referring now to the drawings , there is seen in fig1 the preferred embodiment of the device referenced generally by the numeral 10 which is inserted into the birth canal of a pregnant livestock or zoological animal . housing 12 is configured to have an internal cavity 11 to hold the elements to be described . a removable cap 14 is provided to cover open end 15 of housing 12 and to protect the elements therein . it is also conceivable that housing 12 be formed as a unitary piece ; however , it is preferred that the elements within cavity 11 be accessible so that they may be easily replaced . housing 12 is preferably made of a non - toxic , biomedically acceptable material suitable for insertion into the birth canal of a pregnant animal . for example , the material may be latex rubber , medical grade abs , polyethelene or other material suitable for this purpose and which may be sterilized by known means . also , it is desirable that the outer perimeter of housing 12 be configured such that it may be easily inserted and held in the birth canal , not to be expelled therefrom until the fetus forcefully pushes it out at parturition . means of holding housing 12 in the birth canal may be by the shape of the device itself whereby the walls of the birth canal frictionally hold device 10 in place , by suitable coverings to augment the friction of the walls of the birth canal , by tethering device 10 to outside the birth canal , or by yet other suitable means for this purpose . it should therefore be obvious that casing 12 may be configured in many shapes and sizes depending on the type of animal , the size of the birth canal and the desired means of holding device 10 within the birth canal . held within the internal cavity 11 of housing 12 is a power source such as battery 16 to drive associated circuitry mounted on circuit board 18 . in the preferred embodiment as shown in the drawings , the circuitry includes photo - detector 20 which is electrically responsive to radiation of predetermined intensity and wave - length . in this embodiment , it is therefore required that casing 12 either be translucent , transparent or be provided with a window ( not shown ) adjacent to photo - detector 20 such that light or infrared radiation may enter cavity 11 to activate photo - detector 20 in the intended manner . examples of photo - detectors which may be used for this purpose include photo - resistors , photo - semiconductors and photo - voltaic cells , which are relatively cheap and small in size . thus , when the device 10 is inserted into the dark birth canal of a pregnant animal , photo - detector 20 is not receiving any light and is therefore unactivated . when device 10 is expelled from the birth canal by a fetus pushing it out at parturition , the level of ambient brightness increases and is allowed to pass through casing 12 to activate photo - detector 20 . if the animal is normally held within an otherwise unlit stall , a suitable light source should be provided therein sufficient to activate photo - detector 20 , which normally requires only a very slight level change in brightness to activate . activation of photo - detector 20 may by itself be used to activate an alerting device as seen in the prior art but , in the preferred embodiment , drives circuitry which , in turn , activates tone generator 22 . it is also noted that other means may be employed to activate tone generator 22 upon expulsion by a fetus , such as those seen in the aforementioned prior art wherein photo - detector 20 of the present invention is merely the preferred method of activating tone generator 22 . describing the preferred embodiment of the present invention now in more particularity , as seen in the functional block diagram of fig2 when device 10 is expelled from the birth canal of the animal , any light present at the place of birth will pass through the walls of transparent casing 12 ( or window therein ) and activate photo - detector 20 . upon activation , photo - detector 20 will activate a circuit designed to allow tone generator 22 to be activated for a substantial duration from even a momentary change in the level of brightness . this circuit may take the form of a one - shot multivibrator , indicated by reference numeral 24 , or other means to extend the decay time constant of photo - detector 20 . upon activation by photo - detector 20 , one - shot multivibrator 24 will feed a signal to astable multivibrator 26 which will then generate a low frequency , rectangular waveform . the waveform thus generated by astable multivibrator 26 causes high frequency oscillator 28 to pulse on and off repetitively . oscillator 28 drives a transducer or tone generator 22 which thereby radiates a pulsed , high frequency tone which may be either in the sonic or ultrasonic range . the purpose of pulsing the tone is to assist in the signal processing in the receiver ( described below ) so that a steady , high frequency tone , which might be present as background noise , will not give a false alarm . only the properly pulsed , high frequency sound from ejected device 10 will be detected . it is contemplated that other known techniques and circuitry may be employed to assist the receiver to discriminate , in varying accuracy , between a tone emitted from ejected device 10 and background noise to reduce the possibility of false alarms . in this regard , pulsing the tone by use of oscillator 28 is one such way to achieve this objective and is preferred . referring now to fig3 there is seen a functional block diagram of the tone receiving apparatus of the invention . the microphone 32 for the sonic or ultrasonic receiver seen in fig3 is located at some location near the birth place such that it may pick up the pulsed tone generated by expelled device 10 . it is thus understood that the sound generated by tone generator 30 serves only as a short duration , data link to the receiving system of fig3 and therefore device 10 is more simple in design than the radio transmitter of prior art and requires much less power to operate than the buzzer alerting system of prior art , which must generate tones of sufficient loudness and duration so that they may be directly heard by the attendants . a microphone indicated by reference numeral 32 is provided to pick up the pulsed tone generated by tone generator 22 of expelled device 10 . a plurality of microphones may be provided in a number of different birthing areas or stalls , each being connected to the receiving system of fig3 such that more than one pregnant animal may be monitored on the same receiving system . it is contemplated that a purality of the receiving system seen in fig2 may be connected to a single alerting system such as a multichannel , radio paging system . each such receiving system would activate only its own channel and therefore only the corresponding paging devices would be triggered . the signal thus received can then be matched with a corresponding channel of the ultimate alerting device . this arrangement would be especially useful for zoos where many different types of animals are kept . specific attendants are usually assigned to tend to specific animals and by utilizing the capabilities of a multi - channel alerting system , a specific attendant can be summoned to attend to the specific animal for which they care . microphone 32 feeds the high frequency , pulsed tone to mixer / amplifier 34 . mixing is provided when multiple inputs are fed into the receiving system seen in fig3 . the amplified signal passes through a narrow , high - frequency , band - pass filter 36 to reject to some degree all pickup but the desired high frequency tone emitted by tone generator 22 . the output of band - pass filter 36 is then detected by detector 38 , resulting in only the envelope of the high frequency waveforms remaining . this envelope signal passes through a low frequency , band - pass filter 40 , which will reject any steady waveform and pass only the pulsed frequency . the integrator and threshold detector 42 reject transient signals that may momentarily pass through the filters 36 and 40 . in this manner , all signals are more or less rejected except those of the correct tone and correct keying frequency , resulting in few , if any , false alarms . a correct tone signaling the ejection of device 10 from the birth canal of a pregnant animal is fed from integrator / threshold detector 42 to relay 44 . relay closure 44 is used to trigger any one or more of known , ultimate alerting devices that are capable of alerting an attendant a significant distance from the place of birth . other means may be used , as desired , to trigger the ultimate alerting device , such as an electronic switch , for example . examples of possible ultimate alerting devices include a radio paging system , a telephone message dialing system , an audible bell or buzzer , a light , a carrier current communicating system , or any such alerting device . many of these devices can accommodate multiple inputs which may be sent to multiple locations . several of the described receiver systems could thereby be connected to a single multi - channel radio paging system , for example , as described earlier . while the foregoing has illustrated and described what is now contemplated to be the best mode of carrying out the invention , the construction is , of course , subject to modification without departing from the spirit and scope of the invention . therefore , it is not desired to restrict the invention to the particular form of construction illustrated and described , but to cover all modifications that may fall within the scope of the appended claims .