Giraffe exhibit a mating system of overlap promiscuity, meaning that individuals mate haphazardly with others within their home ranges. Males and females interact during courtship and mating, but there is no prolonged relationship between mates, and males do not defend or care for young.
Why overlap promiscuity?
The main advantage of overlap promiscuity is that it requires little effort on the part of either males or females to maintain. Both sexes mate when the opportunity arises, but most of their energy can be spent foraging. Overlap promiscuity is the ultimate mating strategy for males, since they can mate with as many females as possible without having to defend any resources for females or young. Because they do not play a part in infant care or defense, males have no need to keep track of paternity (class discussion 2004).
Overlap promiscuity is more of a compromise for females, since it is a system that affords them no help from males. However, for giraffe females, it is a feasible system for several reasons:
Ultimately, giraffe exhibit overlap promiscuity because no other (more involved) system would give them enough advantages to offset the energetic costs (Dagg 1976; Leuthold 1979; Pratt 1985; class discussion 2004).
There is no mating season among giraffe, so mating occurs year-round. Female estrus occurs every two weeks, and lasts around a day. Males constantly roam around in search of estrous females, and when they come across a female, they often perform a sequence of behaviors known as flehmen to test her receptiveness (Dagg 1976).
Male giraffe testing the urine of a female to see if she is in estrus. This interaction is known as the flehmen sequence.
First, the male induces the female to urinate (left), then he taste-tests the urine (right).
Images taken with permission from Liz Leyden
Flehmen, or urine-testing, consists of several steps. First, the male approaches the female and nudges her rump to induce urination. If the female urinates, the male giraffe scoops up some of the urine in his mouth and raises his head to taste it. Presumably, the urine of estrus females has a distinct taste, and if the male determines that the female is receptive, he proceeds to court her (Dagg 1976; Pratt 1985).
Once a male decides to court a female, he follows her determinedly over a period of hours or even days in order to mate with her. The courtship of giraffe is not elaborate, and mostly consists of the male trailing behind the female and occasionally attempting to mount her (Pratt 1985). The female may repeatedly resist these mounting attempts by walking forward unconcernedly, until finally copulation occurs when she stands still for the male. Copulation among giraffe is brief, so it is rarely observed by researchers (Dagg 1976; Pratt 1985).
According to Pratt (1985), males choose their mates based on age and receptiveness. Males use the flehmen sequence to determine which females are in estrus, since courting a non-receptive female would not be worth the energy. Further, males exhibit more interest toward younger females than older ones, perhaps because young females have high fertility, or perhaps because they are less likely to already be pregnant. Male choice is essential to determining who will mate, but ultimately, giraffe females have choices for mating as well.
According to Pratt (1985), female giraffes prefer older, more dominant males, and they exhibit choosiness at two distinct stages: during flehmen and during courtship. When approached for urine-testing, Pratt found that females urinated more often for dominant males than subordinate males. Female giraffe may also approach dominant males and rub their necks on the males' flanks, inviting flehmen (Leuthold 1979; Pratt 1985). Thus, dominant males have more success in determining which females are receptive.
During courtship, females are choosy as well. They often 'play hard to get,' prolonging the courtship period so that if a more dominant male comes along, he can displace the male who is courting her (Pratt 1985). This 'He-man' strategy (allowing the strongest male to mate) is advantageous for the female because it allows her to get the best genes for her offspring: the most dominant male is the one who ultimately fertilizes her eggs (class discussion 2004).
In summary, giraffes are promiscuous, with no long-term relationships observed between males and females. Males wander through their home ranges looking for females and testing their receptiveness through flehmen. Males prefer young females who are in estrus, and females prefer older, dominant males (Dagg 1976; Leuthold 1979; Pratt 1985).
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