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Mating and Young

Young kangaroo rat and Banner-tail nest. Lidiker 1960.

Mating in the kangaroo rat can be described as overlap promiscuity. Reproductive rates of kangaroo rats will vary according to the species, food availability and density of the rodent population in their habitat. If there is a drought of food is short in supply only a few females will make the decision to breed (Howard 1994). On the other hand, the reproductive output is highest following summers with heavy rainfall (Waser and Jones 1991). Waser and Jones hypothesize that kangaroo rats base their reproduction on “phenotypic plasticity”. This hypothesis states that individuals asses their local conditions and adjust their reproductive efforts accordingly. If this hypothesis were true in the kangaroo rat then there is not a relationship between reproductive effort and survival. Thus, selection would be based on the ability of the kangaroo rat to asses and adjust reproductive effort to local conditions. Since the nature of the environment, in which having a productive home range one year will not guarantee a productive home range in the next year,“phenotypic plasticity” is a strategy that would be beneficial to the kangaroo rat (Waser and Jones 1991).

Why overlap promiscuity?

Kangaroo rats have no bonds between the males and females outside of the mating experience. Male and female kangaroo rats live separately and only come together for the purpose of mating. The female kangaroo rat can raise the young on her own, so the male kangaroo rat has no parental investment in the young and leaves them to the mother to raise in her burrow. As discussed above, females will asses their conditions before mating so that they always have enough resources to provide young if they choose to mate. This means that they will not need males to provide resources. Since dominance hierarchy cannot be firmly established in the kangaroo rats, and is only believed to be present in males, hierarchical promiscuity is not an option for their mating system. Furthermore, females will mate with multiple males. Since lots of males will get the opportunity to mate, rank does not seem to be criteria for mating, although females will mate with the more dominant of two competing kangaroo rats. Also, although kangaroo rats have various predators, due to the establishment of burrows, the female kangaroo rats do not need protection from the male kangaroo rats in order to survive. Adaptations to predation have allowed female kangaroo rats to dig and defend their own burrows and forage alone so the male does not provide anything necessary to raise the young.

Merriam kangaroo rats exhibit a long reproductive activity in which they breed between Febuary and May (Smithsonian Institute Press 1999) that results in two or possibly three litters per year (Nader 1978). In order to mate, a male Merriam kangaroo rat will enter the home area of an estrous female. In both species if there are two males present then there is a competitive encounter for access to the female. Before mounting males and females will perform naso-anal circling of each other until the female stops circling and allows the male to mount her. During mating in the Merriam kangaroo rat, the female will allow multiple mounts with multiple males in a short period of time. Females also are the first to terminate the mating encounter with the male. Males will often remain in the female's area, even though the females have left, and perform "sun bathing" to mark the area (Randall 1987).

Why multiple mounts in Merriam?

The female will allow multiple mounts by multiple males because it ensures an offspring will result from her reproductive activity. The female will want to have the best genes possible so it is beneficial for her to perform naso-anal circling before mating to prolong the process in case a more dominant male comes to mate. Males will mate with a female even though she has just mated with a male because they too will want to spread their genes to as many females as possible. Also, since the mating system is not monogamy, the male has little investment in the female so it is not as important to the male to prevent the female from mating with others. It is also beneficial for a male to mate with any female that he runs into because they live in home ranges and they do not know when they will run into another mating partner again.

Reproduction in the Banner-tail occurs year round (Smithsonian Institute Press 1999) females have 0-4 litters with 0- 3 offspring each year (Waser and Jones 1991). Banner-tail kangaroo rat males also will enter the females mound to mate. The male Banner-tail will use the scent of the females urine on their mounds to determine her reproductive condition and then make the decision to enter. The kangaroo rats that would enter are either territorial neighbors or had overlapping home ranges with the female . Unlike Merriam kangaroo rats, Banner-tail’s only have one mounting per reproductive encounter and the reproductive encounter lasts much longer. There is much more chasing and foot drumming in the male Banner-tail before the female will accept him to mate (Randall 1987). Gestation lasts from 22 to 27 weeks Smithsonian Institute Press 1999).

Why not multiple mounts in Banner-tail?

Banner-tails individually occupy a mound and it serves as a mating territory for them. It is uncommon to find more than one kangaroo rat in a mound at a time and if it occurs it is almost always a mating encounter (Randall 1987). When mating occurs in a mound, it is easier for the male in the mound to chase other males away from the territory while he is there. Where as if mating is out in the open like it is for the Merriam kangaroo rat, it is harder for the male to defend the female while mating. Mating also takes longer in the Banner-tail kangaroo rat so there is less time and energy for the female to have multiple mounts. Since the Banner-tail female requires more courtship behavior than the Merriam which will make the encounter last longer, the Banner-tail females ensure that they are getting strong genes. The males will be dominant males who have chased others away and they have the energy and strength to invest in courting behavior.

Young

Kangaroo rats are born in the burrow in a fur- lined nest made for the purpose of nesting. They are hairless and blind when they are delivered. Young male kangaroo rats develop faster than young females (Nader 1978). The kangaroo rat has four stages of maturity: Juvenile, Immature, Sub adult and Adult, and there are no significant morphological differences between the age groups (Nader 1978). Merriam kangaroo rats' young crawl for about one week after birth until their hind legs are fully functional after two weeks. Merriam kangaroo rats will wean their young between two and three weeks after birth (Reynolds 1958). After some time, the young will move out of the burrow or inherit the burrow from their mother. Weaning of Banner-tails occurs fairly soon after birth, about 22 to 25 days (Smithsonian Institute Press 1999) and offspring reside in the mound for 1 to 6 months after weaning where they share the maternal seed caches (Waser and Jones 1991). Juveniles become independent usually between March and August when they wither move to another mound, inherit a maternal mound after the mother dies or inherit the maternal mound after the other moves away (Waser and Jones 1991).Thus, it is the female kangaroo rat who puts all the investment into her young. Most of the investment she provides comes in the form of a food source and protection within the burrow, rather than grooming behavior.

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