poses a mystery. Whereas smell is a very well developed sense in mammals, it has long been thought that birds lack such keen olfactory abilities. Although a growing body of research is showing that birds can discriminate odors more than previously thought, none had shown that birds can do as mammals and use odor to compare their MHC composition with that of prospective mates. This mystery appears to have been solved by a group of researchers from Austria and France. Team leader Richard H. Wagner and behavioral geneticist Wouter van Dongen of the Konrad Lorenz Institute of Ethology, a part of the Veterinary Medicine University Vienna, have been collabortating with French colleagues on a long-term study of a cliff-nesting gull, the black-legged kittiwake, breeding in Anchorage Bay, Alaska.

When birds groom themselves with their bills, they spread chemical compounds from their preen glands throughout their plumage. These chemicals produce odors that appear to be unique to each individual, providing an olfactory fingerprint. The team suspected that, just as in mammals, these odors may be used by kittiwakes to assess their relatedness to other individuals.

To test this idea, the researchers collected both DNA samples and preen gland odor samples from nesting kittiwakes. The project then entailed two kinds of laboratory work: while Sarah Leclaire at the University of Toulouse conducted the analyses of the preen gland chemicals to characterise the odor signatures of each individual, van Dongen analyzed the MHC of the kittiwakes in the Vienna lab. The team had previously discovered that kittiwakes avoided pairing with relatives, but the mechanism by which the birds recognized their relatedness to each other had remained unknown until now. Their new finding is that individual kittiwakes that smell similarly to each other (i.e. have similar preen gland chemicals) also have similar MHC genes. Closer relatives therefore have more similar odors than distantly related individuals. This suggests that birds may be able to compare their own odor with those of potential mates, and to choose unrelated individuals as breeding partners. Quips ornithologist Wagner, "the more research that is performed on smell, the more it appears that anything mammals can do, birds can do too." The new findings, moreover, open the door for further work linking mate choice and disease-resistance in birds.

Selengkapnya »»  

Bad news for kids: Parents do not defend their offspring at all cost, bird study shows

Blue tit parents weighs the risks for themselves before defending their offspring.

Do parents defend their offspring whenever necessary, and do self-sacrificing parents really exist? To answer this question, researchers of the University of Veterinary Medicine, Vienna examined defence behaviours of parent blue tits. They investigated whether birds would risk everything to protect their young from predators. Their conclusion: parents weigh the risks. It is not only the risk to the nestlings, but also their own risk that plays a role when defending their nests. The results were published in the journal Proceedings of the Royal Society B.

The degree to which parents sacrifice themselves for their children depends on a variety of factors. On the one hand nest predators pose a threat to the young and the parent birds. But also the time of hatching plays a role. Earlier studies have shown that birds born late in the season are more likely to be protected by their parents, as the adult birds often do not have the chance to produce replacement clutches. Older offspring also tend to be protected more readily than younger ones, as much more parental care and energy have already been invested.

"Studies on the risk behaviour of birds already exist. But there are only a few in which several factors have been investigated at the same time. We are upon the first showing how parents respond to more or less dangerous predators when defending offspring of different ages in the field says first author Katharina Mahr.

Risk research using predator models

Together with Herbert Hoi from the Konrad Lorenz Institute of Ethology they observed blue tit parents defending their nestlings aged between five and twelve days. The parents were confronted with different predators of varying threat levels. A stuffed sparrowhawk represented a predator with great risk to adult blue tits. A sparrowhawk is not considered dangerous for very young blue tits, as it cannot reach into the nest-box. However, sparrowhawks become more relevant predators for older nestlings. A rubber snake represented a predator of medium risk. Snakes are very dangerous for the young, but due to their low mobility represent a low risk to parents. A stuffed woodpecker was used as a third predator. Woodpeckers are able to open nest-boxes and therefore represent the greatest danger to very young nestlings, but not to older juveniles.

When exposed to a snake, blue tit parents attacked the predator. Since alarm calls against snakes have little effect, as snakes don't hear those calls, parents rather attack the enemy. Furthermore, snakes are less mobile than birds. A targeted attack is therefore the best choice with the lowest risk for the parents.

Direct attacks on predators are, according to Mahr, the riskiest behaviour bird parents can exhibit. When exposed to the sparrowhawk or the woodpecker, the parents neither attacked nor approached the predators. Instead, the parents responded with loud alarm calls. "Attacking a sparrowhawk is life-threatening for blue tits," Mahr points out. "That's why they probably do not attack. Their own risk is too high."

Blue tits keep cool

"Defence behaviour is apparently not based on rash decisions, but is subject to a cascade of considerations," Mahr concludes. The age of the nestlings seems to play a role in the less risky alarm calling strategy. Parents warned their twelve-day-old nestlings more often using alarm calls than their five-day-old offspring.

Moreover, nestlings are extremely dependent on their parents. "Self-sacrifice only happens, when the nestlings are able to survive on their own. Otherwise the sacrifice makes no sense," Mahr explains. "That's probably why parents defend older offspring more vigorously than young."

"Our findings could have general implications for the economy of risk management and decision-making strategies in living beings including humans. Humans do react rashly at times. But probably unconscious decisions don't always turn out to be the worst," says Mahr.

In the future, Mahr and Hoi would like to filter out the differences, if any, between maternal and paternal behaviours. The data have already been collected and must now be evaluated.

This study was based on field observations by Julia Thoma, who provided the first data for this project as part of her pre-academic paper for the project Sparkling Science.

Selengkapnya »»