The following article was featured in the July/August issue of PORK Network magazine.
Pigs don’t exactly see eye to eye with humans; in fact, they see about one-sixth the resolution we do. Understanding how pigs see can help producers manage their indoor lighting systems to give pigs the most efficient and comfortable visual environment.
Pigs have two pigment cones in each eye, which helps them decipher color vision. Humans, on the other hand, have three types of cones in our eyes. Animals with only one pigment cone (called monochromatic vision) can only see in black and white. So, pigs are not colorblind, but they don’t have the same clarity as humans.
Difficulty with red and green
In some studies, pigs are thought to be colorblind to red and green. Research compiled by Dr. Nina Taylor Wainwright for the British Pig Executive group (made up of United Kingdom pig levy payers), states, “Experimental evidence suggests that pigs have poorer color perception than humans, and despite similar detection of blue wavelengths, pigs show reduced sensitivity to the red end of the spectrum.”
Apisit Kittawornrat and Dr. Jeff Zimmerman, both with Iowa State University at the time of the study, found that “Pigs generally prefer lighted areas as opposed to darkness. For example, Tanida et al. (1996) found that piglets tended to move toward more brightly illuminated areas and recommended even, diffuse lighting to encourage movement into poorly illuminated areas. Likewise, Grandin (1982) noted that pigs tended to move toward more brightly lighted areas and suggested shining light into the interior space, rather than into the pigs’ ‘eyes, to direct and encourage their movement, e.g. into the unlighted interior of a truck to facilitate loading.”
The researchers continue, “Pigs do respond to color, e.g. the change in the color of the handlers’ uniform (Hemsworth, 2007), but the presence of a particular photo pigment is a weak indicator of the information actually supplied by activation of this pigment. Tanida et al. (1991) concluded that pigs can identify ‘blue,’ but suggested that pigs discriminated blue from other colors on the basis of hue rather than brightness.”
“Most mammalian orders have two cone types, one with peak sensitivity in the short wavelength region (blue), and another in the medium to long-range region (green – yellow - red),” notes Wainwright. “This allows comparison of the short wavelength component against medium to long wavelengths, giving color vision, but most species will be red-green “color blind,” being unable to distinguish red wavelengths from green ones. In contrast, humans and some other primates have three types, enabling additional contrasts in the medium to long range contrast (i.e. green v red differentiation).
“Mammalian retinae generally contain low numbers of S cones (short wavelength receptive) and higher numbers of M (medium wavelength) or L (Long wavelength) cones,” adds Wainwright.
From Hutson et al. (1993), “pigs showed a consistent startle response to the warning colors of black and yellow, although this could reflect detection of the high contrast pattern, rather than just colors.”
Studies have also shown that when pigs forage, they associate colors to certain foods. Products today are using knowledge of the pig’s color vision to better serve the pig and the producer.
A pig’s reduced red sensitivity is being utilized in the lighting industry by companies such as ONCE Inc. It developed an LED light that has a “Dim to Red” capability. The pigs perceive the red light as darkness. A red light can be used as a sunrise and sunset to “reduce the stress of sudden changes in light.” Another aspect of the light is a red service light. It allows workers to see and make their way around, while the pigs are not disturbed with sudden changes in light.
Duration of light a factor
Besides color, the duration of light can greatly affect pigs’ welfare and seasonality. Research has shown that a straight 24 hours of light is not good for pigs and can have detrimental effects, though extended hours of light may be beneficial from the standpoint of appetite and rate of gain. Conversely, a 24-hour dark setting can be detrimental, but to a lesser extent.
Producers should examine the type of light used in their facilities, because a pig’s welfare can be impacted by the lighting setup. According to Wainwright, continuous lighting results in “increasing agnostic behaviors (indicating stress), and at bright illuminence also resulted in eye damage and weight loss; intermittent photo-patterns also agitated pigs.”
Her research suggests:
· Piglets benefit from increasing or longer day lengths (15-18 hours) due to increased suckling and heavier and larger weaned litters
· Long or lengthening hours of light will increase food intake in grower/finishers
· Short or decreasing hours of light will decrease time taken to reach puberty in males and females, and for sows to return to estrus; these day lengths could therefore be a useful tool in breeding facilities,
Short or decreasing hours of light should be avoided in boar housing to reduce mounting activity, aggression and to reduce risk of boar taint. Lighting not only affects the visual perception of pigs but also their seasonality. Research with boars shows that environmental lighting can have an effect on their physiology.
In Once research, LED lights can be used to adjust the intensity, wavelength and duration of light to meet the animals’ needs. In boar holding units, this can help boost the production of semen. Changing the photoperiod, or number of hours of light in a day, in different seasons can increase productivity. For example, decreasing photoperiods in autumn and winter has been shown to enhance boar reproductive performance.
More research needed
While initial studies show that the duration and color of light can affect pigs, more study is necessary to identify the impact of changing lighting patterns on physiology, reproduction and other biological systems. Opportunity exists to maximize new technology as a result.
For more articles and features from the July/August issue of PORK Network, click here.