Behavioral Genetics in Pigs and Relations to Welfare - Genetics and the Behavior of Domestic Animals - page 397

In general, pigs search for positive and close interactions with humans.
Fear of humans is an indicator of low welfare and different methods are used
to record fear. One way, used by Velie et al. (2009) and others, is to let
a person unfamiliar to the pigs enter the pen and stand there motionless,
and the latency for pigs to approach and touch the human is recorded. The
success of this test relies on the pigs' motivation to voluntarily approach
the human. The trait “easy to handle” can be measured during routine work,
e.g., when pigs are moved between pens or weighed.
Tail Biting
Pigs' behavioral response to chronic stressors in the environment may trans-
late into abnormal behavior such as tail biting. It could be a redirected
behavior related to the need to explore and forage. Even so, tail biting can
also be observed in outdoor production ( Walker and Bilkei, 2006 ). Pigs
performing tail biting with a high frequency also perform much ear-biting
and belly-nosing. Brunberg et al. (2011) identified three types of pigs: biters,
victims, and neutral pigs that never perform nor receive tail biting even
during an ongoing outbreak. The frequency of biters and the severity of tail
biting are highly influenced by the environment. The number of injured tails
is easy to record, but to identify the biters is quite complicated. Breuer et al.
(2003) used pigs' motivation to chew a rope as an indirect measure of their
biting tendency.
Sexual Behavior
In most countries males raised for slaughter are castrated, but there are
exceptions like the U.K. Due to welfare reasons, the European Union wants
to ban surgical castration from 2018. Entire males are more likely to
show aggressive behavior and are more active than castrates ( Cronin et al.,
2003 ). They also display more sexual behavior such as mounting ( Rydhmer
et al., 2010 ). In boars used for mating and semen collection, there is genetic
variation in libido traits, with heritabilities around 0.15 ( Rothschild and
Bidanel, 1998 ).
By means of pheromonal communication, gilts and sows grouped
together synchronize estrus. During estrus, the female shows standing reflex
and there is a genetic variation in the ability to show this sexual behavior.
The heritability of this categorical trait was estimated at 0.3 on the underly-
ing scale in a research herd ( Rydhmer et al., 1994 ). Recording of estrus
symptoms was later performed in Norwegian nucleus herds, but almost no
genetic variance was found in that environment ( Holm, 2004 ).
Maternal Behavior
The pig is an exception among the ungulates regarding prolificacy; it has
more in common with rodents and many carnivores. The sow produces a