This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Article written by Maria Tello-Ramos, edited by Felicity Muth.
Like it or not, male and females differ from each other in a number of ways. One way that comes up again and again, in a number of species including our own, is males’ ability to out-perform females in spatial tasks. That is, males are generally better at finding their way back to a location (normally to get a food reward) than females. Why might this be? One reason may be that males and females use different cues to each other when navigating. Males can use both ‘spatial cues’ (distance and direction) as well as ‘feature cues’ (colour, landmarks or the shape of an object). Females, on the other hand, tend to preferentially use feature cues when returning to a previous location. Being able to use two different types of information allows males to use the type of cue that is more relevant to a particular task. For example, if you stop a man and ask him for directions, he is as likely to tell you what direction to take and how far to go, as he is to tell you about the buildings you should look for. On the other hand, a woman will be more likely to give you a detailed account of the buildings you will find along the way to your destination. This difference between male and females has been accepted as a general rule, yet there are almost as many exceptions to this rule as there are agreements.
In cases where males have better spatial ability than females, why might this be? One hypothesis, centered on the observation that sexes also often differ in home range, is that males travel farther than females. Therefore, it would be more useful for males to have better spatial abilities.
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To test this idea, myself and colleagues from the University of St Andrews and the University of Lethbridge, decided to look into this question by studying hummingbirds’ foraging behavior. We did this by training wild, free living hummingbirds in the Canadian Rockies and in central Mexico to feed from artificial flowers. Hummingbirds are a useful animal group to look at this question because males and females are thought to differ in their use of space. Males of many hummingbird species are classified as territorial, generally staying in one location for several weeks, while females are thought to be opportunistic foragers that feed from the flowers they find along their flying paths. Males, being territorial, will benefit from learning the location of the flowers within their territories that do contain nectar and return to them in consecutive visits. If during foraging females, do not return to previously visited flowers, then learning the location of these might not be important for females. Instead, females might benefit more from learning to recognize the visual cues that signal a nectar containing flower in order to spot it from the distance, avoiding visiting empty flowers.
From previous experiments we know that male rufous hummingbirds, as with other animals, preferably use spatial cues (the location) when returning to feed from an artificial flower, but will also use feature cues when spatial cues are not reliable. Since little is known about female hummingbirds, we decided to start by finding out which cues females preferred to use when navigating. We focused on three hummingbird species (rufous, the magnificent and white-eared hummingbird) that are practical to test in the wild.
To test which cues females used, we first trained the female hummingbirds to feed from artificial “flowers” made out of a plastic tube surrounded by a colored cardboard circle mounted on a wooden stick. We then presented a bird with a square array composed of four artificial flowers, each flower a different color. Only one of these flowers was rewarded with sugar solution (a surrogate to nectar). We found that birds would come to the array, visit most of the flowers until it had found the rewarded one, feed from it and then leave for about ten minutes. The flower had enough nectar that a hummingbird could not empty it in one visit, so upon return the bird was expected to return to that flower. While the bird was away, however, we emptied the previously rewarded flower and switched it with another flower on the array. Therefore, when the bird returned to the array, the flower she had visited before had moved. She now had to make a choice about which flower to visit. She could either visit the flower that was in the same spatial location as before but was now a different colour, or she could visit the flower with the same colour as before that was now at a different location. Using the colour cue would show that she paid more attention to the feature of the flower (the colour) rather than the absolute spatial location.
We were surprised to find that the great majority of female hummingbirds (of all three species) first visited the flower at the correct previous location. This is exactly what male hummingbirds do too when tested with the same experimental design.
This means that just like male hummingbirds, female hummingbirds also use spatial cues to find previously rewarding flowers. So why do males and females of this species use the same types of cues to relocate a reward, when this isn’t the case for other species? We think this may be because both male and female hummingbirds need to visit hundreds of flowers every day. These flowers might vary in colour or shape over time, but they do not normally move location. Therefore, natural selection has favored the use of spatial cue in both sexes. While it is possible that the home range or foraging behavior of males and females affects which types of cues are more relevant to each sex, our data suggests that not all species present a sex difference in spatial abilities. Further research into the foraging behavior of wild animals will allows us to truly compare when and in which species there are sex differences in spatial and other cognitive abilities.
Reference
Tello-Ramos, Maria. C., Hurly, T. Andrew & Healy, Susan D. 2014. Female hummingbirds do not relocate rewards using colour cues. Animal Behaviour. 93: 129-133.