AboutZoos, Since 2008


Birth con­trol at the zoo: vets meet the elu­sive goal of hippo castration

pub­lished 23 Decem­ber 2013 | mod­i­fied 03 Novem­ber 2014

One method for con­trol­ling zoo ani­mal pop­u­la­tions is male cas­tra­tion. For hip­popotami, how­ever, this is noto­ri­ously dif­fi­cult, as the per­ti­nent male repro­duc­tive anatomy proves sin­gu­larly elu­sive. Vet­eri­nar­i­ans from the Research Insti­tute of Wildlife Ecol­ogy of the Uni­ver­sity of Vet­eri­nary Med­i­cine, Vienna, and col­leagues, have demon­strated a suc­cess­ful method for cas­trat­ing male hip­pos. Their results are pub­lished on 26 Octo­ber online in the jour­nal The­ri­ogenol­ogy.

Hippos Emmen zooCom­mon hip­popotami (Hip­popota­mus amphibius) are vul­ner­a­ble to extinc­tion in the wild, but repro­duce extremely well under cap­tive breed­ing con­di­tions. Females can give birth to up to 25 young over their 40 year lifes­pan — evi­dently too many for zoos to accom­mo­date. Cap­tive pop­u­la­tions of hip­popota­mus must there­fore be con­trolled. Male cas­tra­tion is use­ful in this respect and it can reduce inter-​male aggres­sion as well. How­ever, doc­u­mented cases of suc­cess­ful hippo cas­tra­tions are scant. The pro­ce­dure is noto­ri­ously dif­fi­cult due to prob­lems with anaes­the­sia and dif­fi­cul­ties in locat­ing the testes.

We used an adapted ver­sion of a cas­tra­tion tech­nique com­monly used for horses
Chris Walzer, Research Insti­tute of Wildlife Ecol­ogy, Vetmeduni Vienna »

Hid­ing tes­ti­cles
Hip­pos are well adapted to an aquatic lifestyle (indeed they are dis­tantly related to whales and other cetaceans), and this is reflected in pecu­liar phys­i­cal and phys­i­o­log­i­cal fea­tures. While whales have truly inter­nal testes that remain in the abdomen at all times, hip­pos “hide” them in the inguinal canal, a pas­sage in the frontal abdom­i­nal wall. Thus the testes are exter­nally dif­fi­cult to locate by sight or touch. A team sup­port­ing Chris Walzer from the Research Insti­tute of Wildlife Ecol­ogy of the Vetmeduni Vienna as well as col­leagues from three zoo­log­i­cal insti­tu­tions and the Leib­niz Zoo for Zoo and Wildlife Research (IZW) eval­u­ated sur­gi­cal cas­tra­tion in ten adult com­mon hip­popotami held at var­i­ous zoos.

Walzer and col­leagues had pre­vi­ously devel­oped a suc­cess­ful hippo anaes­the­sia pro­to­col, a chal­lenge in itself, because deliv­ery of suf­fi­cient anaes­thetic is ham­pered by the hippo’s thick skin and dense sub­cu­ta­neous tis­sue. Also, the anaes­thet­ics can cause breath­ing irreg­u­lar­i­ties and some­times even death.

Imag­ing tech­nique
Hippo castrationAll ani­mals in the study were anaes­thetized with close mon­i­tor­ing of the vital func­tions. The testes were located using ultra­sound. Locat­ing the testes once, how­ever, is not suf­fi­cient, as they are some­times retracted fol­low­ing inci­sion, mak­ing intra-​surgical ultra­sound loca­tion nec­es­sary. “We used an adapted ver­sion of a cas­tra­tion tech­nique com­monly used for horses“, explains Walzer. “In horses loca­tion is eas­ier, because they have an exter­nal scro­tum, but in hip­pos you can­not really see any­thing. We had not seen any pre­vi­ous reports of the use of ultra­sound for locat­ing the tes­ti­cles, but this proved essen­tial, because the loca­tion of the testes is highly vari­able and can change from one moment to the next.” All surgery and lig­a­tions had to be per­formed under con­di­tions of extremely low vis­i­bil­ity and with dif­fi­culty of mobil­is­ing and grasp­ing the elu­sive testicles.

Excel­lent wound heal­ing
Like their whale cousins, hip­pos have excel­lent wound heal­ing which is not sur­pris­ing, given that they often injure one another in fights and would oth­er­wise suc­cumb to bac­te­r­ial wound infec­tions in an aquatic envi­ron­ment. This is based on the secre­tion of a red sweat, with antibiotic-​like prop­er­ties. Fol­low­ing surgery, the hip­pos were sim­ply returned to their respec­tive pools with no ill-​effects to wound healing.

(Source: Vetmeduni-​Vienna press release, 20.12.2013)

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