The sex determination system of XX / XY is applicable on Black rhino, where the males have XY and females have XX chormosomes. Black rhinos have a total of 84 chromosomes.
In wild males sexual maturity is reached between seven to nine years and in wild females at between four to six years. The first parturition of females in the wild is estimated around five years, but in some populations it might take up to twelve years. Research found that in two South African subpopulations of Black rhino, in which there were 0.1 / km2, that in these two subpopulations females reached sexual maturity at a younger age then in a third subpopulation with a Black rhino density of 0.7 / km2 .
Records from captive Black rhinos in the EEP show the youngest male at first reproduction was three years old. However, more generally (the youngest ten males at first reproduction, out of the total past EEP dataset) range between four and five years old. The average age at first reproduction is ten years and one month.
The youngest female at first reproduction was five years old, the youngest ten females at first reproduction ranged between five and six years old. The average age for a female at first reproduction is ten years and four months.
Seasonality of Cycling
The recurring of the cycle in regular intervals is more important than the length of the cycle in days. Oestrous cycling begins while the mother is still nursing. Generally, breeding occurs throughout the year; however there may be mating peaks in some areas. In Kenya mating peaks occur during September till November and during March till April. In Zululand mating peaks are during October till November and during April till July. These indications suggest that most births take place in the rainy season (EAZA yearbook, 1995; Hutchins, 2003; Nowak, 1999).
Irregular oestrous cyclicity patterns are relatively common among captive black rhinos, and in many females behavioural expression of oestrus can also be difficult to detect. It is a recommended that collections use hormone analysis as a management tool as this can allow the prediction of when a female will be receptive to the male based on her hormone profile, giving keepers extra information to help introduce the rhinos at the right time. Furthermore, once pregnancies have been confirmed, this allows the male to be separated and mixed with another female at an earlier stage of gestation.
As oestrous cycle length can be highly variable both between females and within an individual female over time, long-term sample collection is the best way to make accurate predictions. At Chester Zoo, since the initiation of the endocrine programme there have been six pregnancies and five births in six years, highlighting the impact this additional tool can have on facilitating breeding management.
Reproductive Cyclicity in Females
Black rhinos are polyoestrous, meaning that they will come into oestrus, i.e. the time of receptivity to the male, on multiple occasions throughout the year. Although there is some evidence of seasonality of births in the wild due to weather conditions (Hitchins and Anderson 1983), and perhaps in zoos due to management constraints, there is no evidence of seasonality in oestrous cycles.
The oestrous cycle has previously been characterised in this species using steroid hormone analyses, either using blood (Berkeley et al. 1997), urine (Hindle et al. 1992) or faeces (Brown et al. 2001; Edwards et al. 2013), and saliva could potentially also be used as a sampling medium. Changes in the ovary during the oestrous cycle have also been previously described using ultrasound (Radcliffe et al. 2001).
An average oestrous cycle length of the Black rhino has previously been described as around 26 days in length; however, research on wild (Garnier et al 2002), and captive rhinos in America (Brown et al. 2001), and more recently in Europe (Edwards et al. 2013) have revealed that oestrous cycles are more variable in length, with typical oestrous cycles lasting between 20 – 40 days. In addition, short (< 20 days), and long (> 40 days) cycles, and acyclic periods with no evidence of oestrous cyclicity are also relatively common. The causes of these different cycle types are yet to be fully understood, but early indications suggest that these may not reflect normal reproductive function, and to-date pregnancies have only been reported associated with 20 – 40 day cycle types. Therefore oestrous cycles of 20 – 40 days in length are considered to be normal, and although females may vary in their typical cycle length, within an individual cycle length tends to be relatively consistent. Indeed, the regularity of oestrus on an approximately monthly basis is perhaps more important than the exact length of the oestrous cycle.
Females in captivity may commence oestrous cyclicity between three and four years of age, with one female age three years eight months of age exhibiting clear oestrous cyclicity (Edwards et al. 2013). The oldest female to have reproduced in captivity was aged 32 years, but females may continue to cycle after this time, and in the wild have been reported to continue producing offspring after 30 – 35 years.
Reproductive Hormones in Males
Testosterone can be measured in blood (Christensen et al. 2009) and faeces (Brown et al. 2001; Edwards et al. 2013), and can be used as an indicator of reproductive function. Testosterone concentration increases with age (Edwards et al. 2013), and may also vary according to prior reproductive success (Edwards et al. 2013), or due to the sociosexual environment (Christensen et al. 2009).
Gestation Period / Birth Rate
The gestation period is around 15 to 16 months or 440 to 460 days. The inter-birth period in the wild is somewhere around 27 months. The inter-birth period in captivity is 40 months. This may be due to delayed reintroduction to a mate postpartum (Carlstead et al, 1999; EAZA yearbook, 1995; Hutchins, 2003).
After the gestation period there is a single calf born, which weighs around 40 kg. Birth usually takes place in the early morning (Nowak, 1999).
Nursing generally continues for over one year, and the older calf is driven away by the mother around the time that the next offspring is due. Some solid food may be taken within a few weeks of birth, weaning is completed after about two years; independence is achieved between two and a half and three and a half years old (Hutchins, 2003; Nowak, 1999).